Stacker: An Example Of Using LLVM

+ +

+ -

This document is another way to learn about LLVM. Unlike the LLVM Reference Manual or @@ -128,31 +127,28 @@ 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)). 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:


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

"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, GlobalVariables, or @@ -202,7 +198,7 @@ should be constructed. In general, here's what I learned:

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 - statements in C/C++ all need multiple blocks for expression in LVVM. + statements in C/C++ all need multiple blocks for expression in LLVM. The rule is, create them early.

Terminate your blocks early. This just reduces the chances that you forget to terminate your blocks which is required (go @@ -222,18 +218,18 @@ should be constructed. In general, here's what I learned: 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();
++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:

 using namespace llvm;
 BasicBlock*
-MyCompiler::handle_if( BasicBlock* bb, SetCondInst* condition )
+MyCompiler::handle_if( BasicBlock* bb, ICmpInst* condition )
 {
     // Create the blocks to contain code in the structure of if/then/else
     BasicBlock* then_bb = new BasicBlock(); 
@@ -301,20 +297,21 @@ read the Language Reference and Programmer's Manual a couple times each, I still
 missed a few very key points:
 
 
-    GetElementPtrInst gives you back a Value for the last thing indexed.
-    
All global variables in LLVM  are pointers.
-    
Pointers must also be dereferenced with the GetElementPtrInst instruction.
+
GetElementPtrInst gives you back a Value for the last thing indexed.
+All global variables in LLVM  are pointers.
+Pointers must also be dereferenced with the GetElementPtrInst
+instruction.
 
 This means that when you look up an element in the global variable (assuming
 it's a struct or array), you must deference the pointer first! For many
 things, this leads to the idiom:
 
-
-std::vector index_vector;
-index_vector.push_back( ConstantSInt::get( Type::LongTy, 0 );
++std::vector<Value*> index_vector;
+index_vector.push_back( ConstantInt::get( Type::LongTy, 0 );
 // ... push other indices ...
 GetElementPtrInst* gep = new GetElementPtrInst( ptr, index_vector );
-
+
 For example, suppose we have a global variable whose type is [24 x int]. The
 variable itself represents a pointer to that array. To subscript the
 array, we need two indices, not just one. The first index (0) dereferences the
@@ -346,7 +343,7 @@ different. I recommend you read the
 carefully. Then, read it again.

 
Here are some handy tips that I discovered along the way:
 
-    Unitialized means external. That is, the symbol is declared in the current
+    
Uninitialized means external. That is, the symbol is declared in the current
     module and can be used by that module, but it is not defined by that module.
     Setting an initializer changes a global' linkage type. Setting an 
     initializer changes a global's linkage type from whatever it was to a normal, 
@@ -370,9 +367,9 @@ functions in the LLVM IR that make things easier. Here's what I learned: 
 
  Constants are Values like anything else and can be operands of instructions
  Integer constants, frequently needed, can be created using the static "get"
- methods of the ConstantInt, ConstantSInt, and ConstantUInt classes. The nice thing
- about these is that you can "get" any kind of integer quickly.
- There's a special method on Constant class which allows you to get the null 
+ methods of the ConstantInt class. The nice thing about these is that you can 
+ "get" any kind of integer quickly.
+ There's a special method on Constant class which allows you to get the null
  constant for any type. This is really handy for initializing large 
  arrays or structures, etc.
 
@@ -466,6 +463,11 @@ unit. Anything declared with FORWARD is an external symbol for
 linking.

- - - - - - - - - - - - -

Definition Of Operation Of Built In Words

LOGICAL OPERATIONS

Word Name Operation Description

w1 w2 -- b

Two values (w1 and w2) are popped off the stack and + + + + + + + + + + + + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + + - - - - + + + + - + - - - - - - - - - + + + + + + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - + - - - - - - - - - + + + + + + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - + - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - + - - - - - - - - - + + + + + + + + - - - - + + + - - - - + + + - - - - + + + - + - - - - - - - - - + + + + + + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - - - + + + - - + 10 WHILE >d -- END
+ This will print the numbers from 10 down to 1. 10 is pushed on the + stack. Since that is non-zero, the while loop is entered. The top of + the stack (10) is printed out with >d. The top of the stack is + decremented, yielding 9 and control is transfered back to the WHILE + keyword. The process starts all over again and repeats until + the top of stack is decremented to 0 at which point the WHILE test + fails and control is transfered to the word after the END. + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

Definition Of Operation Of Built In Words
LOGICAL OPERATIONS
Word	Name	Operation	Description
<	LT	w1 w2 -- b	Two values (w1 and w2) are popped off the stack and compared. If w1 is less than w2, TRUE is pushed back on the stack, otherwise FALSE is pushed back on the stack.
>	GT	w1 w2 -- b	Two values (w1 and w2) are popped off the stack and +
>	GT	w1 w2 -- b	Two values (w1 and w2) are popped off the stack and compared. If w1 is greater than w2, TRUE is pushed back on the stack, otherwise FALSE is pushed back on the stack.
>=	GE	w1 w2 -- b	Two values (w1 and w2) are popped off the stack and +
>=	GE	w1 w2 -- b	Two values (w1 and w2) are popped off the stack and compared. If w1 is greater than or equal to w2, TRUE is pushed back on the stack, otherwise FALSE is pushed back on the stack.
<=	LE	w1 w2 -- b	Two values (w1 and w2) are popped off the stack and +
<=	LE	w1 w2 -- b	Two values (w1 and w2) are popped off the stack and compared. If w1 is less than or equal to w2, TRUE is pushed back on the stack, otherwise FALSE is pushed back on the stack.
=	EQ	w1 w2 -- b	Two values (w1 and w2) are popped off the stack and +
=	EQ	w1 w2 -- b	Two values (w1 and w2) are popped off the stack and compared. If w1 is equal to w2, TRUE is pushed back on the stack, otherwise FALSE is pushed back
<>	NE	w1 w2 -- b	Two values (w1 and w2) are popped off the stack and +
<>	NE	w1 w2 -- b	Two values (w1 and w2) are popped off the stack and compared. If w1 is equal to w2, TRUE is pushed back on the stack, otherwise FALSE is pushed back
FALSE	FALSE	-- b	The boolean value FALSE (0) is pushed on to the stack.
FALSE	FALSE	-- b	The boolean value FALSE (0) is pushed on to the stack.
TRUE	TRUE	-- b	The boolean value TRUE (-1) is pushed on to the stack.
TRUE	TRUE	-- b	The boolean value TRUE (-1) is pushed on to the stack.
BITWISE OPERATORS
BITWISE OPERATORS
Word	Name	Operation	Description
<<	SHL	w1 w2 -- w1<<w2	Two values (w1 and w2) are popped off the stack. The w2 +	Word	Name	Operation	Description
<<	SHL	w1 w2 -- w1<<w2	Two values (w1 and w2) are popped off the stack. The w2 operand is shifted left by the number of bits given by the w1 operand. The result is pushed back to the stack.
>>	SHR	w1 w2 -- w1>>w2	Two values (w1 and w2) are popped off the stack. The w2 +
>>	SHR	w1 w2 -- w1>>w2	Two values (w1 and w2) are popped off the stack. The w2 operand is shifted right by the number of bits given by the w1 operand. The result is pushed back to the stack.
OR	OR	w1 w2 -- w2\|w1	Two values (w1 and w2) are popped off the stack. The values +
OR	OR	w1 w2 -- w2\|w1	Two values (w1 and w2) are popped off the stack. The values are bitwise OR'd together and pushed back on the stack. This is not a logical OR. The sequence 1 2 OR yields 3 not 1.
AND	AND	w1 w2 -- w2&w1	Two values (w1 and w2) are popped off the stack. The values +
AND	AND	w1 w2 -- w2&w1	Two values (w1 and w2) are popped off the stack. The values are bitwise AND'd together and pushed back on the stack. This is not a logical AND. The sequence 1 2 AND yields 0 not 1.
XOR	XOR	w1 w2 -- w2^w1	Two values (w1 and w2) are popped off the stack. The values +
XOR	XOR	w1 w2 -- w2^w1	Two values (w1 and w2) are popped off the stack. The values are bitwise exclusive OR'd together and pushed back on the stack. For example, The sequence 1 3 XOR yields 2.
ARITHMETIC OPERATORS
ARITHMETIC OPERATORS
Word	Name	Operation	Description
ABS	ABS	w -- \|w\|	One value s popped off the stack; its absolute value is computed +	Word	Name	Operation	Description
ABS	ABS	w -- \|w\|	One value s popped off the stack; its absolute value is computed and then pushed on to the stack. If w1 is -1 then w2 is 1. If w1 is 1 then w2 is also 1.
NEG	NEG	w -- -w	One value is popped off the stack which is negated and then +
NEG	NEG	w -- -w	One value is popped off the stack which is negated and then pushed back on to the stack. If w1 is -1 then w2 is 1. If w1 is 1 then w2 is -1.
+	ADD	w1 w2 -- w2+w1	Two values are popped off the stack. Their sum is pushed back +
+	ADD	w1 w2 -- w2+w1	Two values are popped off the stack. Their sum is pushed back on to the stack
-	SUB	w1 w2 -- w2-w1	Two values are popped off the stack. Their difference is pushed back +
-	SUB	w1 w2 -- w2-w1	Two values are popped off the stack. Their difference is pushed back on to the stack
*	MUL	w1 w2 -- w2*w1	Two values are popped off the stack. Their product is pushed back +
*	MUL	w1 w2 -- w2*w1	Two values are popped off the stack. Their product is pushed back on to the stack
/	DIV	w1 w2 -- w2/w1	Two values are popped off the stack. Their quotient is pushed back +
/	DIV	w1 w2 -- w2/w1	Two values are popped off the stack. Their quotient is pushed back on to the stack
MOD	MOD	w1 w2 -- w2%w1	Two values are popped off the stack. Their remainder after division +
MOD	MOD	w1 w2 -- w2%w1	Two values are popped off the stack. Their remainder after division of w1 by w2 is pushed back on to the stack
*/	STAR_SLAH	w1 w2 w3 -- (w3*w2)/w1	Three values are popped off the stack. The product of w1 and w2 is +
*/	STAR_SLAH	w1 w2 w3 -- (w3*w2)/w1	Three values are popped off the stack. The product of w1 and w2 is divided by w3. The result is pushed back on to the stack.
++	INCR	w -- w+1	One value is popped off the stack. It is incremented by one and then +
++	INCR	w -- w+1	One value is popped off the stack. It is incremented by one and then pushed back on to the stack.
--	DECR	w -- w-1	One value is popped off the stack. It is decremented by one and then +
--	DECR	w -- w-1	One value is popped off the stack. It is decremented by one and then pushed back on to the stack.
MIN	MIN	w1 w2 -- (w2<w1?w2:w1)	Two values are popped off the stack. The larger one is pushed back +
MIN	MIN	w1 w2 -- (w2<w1?w2:w1)	Two values are popped off the stack. The larger one is pushed back on to the stack.
MAX	MAX	w1 w2 -- (w2>w1?w2:w1)	Two values are popped off the stack. The larger value is pushed back +
MAX	MAX	w1 w2 -- (w2>w1?w2:w1)	Two values are popped off the stack. The larger value is pushed back on to the stack.
STACK MANIPULATION OPERATORS
STACK MANIPULATION OPERATORS
Word	Name	Operation	Description
DROP	DROP	w --	One value is popped off the stack.
DROP2	DROP2	w1 w2 --	Two values are popped off the stack.
NIP	NIP	w1 w2 -- w2	The second value on the stack is removed from the stack. That is, +	Word	Name	Operation	Description
DROP	DROP	w --	One value is popped off the stack.
DROP2	DROP2	w1 w2 --	Two values are popped off the stack.
NIP	NIP	w1 w2 -- w2	The second value on the stack is removed from the stack. That is, a value is popped off the stack and retained. Then a second value is popped and the retained value is pushed.
NIP2	NIP2	w1 w2 w3 w4 -- w3 w4	The third and fourth values on the stack are removed from it. That is, +
NIP2	NIP2	w1 w2 w3 w4 -- w3 w4	The third and fourth values on the stack are removed from it. That is, two values are popped and retained. Then two more values are popped and the two retained values are pushed back on.
DUP	DUP	w1 -- w1 w1	One value is popped off the stack. That value is then pushed on to +
DUP	DUP	w1 -- w1 w1	One value is popped off the stack. That value is then pushed on to the stack twice to duplicate the top stack vaue.
DUP2	DUP2	w1 w2 -- w1 w2 w1 w2	The top two values on the stack are duplicated. That is, two vaues +
DUP2	DUP2	w1 w2 -- w1 w2 w1 w2	The top two values on the stack are duplicated. That is, two vaues are popped off the stack. They are alternately pushed back on the stack twice each.
SWAP	SWAP	w1 w2 -- w2 w1	The top two stack items are reversed in their order. That is, two +
SWAP	SWAP	w1 w2 -- w2 w1	The top two stack items are reversed in their order. That is, two values are popped off the stack and pushed back on to the stack in the opposite order they were popped.
SWAP2	SWAP2	w1 w2 w3 w4 -- w3 w4 w2 w1	The top four stack items are swapped in pairs. That is, two values +
SWAP2	SWAP2	w1 w2 w3 w4 -- w3 w4 w2 w1	The top four stack items are swapped in pairs. That is, two values are popped and retained. Then, two more values are popped and retained. The values are pushed back on to the stack in the reverse order but - in pairs. + in pairs.
OVER	OVER	w1 w2-- w1 w2 w1	Two values are popped from the stack. They are pushed back +
OVER	OVER	w1 w2-- w1 w2 w1	Two values are popped from the stack. They are pushed back on to the stack in the order w1 w2 w1. This seems to cause the top stack element to be duplicated "over" the next value.
OVER2	OVER2	w1 w2 w3 w4 -- w1 w2 w3 w4 w1 w2	The third and fourth values on the stack are replicated on to the +
OVER2	OVER2	w1 w2 w3 w4 -- w1 w2 w3 w4 w1 w2	The third and fourth values on the stack are replicated on to the top of the stack
ROT	ROT	w1 w2 w3 -- w2 w3 w1	The top three values are rotated. That is, three value are popped +
ROT	ROT	w1 w2 w3 -- w2 w3 w1	The top three values are rotated. That is, three value are popped off the stack. They are pushed back on to the stack in the order w1 w3 w2.
ROT2	ROT2	w1 w2 w3 w4 w5 w6 -- w3 w4 w5 w6 w1 w2	Like ROT but the rotation is done using three pairs instead of +
ROT2	ROT2	w1 w2 w3 w4 w5 w6 -- w3 w4 w5 w6 w1 w2	Like ROT but the rotation is done using three pairs instead of three singles.
RROT	RROT	w1 w2 w3 -- w2 w3 w1	Reverse rotation. Like ROT, but it rotates the other way around. +
RROT	RROT	w1 w2 w3 -- w3 w1 w2	Reverse rotation. Like ROT, but it rotates the other way around. Essentially, the third element on the stack is moved to the top of the stack.
RROT2	RROT2	w1 w2 w3 w4 w5 w6 -- w3 w4 w5 w6 w1 w2	Double reverse rotation. Like RROT but the rotation is done using +
RROT2	RROT2	w1 w2 w3 w4 w5 w6 -- w3 w4 w5 w6 w1 w2	Double reverse rotation. Like RROT but the rotation is done using three pairs instead of three singles. The fifth and sixth stack elements are moved to the first and second positions
TUCK	TUCK	w1 w2 -- w2 w1 w2	Similar to OVER except that the second operand is being +
TUCK	TUCK	w1 w2 -- w2 w1 w2	Similar to OVER except that the second operand is being replicated. Essentially, the first operand is being "tucked" in between two instances of the second operand. Logically, two values are popped off the stack. They are placed back on the stack in the order w2 w1 w2.
TUCK2	TUCK2	w1 w2 w3 w4 -- w3 w4 w1 w2 w3 w4	Like TUCK but a pair of elements is tucked over two pairs. +
TUCK2	TUCK2	w1 w2 w3 w4 -- w3 w4 w1 w2 w3 w4	Like TUCK but a pair of elements is tucked over two pairs. That is, the top two elements of the stack are duplicated and inserted into the stack at the fifth and positions.
PICK	PICK	x0 ... Xn n -- x0 ... Xn x0	The top of the stack is used as an index into the remainder of +
PICK	PICK	x0 ... Xn n -- x0 ... Xn x0	The top of the stack is used as an index into the remainder of the stack. The element at the nth position replaces the index (top of stack). This is useful for cycling through a set of values. Note that indexing is zero based. So, if n=0 then you get the second item on the stack. If n=1 you get the third, etc. Note also that the index is replaced by the n'th value.
SELECT	SELECT	m n X0..Xm Xm+1 .. Xn -- Xm	This is like PICK but the list is removed and you need to specify +
SELECT	SELECT	m n X0..Xm Xm+1 .. Xn -- Xm	This is like PICK but the list is removed and you need to specify both the index and the size of the list. Careful with this one, the wrong value for n can blow away a huge amount of the stack.
ROLL	ROLL	x0 x1 .. xn n -- x1 .. xn x0	Not Implemented. This one has been left as an exercise to +
ROLL	ROLL	x0 x1 .. xn n -- x1 .. xn x0	Not Implemented. This one has been left as an exercise to the student. See Exercise. ROLL requires a value, "n", to be on the top of the stack. This value specifies how far into the stack to "roll". The n'th value is moved (not @@ -842,25 +845,25 @@ using the following construction: how much to rotate. That is, ROLL with n=1 is the same as ROT and ROLL with n=2 is the same as ROT2.
MEMORY OPERATORS
MEMORY OPERATORS
Word	Name	Operation	Description
MALLOC	MALLOC	w1 -- p	One value is popped off the stack. The value is used as the size +	Word	Name	Operation	Description
MALLOC	MALLOC	w1 -- p	One value is popped off the stack. The value is used as the size of a memory block to allocate. The size is in bytes, not words. The memory allocation is completed and the address of the memory block is pushed on to the stack.
FREE	FREE	p --	One pointer value is popped off the stack. The value should be +
FREE	FREE	p --	One pointer value is popped off the stack. The value should be the address of a memory block created by the MALLOC operation. The associated memory block is freed. Nothing is pushed back on the stack. Many bugs can be created by attempting to FREE something @@ -872,20 +875,20 @@ using the following construction: the stack (for the FREE at the end) and that every use of the pointer is preceded by a DUP to retain the copy for FREE.
GET	GET	w1 p -- w2 p	An integer index and a pointer to a memory block are popped of +
GET	GET	w1 p -- w2 p	An integer index and a pointer to a memory block are popped of the block. The index is used to index one byte from the memory block. That byte value is retained, the pointer is pushed again and the retained value is pushed. Note that the pointer value s essentially retained in its position so this doesn't count as a "use ptr" in the FREE idiom.
PUT	PUT	w1 w2 p -- p	An integer value is popped of the stack. This is the value to +
PUT	PUT	w1 w2 p -- p	An integer value is popped of the stack. This is the value to be put into a memory block. Another integer value is popped of the stack. This is the indexed byte in the memory block. A pointer to the memory block is popped off the stack. The @@ -895,33 +898,33 @@ using the following construction: pushed back on the stack so this doesn't count as a "use ptr" in the FREE idiom.
CONTROL FLOW OPERATORS
CONTROL FLOW OPERATORS
Word	Name	Operation	Description
RETURN	RETURN	--	The currently executing definition returns immediately to its caller. +	Word	Name	Operation	Description
RETURN	RETURN	--	The currently executing definition returns immediately to its caller. Note that there is an implicit `RETURN` at the end of each definition, logically located at the semi-colon. The sequence `RETURN ;` is valid but redundant.
EXIT	EXIT	w1 --	A return value for the program is popped off the stack. The program is +
EXIT	EXIT	w1 --	A return value for the program is popped off the stack. The program is then immediately terminated. This is normally an abnormal exit from the program. For a normal exit (when `MAIN` finishes), the exit code will always be zero in accordance with UNIX conventions.
RECURSE	RECURSE	--	The currently executed definition is called again. This operation is +
RECURSE	RECURSE	--	The currently executed definition is called again. This operation is needed since the definition of a word doesn't exist until the semi colon is reacher. Attempting something like: `: recurser recurser ;` will yield and error saying that @@ -929,102 +932,109 @@ using the following construction: to: `: recurser RECURSE ;`
IF (words...) ENDIF	IF (words...) ENDIF	b --	A boolean value is popped of the stack. If it is non-zero then the "words..." +
IF (words...) ENDIF	IF (words...) ENDIF	b --	A boolean value is popped of the stack. If it is non-zero then the "words..." are executed. Otherwise, execution continues immediately following the ENDIF.
IF (words...) ELSE (words...) ENDIF	IF (words...) ELSE (words...) ENDIF	b --	A boolean value is popped of the stack. If it is non-zero then the "words..." +
IF (words...) ELSE (words...) ENDIF	IF (words...) ELSE (words...) ENDIF	b --	A boolean value is popped of the stack. If it is non-zero then the "words..." between IF and ELSE are executed. Otherwise the words between ELSE and ENDIF are executed. In either case, after the (words....) have executed, execution continues immediately following the ENDIF.
WHILE (words...) END	WHILE (words...) END	b -- b	The boolean value on the top of the stack is examined. If it is non-zero then the - "words..." between WHILE and END are executed. Execution then begins again at the WHILE where another - boolean is popped off the stack. To prevent this operation from eating up the entire - stack, you should push on to the stack (just before the END) a boolean value that indicates - whether to terminate. Note that since booleans and integers can be coerced you can - use the following "for loop" idiom: - `(push count) WHILE (words...) -- END` +
WHILE word END	WHILE word END	b -- b	The boolean value on the top of the stack is examined (not popped). If + it is non-zero then the "word" between WHILE and END is executed. + Execution then begins again at the WHILE where the boolean on the top of + the stack is examined again. The stack is not modified by the WHILE...END + loop, only examined. It is imperative that the "word" in the body of the + loop ensure that the top of the stack contains the next boolean to examine + when it completes. Note that since booleans and integers can be coerced + you can use the following "for loop" idiom: + `(push count) WHILE word -- END` For example: - `10 WHILE DUP >d -- END` - This will print the numbers from 10 down to 1. 10 is pushed on the stack. Since that is - non-zero, the while loop is entered. The top of the stack (10) is duplicated and then - printed out with >d. The top of the stack is decremented, yielding 9 and control is - transfered back to the WHILE keyword. The process starts all over again and repeats until - the top of stack is decremented to 0 at which the WHILE test fails and control is - transfered to the word after the END.
INPUT & OUTPUT OPERATORS
INPUT & OUTPUT OPERATORS
Word	Name	Operation	Description
SPACE	SPACE	--	A space character is put out. There is no stack effect.
TAB	TAB	--	A tab character is put out. There is no stack effect.
CR	CR	--	A carriage return character is put out. There is no stack effect.
>s	OUT_STR	--	A string pointer is popped from the stack. It is put out.
>d	OUT_STR	--	A value is popped from the stack. It is put out as a decimal integer.
>c	OUT_CHR	--	A value is popped from the stack. It is put out as an ASCII character.
<s	IN_STR	-- s	A string is read from the input via the scanf(3) format string " %as". The - resulting string is pushed on to the stack.
<d	IN_STR	-- w	An integer is read from the input via the scanf(3) format string " %d". The - resulting value is pushed on to the stack
<c	IN_CHR	-- w	A single character is read from the input via the scanf(3) format string - " %c". The value is converted to an integer and pushed on to the stack.
DUMP	DUMP	--	The stack contents are dumped to standard output. This is useful for +	Word	Name	Operation	Description
SPACE	SPACE	--	A space character is put out. There is no stack effect.
TAB	TAB	--	A tab character is put out. There is no stack effect.
CR	CR	--	A carriage return character is put out. There is no stack effect.
>s	OUT_STR	--	A string pointer is popped from the stack. It is put out.
>d	OUT_STR	--	A value is popped from the stack. It is put out as a decimal + integer.
>c	OUT_CHR	--	A value is popped from the stack. It is put out as an ASCII + character.
<s	IN_STR	-- s	A string is read from the input via the scanf(3) format string " %as". + The resulting string is pushed on to the stack.
<d	IN_STR	-- w	An integer is read from the input via the scanf(3) format string " %d". + The resulting value is pushed on to the stack
<c	IN_CHR	-- w	A single character is read from the input via the scanf(3) format string + " %c". The value is converted to an integer and pushed on to the stack.
DUMP	DUMP	--	The stack contents are dumped to standard output. This is useful for debugging your definitions. Put DUMP at the beginning and end of a definition to see instantly the net effect of the definition.

Prime: A Complete Example

@@ -1051,7 +1061,7 @@ remainder of the story. ################################################################################ # Utility definitions ################################################################################ -: print >d CR ; +: print >d CR ; : it_is_a_prime TRUE ; : it_is_not_a_prime FALSE ; : continue_loop TRUE ; @@ -1061,10 +1071,10 @@ remainder of the story. # This definition tries an actual division of a candidate prime number. It # determines whether the division loop on this candidate should continue or # not. -# STACK<: +# STACK<: # div - the divisor to try # p - the prime number we are working on -# STACK>: +# STACK>: # cont - should we continue the loop ? # div - the next divisor to try # p - the prime number we are working on @@ -1090,7 +1100,7 @@ remainder of the story. # cont - should we continue the loop (ignored)? # div - the divisor to try # p - the prime number we are working on -# STACK>: +# STACK>: # cont - should we continue the loop ? # div - the next divisor to try # p - the prime number we are working on @@ -1115,9 +1125,9 @@ remainder of the story. # definition which returns a loop continuation value (which we also seed with # the value 1). After the loop, we check the divisor. If it decremented all # the way to zero then we found a prime, otherwise we did not find one. -# STACK<: +# STACK<: # p - the prime number to check -# STACK>: +# STACK>: # yn - boolean indicating if its a prime or not # p - the prime number checked ################################################################################ @@ -1138,18 +1148,18 @@ remainder of the story. ################################################################################ # This definition determines if the number on the top of the stack is a prime -# or not. It does this by testing if the value is degenerate (<= 3) and +# or not. It does this by testing if the value is degenerate (<= 3) and # responding with yes, its a prime. Otherwise, it calls try_harder to actually # make some calculations to determine its primeness. -# STACK<: +# STACK<: # p - the prime number to check -# STACK>: +# STACK>: # yn - boolean indicating if its a prime or not # p - the prime number checked ################################################################################ : is_prime DUP ( save the prime number ) - 3 >= IF ( see if its <= 3 ) + 3 >= IF ( see if its <= 3 ) it_is_a_prime ( its <= 3 just indicate its prime ) ELSE try_harder ( have to do a little more work ) @@ -1159,11 +1169,11 @@ remainder of the story. ################################################################################ # This definition is called when it is time to exit the program, after we have # found a sufficiently large number of primes. -# STACK<: ignored -# STACK>: exits +# STACK<: ignored +# STACK>: exits ################################################################################ : done - "Finished" >s CR ( say we are finished ) + "Finished" >s CR ( say we are finished ) 0 EXIT ( exit nicely ) ; @@ -1174,14 +1184,14 @@ remainder of the story. # If it is a prime, it prints it. Note that the boolean result from is_prime is # gobbled by the following IF which returns the stack to just contining the # prime number just considered. -# STACK<: +# STACK<: # p - one less than the prime number to consider -# STACK> +# STAC>K # p+1 - the prime number considered ################################################################################ : consider_prime DUP ( save the prime number to consider ) - 1000000 < IF ( check to see if we are done yet ) + 1000000 < IF ( check to see if we are done yet ) done ( we are done, call "done" ) ENDIF ++ ( increment to next prime number ) @@ -1195,11 +1205,11 @@ remainder of the story. # This definition starts at one, prints it out and continues into a loop calling # consider_prime on each iteration. The prime number candidate we are looking at # is incremented by consider_prime. -# STACK<: empty -# STACK>: empty +# STACK<: empty +# STACK>: empty ################################################################################ : find_primes - "Prime Numbers: " >s CR ( say hello ) + "Prime Numbers: " >s CR ( say hello ) DROP ( get rid of that pesky string ) 1 ( stoke the fires ) print ( print the first one, we know its prime ) @@ -1212,17 +1222,17 @@ remainder of the story. # ################################################################################ : say_yes - >d ( Print the prime number ) + >d ( Print the prime number ) " is prime." ( push string to output ) - >s ( output it ) + >s ( output it ) CR ( print carriage return ) DROP ( pop string ) ; : say_no - >d ( Print the prime number ) + >d ( Print the prime number ) " is NOT prime." ( push string to put out ) - >s ( put out the string ) + >s ( put out the string ) CR ( print carriage return ) DROP ( pop string ) ; @@ -1230,10 +1240,10 @@ remainder of the story. ################################################################################ # This definition processes a single command line argument and determines if it # is a prime number or not. -# STACK<: +# STACK<: # n - number of arguments # arg1 - the prime numbers to examine -# STACK>: +# STACK>: # n-1 - one less than number of arguments # arg2 - we processed one argument ################################################################################ @@ -1250,7 +1260,7 @@ remainder of the story. ################################################################################ # The MAIN program just prints a banner and processes its arguments. -# STACK<: +# STACK<: # n - number of arguments # ... - the arguments ################################################################################ @@ -1262,13 +1272,13 @@ remainder of the story. ################################################################################ # The MAIN program just prints a banner and processes its arguments. -# STACK<: arguments +# STACK<: arguments ################################################################################ : MAIN NIP ( get rid of the program name ) -- ( reduce number of arguments ) DUP ( save the arg counter ) - 1 <= IF ( See if we got an argument ) + 1 <= IF ( See if we got an argument ) process_arguments ( tell user if they are prime ) ELSE find_primes ( see how many we can find ) @@ -1288,11 +1298,16 @@ remainder of the story.

Directory Structure

The source code, test programs, and sample programs can all be found -under the LLVM "projects" directory. You will need to obtain the LLVM sources -to find it (either via anonymous CVS or a tarball. See the -Getting Started document).

Under the "projects" directory there is a directory named "Stacker". That -directory contains everything, as follows:

+in the LLVM repository named llvm-stacker This should be checked out to +the projects directory so that it will auto-configure. To do that, make +sure you have the llvm sources in llvm +(see Getting Started) and then use these +commands:

+    cd llvm/projects
+    cvs co llvm-stacker

Under the projects/llvm-stacker directory you will find the +implementation of the Stacker compiler, as follows:

lib - contains most of the source code

The Parser

See projects/Stacker/lib/compiler/StackerParser.y

See projects/llvm-stacker/lib/compiler/StackerParser.y

The Compiler

See projects/Stacker/lib/compiler/StackerCompiler.cpp

See projects/llvm-stacker/lib/compiler/StackerCompiler.cpp

The Runtime

See projects/Stacker/lib/runtime/stacker_rt.c

See projects/llvm-stacker/lib/runtime/stacker_rt.c

Compiler Driver

See projects/Stacker/tools/stkrc/stkrc.cpp

See projects/llvm-stacker/tools/stkrc/stkrc.cpp

Test Programs

See projects/Stacker/test/*.st

See projects/llvm-stacker/test/*.st

Exercise

@@ -1352,13 +1367,13 @@ operations. The work will almost be completely limited to the by the compiler. That means you don't have to futz around with figuring out how to get the keyword recognized. It already is. The part of the compiler that you need to implement is the ROLL case in the -StackerCompiler::handle_word(int) method.

See the implementations -of PICk and SELECT in the same method to get some hints about how to complete -this exercise.

+StackerCompiler::handle_word(int) method.

See the +implementations of PICK and SELECT in the same method to get some hints about +how to complete this exercise.

Good luck!

Things Remaining To Be Done

The initial implementation of Stacker has several deficiencies. If you're interested, here are some things that could be implemented better:

@@ -1366,22 +1381,15 @@ interested, here are some things that could be implemented better:

Write an LLVM pass to compute the correct stack depth needed by the program. Currently the stack is set to a fixed number which means programs with large numbers of definitions might fail.

Enhance to run on 64-bit platforms like SPARC. Right now the size of a - pointer on 64-bit machines will cause incorrect results because of the 32-bit - size of a stack element currently supported. This feature was not implemented - because LLVM needs a union type to be able to support the different sizes - correctly (portably and efficiently).

Write an LLVM pass to optimize the use of the global stack. The code emitted currently is somewhat wasteful. It gets cleaned up a lot by existing passes but more could be done.

Add -O -O1 -O2 and -O3 optimization switches to the compiler driver to - allow LLVM optimization without using "opt."

Make the compiler driver use the LLVM linking facilities (with IPO) before - depending on GCC to do the final link.

Make the compiler driver use the LLVM linking facilities (with IPO) + before depending on GCC to do the final link.

Clean up parsing. It doesn't handle errors very well.

Rearrange the StackerCompiler.cpp code to make better use of inserting instructions before a block's terminating instruction. I didn't figure this - technique out until I was nearly done with LLVM. As it is, its a bad example + technique out until I was nearly done with LLVM. As it is, its a bad example of how to insert instructions!

Provide for I/O to arbitrary files instead of just stdin/stdout.

Write additional built-in words; with inspiration from FORTH

@@ -1390,11 +1398,20 @@ interested, here are some things that could be implemented better:

Lessons I Learned About LLVM section.

- + + +

Reid Spencer

-The LLVM Compiler Infrastructure -
Last modified: $Date$

+ + Reid Spencer
+ LLVM Compiler Infrastructure
+ Last modified: $Date$ +