X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=docs%2FStacker.html;h=b8431d2ebe81206fd1ff99d26f65c2aa3d5d11ff;hb=e5109fae718c996ce52b485328f8108728097843;hp=eabccdf6cf10b9aeb3557876bd6aa741502c4d1b;hpb=07e89e43df34ea6c1bfff9e247040f07f59d0d6c;p=oota-llvm.git diff --git a/docs/Stacker.html b/docs/Stacker.html index eabccdf6cf1..b8431d2ebe8 100644 --- a/docs/Stacker.html +++ b/docs/Stacker.html @@ -25,8 +25,10 @@
  1. The Stack
  2. Punctuation +
  3. Comments
  4. Literals
  5. Words +
  6. Standard Style
  7. Built-Ins
@@ -40,6 +42,8 @@
  • The Runtime
  • Compiler Driver
  • Test Programs
    • +
  • Exercise
    • +
  • Things Remaining To Be Done
    • @@ -53,11 +57,11 @@

    This document is another way to learn about LLVM. Unlike the LLVM Reference Manual or -LLVM Programmer's Manual, this -document walks you through the implementation of a programming language -named Stacker. Stacker was invented specifically as a demonstration of +LLVM Programmer's Manual, here we learn +about LLVM through the experience of creating a simple programming language +named Stacker. Stacker was invented specifically as a demonstration of LLVM. The emphasis in this document is not on describing the -intricacies of LLVM itself, but on how to use it to build your own +intricacies of LLVM itself but on how to use it to build your own compiler system.

    @@ -66,21 +70,20 @@ compiler system.

    Amongst other things, LLVM is a platform for compiler writers. Because of its exceptionally clean and small IR (intermediate representation), compiler writing with LLVM is much easier than with -other system. As proof, the author of Stacker wrote the entire -compiler (language definition, lexer, parser, code generator, etc.) in -about four days! That's important to know because it shows -how quickly you can get a new -language up when using LLVM. Furthermore, this was the first +other system. As proof, I wrote the entire compiler (language definition, +lexer, parser, code generator, etc.) in about four days! +That's important to know because it shows how quickly you can get a new +language running when using LLVM. Furthermore, this was the first language the author ever created using LLVM. The learning curve is included in that four days.

    The language described here, Stacker, is Forth-like. Programs -are simple collections of word definitions and the only thing definitions +are simple collections of word definitions, and the only thing definitions can do is manipulate a stack or generate I/O. Stacker is not a "real" -programming language; its very simple. Although it is computationally +programming language; it's very simple. Although it is computationally complete, you wouldn't use it for your next big project. However, -the fact that it is complete, its simple, and it doesn't have +the fact that it is complete, it's simple, and it doesn't have a C-like syntax make it useful for demonstration purposes. It shows -that LLVM could be applied to a wide variety of language syntaxes.

    +that LLVM could be applied to a wide variety of languages.

    The basic notions behind stacker is very simple. There's a stack of integers (or character pointers) that the program manipulates. Pretty much the only thing the program can do is manipulate the stack and do @@ -92,11 +95,11 @@ program in Stacker:

    : MAIN hello_world ;

    This has two "definitions" (Stacker manipulates words, not functions and words have definitions): MAIN and -hello_world. The MAIN definition is standard, it +hello_world. The MAIN definition is standard; it tells Stacker where to start. Here, MAIN is defined to simply invoke the word hello_world. The hello_world definition tells stacker to push the -"Hello, World!" string onto the stack, print it out +"Hello, World!" string on to the stack, print it out (>s), pop it off the stack (DROP), and finally print a carriage return (CR). Although hello_world uses the stack, its net effect is null. Well @@ -106,59 +109,67 @@ written Stacker definitions have that characteristic.

    Lessons I Learned About LLVM
    -

    Stacker was written for two purposes: (a) to get the author over the -learning curve and (b) to provide a simple example of how to write a compiler -using LLVM. During the development of Stacker, many lessons about LLVM were +

    Stacker was written for two purposes:

    +
      +
    1. to get the author over the learning curve, and
      2. +
    2. to provide a simple example of how to write a compiler using LLVM.
      3. +
    +

    During the development of Stacker, many lessons about LLVM were learned. Those lessons are described in the following subsections.

    Everything's a Value!
    -

    Although I knew that LLVM used a Single Static Assignment (SSA) format, +

    Although I knew that LLVM uses a Single Static Assignment (SSA) format, it wasn't obvious to me how prevalent this idea was in LLVM until I really -started using it. Reading the Programmer's Manual and Language Reference I -noted that most of the important LLVM IR (Intermediate Representation) C++ +started using it. Reading the +Programmer's Manual and Language Reference, +I noted that most of the important LLVM IR (Intermediate Representation) C++ classes were derived from the Value class. The full power of that simple design only became fully understood once I started constructing executable expressions for Stacker.

    This really makes your programming go faster. Think about compiling code -for the following C/C++ expression: (a|b)*((x+1)/(y+1)). You could write a -function using LLVM that does exactly that, this way:

    +for the following C/C++ expression: (a|b)*((x+1)/(y+1)). Assuming +the values are on the stack in the order a, b, x, y, this could be +expressed in stacker as: 1 + SWAP 1 + / ROT2 OR *. +You could write a function using LLVM that computes this expression like this: 

    
 Value* 
-expression(BasicBlock*bb, Value* a, Value* b, Value* x, Value* y )
+expression(BasicBlock* bb, Value* a, Value* b, Value* x, Value* y )
 {
     Instruction* tail = bb->getTerminator();
     ConstantSInt* one = ConstantSInt::get( Type::IntTy, 1);
     BinaryOperator* or1 = 
-	new BinaryOperator::create( Instruction::Or, a, b, "", tail );
+	BinaryOperator::create( Instruction::Or, a, b, "", tail );
     BinaryOperator* add1 = 
-	new BinaryOperator::create( Instruction::Add, x, one, "", tail );
+	BinaryOperator::create( Instruction::Add, x, one, "", tail );
     BinaryOperator* add2 =
-	new BinaryOperator::create( Instruction::Add, y, one, "", tail );
+	BinaryOperator::create( Instruction::Add, y, one, "", tail );
     BinaryOperator* div1 = 
-	new BinaryOperator::create( Instruction::Div, add1, add2, "", tail);
+	BinaryOperator::create( Instruction::Div, add1, add2, "", tail);
     BinaryOperator* mult1 = 
-	new BinaryOperator::create( Instruction::Mul, or1, div1, "", tail );
+	BinaryOperator::create( Instruction::Mul, or1, div1, "", tail );
 
     return mult1;
 }
    -

    "Okay, big deal," you say. It is a big deal. Here's why. Note that I didn't +

    "Okay, big deal," you say? It is a big deal. Here's why. Note that I didn't have to tell this function which kinds of Values are being passed in. They could be -instructions, Constants, Global Variables, etc. Furthermore, if you specify Values -that are incorrect for this sequence of operations, LLVM will either notice right -away (at compilation time) or the LLVM Verifier will pick up the inconsistency -when the compiler runs. In no case will you make a type error that gets passed -through to the generated program. This really helps you write a compiler -that always generates correct code!

    +Instructions, Constants, GlobalVariables, or +any of the other subclasses of Value that LLVM supports. +Furthermore, if you specify Values that are incorrect for this sequence of +operations, LLVM will either notice right away (at compilation time) or the LLVM +Verifier will pick up the inconsistency when the compiler runs. In either case +LLVM prevents you from making a type error that gets passed through to the +generated program. This really helps you write a compiler that +always generates correct code!

    The second point is that we don't have to worry about branching, registers, stack variables, saving partial results, etc. The instructions we create are the values we use. Note that all that was created in the above code is a Constant value and five operators. Each of the instructions is -the resulting value of that instruction.

    +the resulting value of that instruction. This saves a lot of time.

    The lesson is this: SSA form is very powerful: there is no difference - between a value and the instruction that created it. This is fully +between a value and the instruction that created it. This is fully enforced by the LLVM IR. Use it to your best advantage.

    @@ -186,12 +197,11 @@ the compiler and the module you just created fails on the LLVM Verifier.

    Concrete Blocks

    After a little initial fumbling around, I quickly caught on to how blocks -should be constructed. The use of the standard template library really helps -simply the interface. In general, here's what I learned: +should be constructed. In general, here's what I learned:

    1. Create your blocks early. While writing your compiler, you will encounter several situations where you know apriori that you will - need several blocks. For example, if-then-else, switch, while and for + need several blocks. For example, if-then-else, switch, while, and for statements in C/C++ all need multiple blocks for expression in LVVM. The rule is, create them early.
    2. Terminate your blocks early. This just reduces the chances @@ -206,19 +216,17 @@ simply the interface. In general, here's what I learned: getTerminator() method on a BasicBlock), it can always be used as the insert_before argument to your instruction constructors. This causes the instruction to automatically be inserted in - the RightPlace&tm; place, just before the terminating instruction. The + the RightPlace™ place, just before the terminating instruction. The nice thing about this design is that you can pass blocks around and insert - new instructions into them without ever known what instructions came + new instructions into them without ever knowing what instructions came before. This makes for some very clean compiler design.

    The foregoing is such an important principal, its worth making an idiom:

    -
    -
+
    
 BasicBlock* bb = new BasicBlock();
 bb->getInstList().push_back( new Branch( ... ) );
 new Instruction(..., bb->getTerminator() );
-
-
    
+

    To make this clear, consider the typical if-then-else statement (see StackerCompiler::handle_if() method). We can set this up in a single function using LLVM in the following way:

    @@ -228,45 +236,47 @@ BasicBlock* MyCompiler::handle_if( BasicBlock* bb, SetCondInst* condition ) { // Create the blocks to contain code in the structure of if/then/else - BasicBlock* then = new BasicBlock(); - BasicBlock* else = new BasicBlock(); - BasicBlock* exit = new BasicBlock(); + BasicBlock* then_bb = new BasicBlock(); + BasicBlock* else_bb = new BasicBlock(); + BasicBlock* exit_bb = new BasicBlock(); // Insert the branch instruction for the "if" - bb->getInstList().push_back( new BranchInst( then, else, condition ) ); + bb->getInstList().push_back( new BranchInst( then_bb, else_bb, condition ) ); // Set up the terminating instructions - then->getInstList().push_back( new BranchInst( exit ) ); - else->getInstList().push_back( new BranchInst( exit ) ); + then->getInstList().push_back( new BranchInst( exit_bb ) ); + else->getInstList().push_back( new BranchInst( exit_bb ) ); // Fill in the then part .. details excised for brevity - this->fill_in( then ); + this->fill_in( then_bb ); // Fill in the else part .. details excised for brevity - this->fill_in( else ); + this->fill_in( else_bb ); // Return a block to the caller that can be filled in with the code // that follows the if/then/else construct. - return exit; + return exit_bb; }

    Presumably in the foregoing, the calls to the "fill_in" method would add the instructions for the "then" and "else" parts. They would use the third part of the idiom almost exclusively (inserting new instructions before the terminator). Furthermore, they could even recurse back to handle_if -should they encounter another if/then/else statement and it will all "just work". -

    +should they encounter another if/then/else statement, and it will just work.

    Note how cleanly this all works out. In particular, the push_back methods on the BasicBlock's instruction list. These are lists of type -Instruction which also happen to be Values. To create +Instruction (which is also of type Value). To create the "if" branch we merely instantiate a BranchInst that takes as -arguments the blocks to branch to and the condition to branch on. The blocks -act like branch labels! This new BranchInst terminates -the BasicBlock provided as an argument. To give the caller a way -to keep inserting after calling handle_if we create an "exit" block -which is returned to the caller. Note that the "exit" block is used as the -terminator for both the "then" and the "else" blocks. This gaurantees that no -matter what else "handle_if" or "fill_in" does, they end up at the "exit" block. +arguments the blocks to branch to and the condition to branch on. The +BasicBlock objects act like branch labels! This new +BranchInst terminates the BasicBlock provided +as an argument. To give the caller a way to keep inserting after calling +handle_if, we create an exit_bb block which is +returned +to the caller. Note that the exit_bb block is used as the +terminator for both the then_bb and the else_bb +blocks. This guarantees that no matter what else handle_if +or fill_in does, they end up at the exit_bb block.

    @@ -277,7 +287,7 @@ One of the first things I noticed is the frequent use of the "push_back" method on the various lists. This is so common that it is worth mentioning. The "push_back" inserts a value into an STL list, vector, array, etc. at the end. The method might have also been named "insert_tail" or "append". -Althought I've used STL quite frequently, my use of push_back wasn't very +Although I've used STL quite frequently, my use of push_back wasn't very high in other programs. In LLVM, you'll use it all the time.

    @@ -286,17 +296,17 @@ high in other programs. In LLVM, you'll use it all the time.

    It took a little getting used to and several rounds of postings to the LLVM -mail list to wrap my head around this instruction correctly. Even though I had +mailing list to wrap my head around this instruction correctly. Even though I had read the Language Reference and Programmer's Manual a couple times each, I still missed a few very key points:

    This means that when you look up an element in the global variable (assuming -its a struct or array), you must deference the pointer first! For many +it's a struct or array), you must deference the pointer first! For many things, this leads to the idiom: 

    
@@ -312,40 +322,43 @@ pointer. The second index subscripts the array. If you're a "C" programmer, this
 will run against your grain because you'll naturally think of the global array
 variable and the address of its first element as the same. That tripped me up
 for a while until I realized that they really do differ .. by type.
-Remember that LLVM is a strongly typed language itself. Absolutely everything
-has a type.  The "type" of the global variable is [24 x int]*. That is, its
+Remember that LLVM is strongly typed. Everything has a type.  
+The "type" of the global variable is [24 x int]*. That is, it's
 a pointer to an array of 24 ints.  When you dereference that global variable with
-a single index, you now have a " [24 x int]" type, the pointer is gone. Although
+a single (0) index, you now have a "[24 x int]" type.  Although
 the pointer value of the dereferenced global and the address of the zero'th element
 in the array will be the same, they differ in their type. The zero'th element has
 type "int" while the pointer value has type "[24 x int]".
    
-
    Get this one aspect of LLVM right in your head and you'll save yourself
+
    Get this one aspect of LLVM right in your head, and you'll save yourself
 a lot of compiler writing headaches down the road.
    Getting Linkage Types Right

    Linkage types in LLVM can be a little confusing, especially if your compiler -writing mind has affixed very hard concepts to particular words like "weak", +writing mind has affixed firm concepts to particular words like "weak", "external", "global", "linkonce", etc. LLVM does not use the precise -definitions of say ELF or GCC even though they share common terms. To be fair, +definitions of, say, ELF or GCC, even though they share common terms. To be fair, the concepts are related and similar but not precisely the same. This can lead you to think you know what a linkage type represents but in fact it is slightly different. I recommend you read the Language Reference on this topic very -carefully.

    +carefully. Then, read it again.

    Here are some handy tips that I discovered along the way:

    @@ -356,7 +369,7 @@ Constants in LLVM took a little getting used to until I discovered a few utility functions in the LLVM IR that make things easier. Here's what I learned: