[LLVMdev] Function pointers bitcasted to varargs

[Arushi Aggarwal]

Hi all,

I had the following function that used function pointers with void arguments,


typedef void (*FP)();

void foo() {
  printf("hello world from foo\n");
}
int main() {
  FP fp;
  fp = foo;
  (*fp)();

}

The corresponding bitcode, with no optimizations is

target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
target triple = "x86_64-unknown-linux-gnu"

@.str = private constant [21 x i8] c"hello world from foo\00", align 1
; <[21 x i8]*> [#uses=1]

define void @foo() nounwind {
entry:
  %0 = call i32 @puts(i8* getelementptr inbounds ([21 x i8]* @.str,
i64 0, i64 0)) nounwind ; <i32> [#uses=0]
  br label %return

return:                                           ; preds = %entry
  ret void
}

declare i32 @puts(i8*)

define i32 @main() nounwind {
entry:
  %retval = alloca i32                            ; <i32*> [#uses=1]
  %fp = alloca void (...)*                        ; <void (...)**> [#uses=2]
  %"alloca point" = bitcast i32 0 to i32          ; <i32> [#uses=0]
  store void (...)* bitcast (void ()* @foo to void (...)*), void
(...)** %fp, align 8
  %0 = load void (...)** %fp, align 8             ; <void (...)*> [#uses=1]
  call void (...)* %0() nounwind
  br label %return

return:                                           ; preds = %entry
  %retval1 = load i32* %retval                    ; <i32> [#uses=1]
  ret i32 %retval1
}

I was wondering why LLVM casts the function pointer to a varargs
function?Is there some reason why it should do that?

Thanks.
Arushi