X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=docs%2FGetElementPtr.html;h=1bf6f432de50f11b18603a6b4433a971fac9a712;hb=6f74f69ff4a6e365272a754f0984c0321755976d;hp=a9905fe6074c71db6742275554af1b6ebea6becc;hpb=f19ccf8b633a4661ef40fbb53570c814313f1750;p=oota-llvm.git diff --git a/docs/GetElementPtr.html b/docs/GetElementPtr.html index a9905fe6074..1bf6f432de5 100644 --- a/docs/GetElementPtr.html +++ b/docs/GetElementPtr.html @@ -17,7 +17,7 @@
  1. Introduction
    2. -
  2. The Questions +
  3. Address Computation
    1. Why is the extra 0 index required?
    2. What is dereferenced by GEP?
      3. @@ -25,6 +25,30 @@ subsequent ones?
    3. Why don't GEP x,0,0,1 and GEP x,1 alias?
    4. Why do GEP x,1,0,0 and GEP x,1 alias?
      5. +
    5. Can GEP index into vector elements? +
    6. Can GEP index into unions? +
    7. What effect do address spaces have on GEPs? +
    8. How is GEP different from ptrtoint, arithmetic, and inttoptr?
      9. +
    9. I'm writing a backend for a target which needs custom lowering for GEP. How do I do this? +
    10. How does VLA addressing work with GEPs? +
    4. +
  4. Rules +
      +
    1. What happens if an array index is out of bounds? +
    2. Can array indices be negative? +
    3. Can I compare two values computed with GEPs? +
    4. Can I do GEP with a different pointer type than the type of the underlying object? +
    5. Can I cast an object's address to integer and add it to null? +
    6. Can I compute the distance between two objects, and add that value to one address to compute the other address? +
    7. Can I do type-based alias analysis on LLVM IR? +
    8. What happens if a GEP computation overflows? +
    9. How can I tell if my front-end is following the rules? +
    5. +
  5. Rationale +
      +
    1. Why is GEP designed this way?
      2. +
    2. Why do struct member indices always use i32?
      3. +
    3. What's an uglygep?
  6. Summary
@@ -37,32 +61,147 @@
Introduction
+

This document seeks to dispel the mystery and confusion surrounding LLVM's - GetElementPtr (GEP) instruction. Questions about the wiley GEP instruction are - probably the most frequently occuring questions once a developer gets down to + GetElementPtr (GEP) instruction. Questions about the wily GEP instruction are + probably the most frequently occurring questions once a developer gets down to coding with LLVM. Here we lay out the sources of confusion and show that the GEP instruction is really quite simple.

-
The Questions
+
Address Computation

When people are first confronted with the GEP instruction, they tend to relate it to known concepts from other programming paradigms, most notably C - array indexing and field selection. However, GEP is a little different and - this leads to the following questions, all of which are answered in the - following sections.

-
    -
  1. Why is the extra 0 index required?
    2. -
  2. What is dereferenced by GEP?
    3. -
  3. Why can you index through the first pointer but not - subsequent ones?
    4. -
  4. Why don't GEP x,0,0,1 and GEP x,1 alias?
    5. -
  5. Why do GEP x,1,0,0 and GEP x,1 alias?
    6. -
+ array indexing and field selection. GEP closely resembles C array indexing + and field selection, however it's is a little different and this leads to + the following questions.

+
+ + +
+ What is the first index of the GEP instruction? +
+
+

Quick answer: The index stepping through the first operand.

+

The confusion with the first index usually arises from thinking about + the GetElementPtr instruction as if it was a C index operator. They aren't the + same. For example, when we write, in "C":

+ +
+
+AType *Foo;
+...
+X = &Foo->F;
+
+
+ +

it is natural to think that there is only one index, the selection of the + field F. However, in this example, Foo is a pointer. That + pointer must be indexed explicitly in LLVM. C, on the other hand, indices + through it transparently. To arrive at the same address location as the C + code, you would provide the GEP instruction with two index operands. The + first operand indexes through the pointer; the second operand indexes the + field F of the structure, just as if you wrote:

+ +
+
+X = &Foo[0].F;
+
+
+ +

Sometimes this question gets rephrased as:

+

Why is it okay to index through the first pointer, but + subsequent pointers won't be dereferenced?

+

The answer is simply because memory does not have to be accessed to + perform the computation. The first operand to the GEP instruction must be a + value of a pointer type. The value of the pointer is provided directly to + the GEP instruction as an operand without any need for accessing memory. It + must, therefore be indexed and requires an index operand. Consider this + example:

+ +
+
+struct munger_struct {
+  int f1;
+  int f2;
+};
+void munge(struct munger_struct *P) {
+  P[0].f1 = P[1].f1 + P[2].f2;
+}
+...
+munger_struct Array[3];
+...
+munge(Array);
+
+
+ +

In this "C" example, the front end compiler (llvm-gcc) will generate three + GEP instructions for the three indices through "P" in the assignment + statement. The function argument P will be the first operand of each + of these GEP instructions. The second operand indexes through that pointer. + The third operand will be the field offset into the + struct munger_struct type, for either the f1 or + f2 field. So, in LLVM assembly the munge function looks + like:

+ +
+
+void %munge(%struct.munger_struct* %P) {
+entry:
+  %tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
+  %tmp = load i32* %tmp
+  %tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
+  %tmp7 = load i32* %tmp6
+  %tmp8 = add i32 %tmp7, %tmp
+  %tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
+  store i32 %tmp8, i32* %tmp9
+  ret void
+}
+
+
+ +

In each case the first operand is the pointer through which the GEP + instruction starts. The same is true whether the first operand is an + argument, allocated memory, or a global variable.

+

To make this clear, let's consider a more obtuse example:

+ +
+
+%MyVar = uninitialized global i32
+...
+%idx1 = getelementptr i32* %MyVar, i64 0
+%idx2 = getelementptr i32* %MyVar, i64 1
+%idx3 = getelementptr i32* %MyVar, i64 2
+
+
+ +

These GEP instructions are simply making address computations from the + base address of MyVar. They compute, as follows (using C syntax): +

+ +
+
+idx1 = (char*) &MyVar + 0
+idx2 = (char*) &MyVar + 4
+idx3 = (char*) &MyVar + 8
+
+
+ +

Since the type i32 is known to be four bytes long, the indices + 0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No + memory is accessed to make these computations because the address of + %MyVar is passed directly to the GEP instructions.

+

The obtuse part of this example is in the cases of %idx2 and + %idx3. They result in the computation of addresses that point to + memory past the end of the %MyVar global, which is only one + i32 long, not three i32s long. While this is legal in LLVM, + it is inadvisable because any load or store with the pointer that results + from these GEP instructions would produce undefined results.

@@ -74,27 +213,36 @@

Quick answer: there are no superfluous indices.

This question arises most often when the GEP instruction is applied to a global variable which is always a pointer type. For example, consider - this:

-  %MyStruct = uninitialized global { float*, int }
-  ...
-  %idx = getelementptr { float*, int }* %MyStruct, long 0, ubyte 1
-

The GEP above yields an int* by indexing the int typed + this:

+ +
+
+%MyStruct = uninitialized global { float*, i32 }
+...
+%idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1
+
+
+ +

The GEP above yields an i32* by indexing the i32 typed field of the structure %MyStruct. When people first look at it, they - wonder why the long 0 index is needed. However, a closer inspection - of how globals and GEPs work reveals the need. Becoming aware of the following - facts will dispell the confusion:

+ wonder why the i64 0 index is needed. However, a closer inspection + of how globals and GEPs work reveals the need. Becoming aware of the following + facts will dispel the confusion:

    -
  1. The type of %MyStruct is not { float*, int } - but rather { float*, int }*. That is, %MyStruct is a +
  2. The type of %MyStruct is not { float*, i32 } + but rather { float*, i32 }*. That is, %MyStruct is a pointer to a structure containing a pointer to a float and an - int.
    3. + i32.
  3. Point #1 is evidenced by noticing the type of the first operand of the GEP instruction (%MyStruct) which is - { float*, int }*.
    4. -
  4. The first index, long 0 is required to dereference the - pointer associated with %MyStruct.
    5. -
  5. The second index, ubyte 1 selects the second field of the - structure (the int).
    6. + { float*, i32 }*. +
  6. The first index, i64 0 is required to step over the global + variable %MyStruct. Since the first argument to the GEP + instruction must always be a value of pointer type, the first index + steps through that pointer. A value of 0 means 0 elements offset from that + pointer.
    7. +
  7. The second index, i32 1 selects the second field of the + structure (the i32).
@@ -105,75 +253,52 @@

Quick answer: nothing.

The GetElementPtr instruction dereferences nothing. That is, it doesn't - access memory in any way. That's what the Load instruction is for. GEP is - only involved in the computation of addresses. For example, consider this:

- 
-  %MyVar = uninitialized global { [40 x int ]* }
-  ...
-  %idx = getelementptr { [40 x int]* }* %MyVar, long 0, ubyte 0, long 0, long 17
+ access memory in any way. That's what the Load and Store instructions are for. + GEP is only involved in the computation of addresses. For example, consider + this:

+ +
+
+%MyVar = uninitialized global { [40 x i32 ]* }
+...
+%idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 17
+
+
+

In this example, we have a global variable, %MyVar that is a pointer to a structure containing a pointer to an array of 40 ints. The - GEP instruction seems to be accessing the 18th integer of of the structure's + GEP instruction seems to be accessing the 18th integer of the structure's array of ints. However, this is actually an illegal GEP instruction. It won't compile. The reason is that the pointer in the structure must be dereferenced in order to index into the array of 40 ints. Since the GEP instruction never accesses memory, it is illegal.

In order to access the 18th integer in the array, you would need to do the following:

- 
-  %idx = getelementptr { [40 x int]* }* %, long 0, ubyte 0
-  %arr = load [40 x int]** %idx
-  %idx = getelementptr [40 x int]* %arr, long 0, long 17
+ +
+
+%idx = getelementptr { [40 x i32]* }* %, i64 0, i32 0
+%arr = load [40 x i32]** %idx
+%idx = getelementptr [40 x i32]* %arr, i64 0, i64 17
+
+
+

In this case, we have to load the pointer in the structure with a load instruction before we can index into the array. If the example was changed to:

- 
-  %MyVar = uninitialized global { [40 x int ] }
-  ...
-  %idx = getelementptr { [40 x int] }*, long 0, ubyte 0, long 17
-

then everything works fine. In this case, the structure does not contain a - pointer and the GEP instruction can index through the global variable pointer, - into the first field of the structure and access the 18th int in the - array there.

-
- -
- Why can you index through the first pointer? +
+
+%MyVar = uninitialized global { [40 x i32 ] }
+...
+%idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17
+
-
-

Quick answer: Because its already present.

-

Having understood the previous question, a new - question then arises:

-
Why is it okay to index through the first pointer, but - subsequent pointers won't be dereferenced?
-

The answer is simply because - memory does not have to be accessed to perform the computation. The first - operand to the GEP instruction must be a value of a pointer type. The value - of the pointer is provided directly to the GEP instruction without any need - for accessing memory. It must, therefore be indexed like any other operand. - Consider this example:

- 
-  %MyVar = unintialized global int
-  ...
-  %idx1 = getelementptr int* %MyVar, long 0
-  %idx2 = getelementptr int* %MyVar, long 1
-  %idx3 = getelementptr int* %MyVar, long 2
-

These GEP instructions are simply making address computations from the - base address of MyVar. They compute, as follows (using C syntax):

- -

Since the type int is known to be four bytes long, the indices - 0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No - memory is accessed to make these computations because the address of - %MyVar is passed directly to the GEP instructions.

-

Note that the cases of %idx2 and %idx3 are a bit silly. - They are computing addresses of something of unknown type (and thus - potentially breaking type safety) because %MyVar is only one - integer long.

+ +

then everything works fine. In this case, the structure does not contain a + pointer and the GEP instruction can index through the global variable, + into the first field of the structure and access the 18th i32 in the + array there.

@@ -185,36 +310,393 @@

If you look at the first indices in these GEP instructions you find that they are different (0 and 1), therefore the address computation diverges with that index. Consider this example:

- 
-  %MyVar = global { [10 x int ] }
-  %idx1 = getlementptr { [10 x int ] }* %MyVar, long 0, byte 0, long 1
-  %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1
+ +
+
+%MyVar = global { [10 x i32 ] }
+%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
+%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1
+
+
+

In this example, idx1 computes the address of the second integer - in the array that is in the structure in %MyVar, that is MyVar+4. The - type of idx1 is int*. However, idx2 computes the - address of the next structure after %MyVar. The type of - idx2 is { [10 x int] }* and its value is equivalent - to MyVar + 40 because it indexes past the ten 4-byte integers - in MyVar. Obviously, in such a situation, the pointers don't - alias.

+ in the array that is in the structure in %MyVar, that is + MyVar+4. The type of idx1 is i32*. However, + idx2 computes the address of the next structure after + %MyVar. The type of idx2 is { [10 x i32] }* and its + value is equivalent to MyVar + 40 because it indexes past the ten + 4-byte integers in MyVar. Obviously, in such a situation, the + pointers don't alias.

+
- Why do GEP x,1,0,0 and GEP x,1 alias? + Why do GEP x,1,0,0 and GEP x,1 alias?

Quick Answer: They compute the same address location.

These two GEP instructions will compute the same address because indexing through the 0th element does not change the address. However, it does change the type. Consider this example:

- 
-  %MyVar = global { [10 x int ] }
-  %idx1 = getlementptr { [10 x int ] }* %MyVar, long 1, byte 0, long 0
-  %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1
+ +
+
+%MyVar = global { [10 x i32 ] }
+%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
+%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1
+
+
+

In this example, the value of %idx1 is %MyVar+40 and - its type is int*. The value of %idx2 is also - MyVar+40 but its type is { [10 x int] }*.

+ its type is i32*. The value of %idx2 is also + MyVar+40 but its type is { [10 x i32] }*.

+
+ + + +
+ Can GEP index into vector elements? +
+
+

This hasn't always been forcefully disallowed, though it's not recommended. + It leads to awkward special cases in the optimizers, and fundamental + inconsistency in the IR. In the future, it will probably be outright + disallowed.

+ +
+ + + +
+ Can GEP index into unions? +
+
+

Unknown.

+ +
+ + + +
+ What effect do address spaces have on GEPs? +
+
+

None, except that the address space qualifier on the first operand pointer + type always matches the address space qualifier on the result type.

+ +
+ + + +
+ How is GEP different from ptrtoint, arithmetic, + and inttoptr? +
+
+

It's very similar; there are only subtle differences.

+ +

With ptrtoint, you have to pick an integer type. One approach is to pick i64; + this is safe on everything LLVM supports (LLVM internally assumes pointers + are never wider than 64 bits in many places), and the optimizer will actually + narrow the i64 arithmetic down to the actual pointer size on targets which + don't support 64-bit arithmetic in most cases. However, there are some cases + where it doesn't do this. With GEP you can avoid this problem. + +

Also, GEP carries additional pointer aliasing rules. It's invalid to take a + GEP from one object, address into a different separately allocated + object, and dereference it. IR producers (front-ends) must follow this rule, + and consumers (optimizers, specifically alias analysis) benefit from being + able to rely on it. See the Rules section for more + information.

+ +

And, GEP is more concise in common cases.

+ +

However, for the underlying integer computation implied, there + is no difference.

+ +
+ + + +
+ I'm writing a backend for a target which needs custom + lowering for GEP. How do I do this? +
+
+

You don't. The integer computation implied by a GEP is target-independent. + Typically what you'll need to do is make your backend pattern-match + expressions trees involving ADD, MUL, etc., which are what GEP is lowered + into. This has the advantage of letting your code work correctly in more + cases.

+ +

GEP does use target-dependent parameters for the size and layout of data + types, which targets can customize.

+ +

If you require support for addressing units which are not 8 bits, you'll + need to fix a lot of code in the backend, with GEP lowering being only a + small piece of the overall picture.

+ +
+ + + +
+ How does VLA addressing work with GEPs? +
+
+

GEPs don't natively support VLAs. LLVM's type system is entirely static, + and GEP address computations are guided by an LLVM type.

+ +

VLA indices can be implemented as linearized indices. For example, an + expression like X[a][b][c], must be effectively lowered into a form + like X[a*m+b*n+c], so that it appears to the GEP as a single-dimensional + array reference.

+ +

This means if you want to write an analysis which understands array + indices and you want to support VLAs, your code will have to be + prepared to reverse-engineer the linearization. One way to solve this + problem is to use the ScalarEvolution library, which always presents + VLA and non-VLA indexing in the same manner.

+
+ + +
Rules
+ + + + +
+ What happens if an array index is out of bounds? +
+
+

There are two senses in which an array index can be out of bounds.

+ +

First, there's the array type which comes from the (static) type of + the first operand to the GEP. Indices greater than the number of elements + in the corresponding static array type are valid. There is no problem with + out of bounds indices in this sense. Indexing into an array only depends + on the size of the array element, not the number of elements.

+ +

A common example of how this is used is arrays where the size is not known. + It's common to use array types with zero length to represent these. The + fact that the static type says there are zero elements is irrelevant; it's + perfectly valid to compute arbitrary element indices, as the computation + only depends on the size of the array element, not the number of + elements. Note that zero-sized arrays are not a special case here.

+ +

This sense is unconnected with inbounds keyword. The + inbounds keyword is designed to describe low-level pointer + arithmetic overflow conditions, rather than high-level array + indexing rules. + +

Analysis passes which wish to understand array indexing should not + assume that the static array type bounds are respected.

+ +

The second sense of being out of bounds is computing an address that's + beyond the actual underlying allocated object.

+ +

With the inbounds keyword, the result value of the GEP is + undefined if the address is outside the actual underlying allocated + object and not the address one-past-the-end.

+ +

Without the inbounds keyword, there are no restrictions + on computing out-of-bounds addresses. Obviously, performing a load or + a store requires an address of allocated and sufficiently aligned + memory. But the GEP itself is only concerned with computing addresses.

+ +
+ + +
+ Can array indices be negative? +
+
+

Yes. This is basically a special case of array indices being out + of bounds.

+ +
+ + +
+ Can I compare two values computed with GEPs? +
+
+

Yes. If both addresses are within the same allocated object, or + one-past-the-end, you'll get the comparison result you expect. If either + is outside of it, integer arithmetic wrapping may occur, so the + comparison may not be meaningful.

+ +
+ + +
+ Can I do GEP with a different pointer type than the type of + the underlying object? +
+
+

Yes. There are no restrictions on bitcasting a pointer value to an arbitrary + pointer type. The types in a GEP serve only to define the parameters for the + underlying integer computation. They need not correspond with the actual + type of the underlying object.

+ +

Furthermore, loads and stores don't have to use the same types as the type + of the underlying object. Types in this context serve only to specify + memory size and alignment. Beyond that there are merely a hint to the + optimizer indicating how the value will likely be used.

+ +
+ + +
+ Can I cast an object's address to integer and add it + to null? +
+
+

You can compute an address that way, but if you use GEP to do the add, + you can't use that pointer to actually access the object, unless the + object is managed outside of LLVM.

+ +

The underlying integer computation is sufficiently defined; null has a + defined value -- zero -- and you can add whatever value you want to it.

+ +

However, it's invalid to access (load from or store to) an LLVM-aware + object with such a pointer. This includes GlobalVariables, Allocas, and + objects pointed to by noalias pointers.

+ +

If you really need this functionality, you can do the arithmetic with + explicit integer instructions, and use inttoptr to convert the result to + an address. Most of GEP's special aliasing rules do not apply to pointers + computed from ptrtoint, arithmetic, and inttoptr sequences.

+ +
+ + +
+ Can I compute the distance between two objects, and add + that value to one address to compute the other address? +
+
+

As with arithmetic on null, You can use GEP to compute an address that + way, but you can't use that pointer to actually access the object if you + do, unless the object is managed outside of LLVM.

+ +

Also as above, ptrtoint and inttoptr provide an alternative way to do this + which do not have this restriction.

+ +
+ + +
+ Can I do type-based alias analysis on LLVM IR? +
+
+

You can't do type-based alias analysis using LLVM's built-in type system, + because LLVM has no restrictions on mixing types in addressing, loads or + stores.

+ +

It would be possible to add special annotations to the IR, probably using + metadata, to describe a different type system (such as the C type system), + and do type-based aliasing on top of that. This is a much bigger + undertaking though.

+ +
+ + + +
+ What happens if a GEP computation overflows? +
+
+

If the GEP has the inbounds keyword, the result value is + undefined.

+ +

Otherwise, the result value is the result from evaluating the implied + two's complement integer computation. However, since there's no + guarantee of where an object will be allocated in the address space, + such values have limited meaning.

+ +
+ + + +
+ How can I tell if my front-end is following the + rules? +
+
+

There is currently no checker for the getelementptr rules. Currently, + the only way to do this is to manually check each place in your front-end + where GetElementPtr operators are created.

+ +

It's not possible to write a checker which could find all rule + violations statically. It would be possible to write a checker which + works by instrumenting the code with dynamic checks though. Alternatively, + it would be possible to write a static checker which catches a subset of + possible problems. However, no such checker exists today.

+ +
+ + +
Rationale
+ + + + +
+ Why is GEP designed this way? +
+
+

The design of GEP has the following goals, in rough unofficial + order of priority:

+ +
+ + +
+ Why do struct member indices always use i32? +
+
+

The specific type i32 is probably just a historical artifact, however it's + wide enough for all practical purposes, so there's been no need to change it. + It doesn't necessarily imply i32 address arithmetic; it's just an identifier + which identifies a field in a struct. Requiring that all struct indices be + the same reduces the range of possibilities for cases where two GEPs are + effectively the same but have distinct operand types.

+ +
+ + + +
+ What's an uglygep? +
+
+

Some LLVM optimizers operate on GEPs by internally lowering them into + more primitive integer expressions, which allows them to be combined + with other integer expressions and/or split into multiple separate + integer expressions. If they've made non-trivial changes, translating + back into LLVM IR can involve reverse-engineering the structure of + the addressing in order to fit it into the static type of the original + first operand. It isn't always possibly to fully reconstruct this + structure; sometimes the underlying addressing doesn't correspond with + the static type at all. In such cases the optimizer instead will emit + a GEP with the base pointer casted to a simple address-unit pointer, + using the name "uglygep". This isn't pretty, but it's just as + valid, and it's sufficient to preserve the pointer aliasing guarantees + that GEP provides.

+
@@ -237,340 +719,14 @@ - -
Appendix: Discussion
- -
-

The following is a real discussion from the - #llvm IRC channel about the GEP - instruction. You may find this instructive as it was the basis for this - document.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
UserComment
YorionIf x & y must alias, are [ getelementptr x,0,0,1,2 ] and [ getelementptr x,1,2 ] aliased? (they obviously have different types, but they should alias...)
Yorionoops, for the second one I meant [ getelementptr y,1,2 ]
ReidI don't see how that could be, Yorion but I'm not the authority on this
Yorionhmm..
Reidthe two geps, by definition, are going to produce different pointers which are not aliased
Yorionwould [ GEP x,1,0 ] and [ GEP y,1 ] be aliased?
Reidif the second gep was [gep y,0,0,1,2] then they should be aliased as well
Reidno, I wouldn't expect that to work either :)
Reidyou can't just arbitrarily drop leading or trailing indices :)
Reid(.. leading or trailing 0 indices, I mean)
Reidthis instruction walks through a data structure and generates a pointer to the resulting thing
Reidif the number of indices are different, you're ending up at a different place and by definition they'll have different addresses
Yorionoh, I see, because of different types, [ GEP x,0,1 ] - & [ GEP x,1 ] actually might refer to different fields, but might also refer to the same ones...
Reidor, at least, that's my crude understanding of it :)
Reidno, they'll definitely refer to different fields
nicholasGEP x,0,1 ==> &((*(x+0))+1)? vs. GEP x,1 ==> &(*(x+1))?
Reidlemme grok that for a sec
Reidthat might be true in some limited definition of x, but it wouldn't be generally
nicholasoh. fields of different sizes in a structure.
Reidyup
Yorionis perhaps the type unification the reason why [ GEP x,0,1 ] and [ GEP x,1 ] cannot alias?
Reidno
Reidthey may or may not have the same type, but they are definitely different pointers
Reidlets use a concrete example for "x"
Reidsuppose x is "struct {int a, float b} *"
ReidGEP X,0,1 is going to return the address of b
ReidGEP X,1 is going to return the address of the *second* "a" (after the first b)
Yorionah, I see...
Yoriontrailing zeros are still a bit confusing...
Reidsame thing .. you're just selecting the 0th member of an array or structure
Yorionyou don't move away from the pointer, only the type is changed
Reidno, you still move away from the pointer .. the type might change, or not
Reidthe pointer definitely changes
Reidlets look at an example for trailing zero
Reidsuppose x is "int x[10][10][10][10]" (in C)
ReidGEP X,0,0 will yield you a 3 dimensional array
ReidGEP X,0,0,0,0,0 will yield you an "int"
Reidmake sense?
Yorionyes
Reidso, I think there's a law here: if the number of indices in two GEP instructions are not equivalent, there is no way the resulting pointers can alias
Reid(assuming the x and y alias)
YorionI was confused with some code in BasicAliasAnalysis that says that two pointers are equal if they differ only in trailing zeros
YorionBasicAliasAnalysis.cpp:504-518
Reidlemme look
nicholasif y1 = GEP X, 0, 0 and y2 = GEP X, 0, 0, 0, 0, 0 (from Reid's example)
nicholasthen doesn't *y1 and *y2 both refer to the same "int"?
Reidthey shouldn't
Reidhmm .. actually, maybe you're right :)
Reidthey definitely have different *types*
Yoriontrue
nicholasdifferent types just doesn't cut it. :)
Reid.. thinking on this :)
nicholassimilarly, i could create a yucky with a struct that has a char *, then have you GEP right through the pointer into the pointed-to data. That could mean that the resulting point might alias anything.
Yorionmy theory (after reading BAA) is that all zeros can be omitted, and that the pointers alias if they have the same sequence of indices
Yorionhowever, this screws the typing, so that's why zeros are for
Yorionnicholas, does that match your hunch?
nicholasI have to admit, I've had much grief with GEPIs already. I wish the semantics were plainly documented as part of their own language, instead of just relying on C aliasing rules and C semantics...
nicholasYorion: leading zeroes can't be omitted.
Reidokay, if you have two GEPs and their leading indices are an exact match, if the one with more indices only has trailing 0s then they should alias
nicholasmust alias, i think.
Reidyes, must alias, sorry
Yorionokay
YorionI'm glad we cleared this up
Reidso, if y1 = GEP X, 1,2,0 and if y2 = GEP X, 1,2,0,0,0 then y1 "must alias" y2 :)
Reidbut that doesn't work for leading 0s :)
Yorionyes, true... I was wrong
ReidI too have been having fun with GEP recently :)
Yorionbut, there're cases like [a = GEP x,1,0; b = GEP a,1,0; c = GEP b,1,0], and that should be equivalent to GEP x,1,0,1,0,1
Reidnot quite
nicholasI'm sure another rule can be written for GEPIs, but they would only apply to type-safe code.
nicholasanother *tautology
YorionReid: why not, only the type should be different...
Reidits should be equivalent to GEP x,1,0,1,0,1,0
Yorionand that must alias GEP x,1,0,1,0,1
Reidhmm, by the previous rule, I guess you're right :)
YorionI'm a bit scared that even you're a bit confused about GEP
ReidI'm glad I'm not the only one that gets a little confused wrapping my head around this stuff :)
ReidGEP has always confused me .. partly because I think its wrong :)
Reidwell, actually, not so much that GEP is wrong, but that gvars being pointers without storage
Reidi.e. when you say "%x = global int" in LLVM, the type of X is int*
Reidyet, there is no storage for that pointer
Reidits magically deduced
nicholaswell, it makes no sense to have globals be SSA...
Reidheh
Reidyeah, well .. practicalities :)
Yoriontrue
Yorionsabre gave me a reference on how globals are handled in SSA
Reidanyway, gotta run
Yorionthe paper was crappy, but I do understand now why is it implemented that way in LLVM
Yorionthx for the interesting discussion Reid
Reidheh .. its one that Chris and I keep having .. he just tells me that C has rotted my brain :)
nicholaslol
Yorionhehehe
Reidhe might be right :)
Yorionsabre: have you seen the discussion on GEP?
sabreno
sabreI'll read the backlog, j/s
sabreok, there's a lot
sabrewhat's the executive summary?
sabredo you have a q?
Yorionis it possible that GEP x,0,0,1 and GEP x,1 alias?
sabreno
Yorionand b) GEP x,1,0,0 and GEP x,1 should alias, right?
sabreI assume you mean for size = 1 ?
sabreb) yes
Yorionalthough they have different types
sabreright
Yorionokay
YorionI'm still not 100% convinced that: a=GEP x,1,0; b=GEP a,1,0; c=GEP b,1,0 cannot alias Z=GEP x,1,1,1
Yorion(that c and z cannot alias)
sabrethat's becuse they do alias
sabremustalias in fact
Yorionbut then: GEP x,1,0,1,0,1,0 = GEP x,1,1,1
sabreYorion: no
sabrec != GEP x,1,0,1,0,1,0
sabrethe first index doesn't work like that
Yorionhow does then the first index work? c and z must alias, but GEP x,1,0,1,0 != GEP x,1,1 ??
sabre*sigh*
Reid:)
Reidwe need an FAQ on this
sabreYorion: how did you get
sabre"GEP x,1,0,1,0"?
Yorionlook
sabreyou can't just concatenate subscripts
Yorionwhy?
sabrebecause... it doesn't work that way?
sabreconsider C
Yorionhow does it work?
sabreif I have blah* P
sabreP[0][1][2][3][4]
sabrethis is *not* the same as:
sabret = &P[0][1][2] ... t[3][4]
sabreYorion: Consider: struct *P
sabreP->X == P[0].X
sabreYou're losing the 0.
sabreP->X->Y == P[0].X[0].Y
sabreNot P.X.Y
sabreactually that's a bad analogy
sabrebecause C dereferences in this case
sabreTry: (&(P->X))->Y
Yorionso, a=GEP x,1,0; b=GEP a,1,0; c=GEP b,1,0, can you construct the definition of c in terms of x?
sabreyes, but you're going out of bounds :)
sabreconsider this:
sabre{ float, { double , { int } } } *P
sabreint *X = gep P, 0, 1, 1, 0
sabredo you understand the leading zero?
sabrealternatively:
sabret = gep P, 0, 1
sabret2 = gep t, 0, 1
sabreX = gep t, 0, 0
Yorionwhat's t2 for?
sabreoh
sabresorry :)
sabreX = gep t2, 0, 0
Yoriongive me a minute please
sabreok
Yorionsabre: shouldn't the type be: { float, { double, { int }* } }* P ?
sabrenope
sabrewhy the extra * ?
sabreif it helps, the type of t is { double, {int}}* and t2 is {int}* and X is int*
Yorionwait... 0 represents dereference, natural number i - represents &A[i] ?
sabregep does no dereferences, ever
sabregep P, 0, 1 is equivalent to &P[0].X
sabreaka &P->X
sabregep P, 1 == &P[1] aka P+1
sabreso gep P, 0, 1 can't alias gep P, 1 just like - &P->Y can't alias P+1
sabreassuming P is a pointer to struct {X, Y }
Yorionsabre: is it possible to come up with a general rule for "arithmetic of GEP indices"?
sabreYorion: of course, it's very simple
sabrejust not what you're expecting :)
sabreSee langref.html
Yorionfor example, a=GEP x,0,0,1 b=GEP a,0,0,1, c=GEP b,0,0,1, that should be equal to GEP x,0,1,1,0, right?
YorionI did
Yorionoops, equal to GEP x,0,1,1,1,0
sabrethat would be:
sabreGEP X, 0, 0, 1, 0, 1, 0, 1
Yorionyou're killing me
sabreThe basic rule when turning: A = GEP B, C D = GEP A, 0, E
sabreis that you drop the 0, turning it into
sabreGEP B, C, E
Yorionokay, that's good. any other rules?
nicholaswhat if it isn't a 0?
sabremore generally: A = GEP Ptr, B, C, ... D = GEP A, 0, E, F, ...
sabreD = GEP Ptr, B, C, ... E, F, ...
sabreif it's not zero, you generally cannot concatenate them
sabreunless the first gep has one subscript
sabrein which case you drop the zero
sabreif you look in InstCombiner::visitGetElementPtrInst, it should have this logic
Yorionwhat if there is no zero? how can I compute the offset from the base pointer in that case?
Yorionlike A=GEP B,C and D=GEP A,E,F
sabreyou don't have to combine them to compute an offset
sabreare you *just* trying to get a byte offset from the pointer?
YorionI'm trying to get offset of D from B
sabrewhy didn't you say so? :)
sabrewith all constant subscripts, it's trivial
sabrelook at SelectionDAGLowering::visitGetElementPtr
sabrein CodeGen/SelectionDAG/SelectionDAGISel.cpp
sabrebasically the rule is that you multiply the index by the size of the thing indexed
sabrethere is also a Support/GetElementPtrIterator or something
sabrethat makes it trivial to see what type is indexed by which subscript
sabrefor each subscript it gives you a type
sabreFor an array subscript you multiply the index by the indexed type
sabrefor a struct subscript, you add the field offset
sabres/array/sequentialtype/ if you're in a pedantic mood
Yorionlet's focus on structs: in that case, the above given example would be: D = GEP B,C,E,F?
sabreno
sabreyou drop the E if it's zero
sabreif it's not you can't concat
sabreare you trying to trick me into saying "yes, just append the indices"? :)
Yorionokay, let's assume E is not zero, how do I compute offset from B for D for a struct?
sabreWhy are you framing this in terms of concatenation?
Yorionno, I'm trying to understand it
sabrecomputing an offset and concatenating are entirely different
sabreLets consider a specific example
Yorionbecause I want to express certain properties in the terms of base pointers either globals or parameters
YorionI want to eliminate locals from my analysis
sabreyou realize that parmeters can point into the middle of structs?
Yorionyes
sabreyou realize invalid access paths can be constructed with geps/
sabre?
Yorionwhat do you mean by invalid access paths?
Yorionlike offseting out of the struct which is passed to the function?
sabreThe case where the subscript isn't zero is invalid code
sabrefrom a type-safety perspective
DannyBhe means untypesafe things that seem valid :)
DannyBIE they point somewhere in the struct, but not to any particular field
DannyB(or whatever)
sabreright
Yorionokay
sabreor they might point in some other struct :)
sabreIt's the equivalent to saying:
sabrestruct Foo { int A, int B; }
sabreFoo* P =
sabreT = &P->B;
sabreS = T+1
sabrethat is:
sabreT = gep 0, 1
sabreS = gep T, 1
sabreyou can't concat those and get a type-safe access path
sabreremember T = &P->B === T = &P[0].B
sabreunderstand?
Yorionlet me think for a minute
sabreConsider what the C case does, it will be most clear if you're used to C
sabre:)
sabreConsider the byte offset and why it doesn't point into P-> anything
sabreit points into P[1] not P[0]
Yorionit's perfectly fine if GEP offsets out of the type. I'd still need to express GEP in the terms of base pointers. Take the example above: T=GEP P,0,1; S=GEP T,1
Yoriontype safety is not crucial in my case
sabreThat specific example is GEP P, 1, 0
sabrehowever, you can form geps that are NOT equivalent to anything else
sabrefor example, consider:
sabrestruct X { int, char}
Yorionthat is fine. they're equivalent to something in the calling context
sabrethe same sequence points into padding
sabreand there is no gep that can do that
Yorionthe bottom line is: if the program is valid, interprocedural analysis will match that offset with something in one of the functions on the call stack
Yorionand that's all I care about
Yorioncan you give me a formula for structs for computing - offsets that takes into account the case GEP T,&lt:non_zeros> and the size of the structs/fields?
sabreyes, I did above
sabreTwo things:
sabreGEP Ptr, A, X, Y, Z
sabreThe result is Ptr + A * sizeof(struct) + fieldoffs(X) + fieldoffs(Y) + fieldoffs(Z)
sabresimple enough?
sabreyou see why "A" is special?
Yoriongive me a min, I'm constructing an example
Reidsabre: I think I finally understand
Reidyour comment that GEP *never* dereferences makes a lot of sense
Reidit is only doing address calculation, so the first one is taking the address of the var
sabreIf C didn't conflate lvalues and rvalues, GEP would be much easier to understand for people
Reidyeah
Yorionso, for example: M=GEP A,B,C; N=GEP M,D,E; N = [ A + B*sizeof(struct) + fieldoffs(C) ]:(of type T) + D*sizeof(T) + fieldoffs(E)
ReidI just remember learning a hard lesson about the difference between char* A and char A[] .. long time ago when I was learning C
sabreof type T*
sabreotherwise fine
Yorionokay, I think I finally understand it
sabrewithout the T* your D sizeof will be wrong
Yoriona suggestion: the formula you gave above explains it all
YorionI'd suggest explaining it that way in documentation
sabreThat's not right though
sabreit doesn't include arrays or packed types
sabreso it is, at best, a half truth
Yoriontell me, how to compute the fieldoffs for an index?
sabrearrays can be in structs :)
Yorionin bytes
sabreidx * sizeof(arrayelt)
sabrejust like for pointers (the first index)
sabreThere are two cases: structs and sequentials
sabrefor sequentials you use idx*sizeof(sequenced type)
sabrefor structs you add their offset
sabreit's really very simple :)
sabrethe first index of a gep is always over the pointer
Yorionno I meant in LLVM, how do I convert the field offset into bytes?
sabrewhich is why it's strange
sabreif you only think about structs
sabreTargetData::getFieldOffset
sabreor something
sabrelook in SelectionDAGISel.cpp (visitGEP) as I suggested.
Yoriondo you still have the energy to go over sequential types? :-)
Yorionwhat is the offset formula for sequential types?
Reidwe just went over that: idx * sizeof(elementType)
Yorionso, if there's an array hidden somewhere in the struct, essentially I need to compute idx*sizeof() instead of fieldoffs() and that's it?
sabreyes
Reidyes
Yorionexcellent.
sabreThere are two cases: structs and sequentials
sabre[9:17pm] sabre: for sequentials you use idx*sizeof(sequenced type)
sabre[9:17pm] sabre: for structs you add their offset
sabre[9:17pm] sabre: it's really very simple :)
Yorionnow when I understand it, it is simple...
Yorionthx
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