[LLVMdev] c const

[Holger Schurig]

> How is c's const keyword translated when compiling c into
> llvm bytecode.

It isn't. You can verify this quite simply with the following
test program:

void a(const void *p)
{
}

void b(void *p)
{
}



$ clang --emit-llvm test.c
; ModuleID = 'foo'

define void @a(i8* %p) {
entry:
        %p.addr = alloca i8*            ; <i8**> [#uses=1]
        %allocapt = bitcast i32 undef to i32            ; <i32> [#uses=0]
        store i8* %p, i8** %p.addr
        ret void
}

define void @b(i8* %p) {
entry:
        %p.addr = alloca i8*            ; <i8**> [#uses=1]
        %allocapt = bitcast i32 undef to i32            ; <i32> [#uses=0]
        store i8* %p, i8** %p.addr
        ret void
}



Now, I tried this with the gcc 4.0 frontend as well:

$ gcc -c --emit-llvm test.c -o - | llvm-dis -o -
; ModuleID = '<stdin>'
target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32"
target triple = "i686-pc-linux-gnu"

define void @a(i8* %p) {
entry:
        %p_addr = alloca i8*            ; <i8**> [#uses=1]
        %"alloca point" = bitcast i32 0 to i32          ; <i32> [#uses=0]
        store i8* %p, i8** %p_addr
        br label %return

return:         ; preds = %entry
        ret void
}

define void @b(i8* %p) {
entry:
        %p_addr = alloca i8*            ; <i8**> [#uses=1]
        %"alloca point" = bitcast i32 0 to i32          ; <i32> [#uses=0]
        store i8* %p, i8** %p_addr
        br label %return

return:         ; preds = %entry
        ret void
}


As you can see, with both C compilers the generated intermediate language
is the same for both function a() and b().