CODEWITHSUNDEEP
c++llvminstrumentation
codebomb
codebomb

Reputation: 270

What does CallInst::Create() return in LLVM?

Considering

static CallInst *Create(Value *Func,
                      ArrayRef<Value *> Args,
                      const Twine &NameStr = "",
                      Instruction *InsertBefore = 0)

this function, I wonder what the return value of this function means.

For example, in following code,

int foo(int a);
...
Function *foo_ptr = ~~;//say, foo is refered through getOrInsertFunction()
CallInst *ptr = CallInst::Create(foo_ptr, .../* properly set */);

the CallInst *ptr is the return value. Abstractly, does ptr mean

  1. an integer value returned by int foo(int);
  2. or CALL instruction

I thought number 2 was the answer, but started to get confused looking at some codes.

Upvotes: 4

Views: 6674

Answers (1)

Mats Petersson
Mats Petersson

Reputation: 129504

Both 1 and 2 are "true". It returns the call instruction, whose "value", when we execute the code, will be the return value of the function.

To illustrate, take this little Pascal program:

program p;

function f: integer;
begin
   f := 42;
end; { f }

begin
   writeln(f);
end.

Which translates to this LLVM-IR:

; ModuleID = 'TheModule'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

%text = type { i32, i8*, i32, i32 }

@input = global %text zeroinitializer, align 8
@output = global %text zeroinitializer, align 8
@UnitIniList = constant [1 x i8*] zeroinitializer

define i32 @P.f() #0 {
entry:
  %f = alloca i32, align 4
  store i32 42, i32* %f
  %0 = load i32, i32* %f
  ret i32 %0
}

define void @__PascalMain() #0 {
entry:
  %calltmp = call i32 @P.f()
  call void @__write_int(%text* @output, i32 %calltmp, i32 0)
  call void @__write_nl(%text* @output)
  ret void
}

declare void @__write_int(%text*, i32, i32)

declare void @__write_nl(%text*)

attributes #0 = { "no-frame-pointer-elim"="true" }

The call i32 @P.f() is generated by:

inst = builder.CreateCall(calleF, argsV, "calltmp");

The contents of inst is %calltmp = call i32 @P.f() - and that is a CallInst 'value'.

and inst is returned to the evaluation of the expression for the argument to writeln.

Upvotes: 4

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