llvm dev - Feb 2013 - [LLVMdev] Does LLVM optimize variable to constant value in the compiling time?

Considering the following codes:

int x = 21;

if(x > 20) {

    p = &C;

} else {

    p = &E;

}


I looked into the bitcode that LLVM generates, it still considers the
'else' branch, which will be never reached during the runtime.


Thanks,

Robert
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> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu]
> On Behalf Of Robert Sandra
> Subject: [LLVMdev] Does LLVM optimize variable to constant value in the
compiling time?
> Considering the following codes:
> int x = 21; 
> if(x > 20) { 
>    p = &C; 
> } else { 
>    p = &E; 
> }
> I looked into the bitcode that LLVM generates, it still considers the
'else'
> branch, which will be never reached during the runtime.
What version of LLVM?  What happens if you enable optimization?

 - Chuck


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On Fri, Feb 22, 2013 at 11:16 PM, Robert Sandra
<robert.sandra0712 at gmail.com> wrote:
> Considering the following codes:
>
> int x = 21;
>
> if(x > 20) {
>
>     p = &C;
>
> } else {
>
>     p = &E;
>
> }
>
>
> I looked into the bitcode that LLVM generates, it still considers the
'else'
> branch, which will be never reached during the runtime.
Did you turn on the optimization?

Dmitri

-- 
main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at
gmail.com>*/

By optimization, do you mean enable the optimization whiling building llvm
or use optimization flag while using clang command to get the bitcode file?

In my case, I disable the optimization when I built llvm, and I use the
following command to get the bitcode:

clang -O0 -emit-llvm hello.c -c -o hello.bc

To run the pass:

opt -load ../../../Debug+Asserts/lib/Hello.so -hello < hello.bc >
/dev/null

The llvm version I am using is 3.3svn.

Thanks.

On Fri, Feb 22, 2013 at 4:25 PM, Caldarale, Charles R <
Chuck.Caldarale at unisys.com> wrote:
> > From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu]
> > On Behalf Of Robert Sandra
> > Subject: [LLVMdev] Does LLVM optimize variable to constant value in
the
> compiling time?
>
> > Considering the following codes:
>
> > int x = 21;
> > if(x > 20) {
> >    p = &C;
> > } else {
> >    p = &E;
> > }
>
> > I looked into the bitcode that LLVM generates, it still considers the
> 'else'
> > branch, which will be never reached during the runtime.
>
> What version of LLVM?  What happens if you enable optimization?
>
>  - Chuck
>
>
> THIS COMMUNICATION MAY CONTAIN CONFIDENTIAL AND/OR OTHERWISE PROPRIETARY
> MATERIAL and is thus for use only by the intended recipient. If you
> received this in error, please contact the sender and delete the e-mail and
> its attachments from all computers.
>
>
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Hello
> I looked into the bitcode that LLVM generates, it still considers the
'else'
> branch, which will be never reached during the runtime.
Have you forgotten about -O2 option?

Here we go:
$ cat test.c
int C;
int E;

int* foo() {
  int*p;
  int x = 21;
  if(x > 20) {
    p = &C;
  } else {
    p = &E;
  }

  return p;
}

$ clang -S -O2 test.c -emit-llvm -o -
; ModuleID = 'test.c'
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-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.7.0"

@C = common global i32 0, align 4
@E = common global i32 0, align 4

define i32* @foo() nounwind uwtable readnone ssp {
  ret i32* @C
}


--
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University