[LLVMdev] c const

[Ryan M. Lefever]

Thanks for the help.  I'm surprised that llvm doesn't preserve the const 
property.  I thought that it was supposed to be useful for various 
compiler optimizations.

Holger Schurig wrote:
>>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().
> 
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-- 
Ryan M. Lefever  [http://www.ews.uiuc.edu/~lefever]