llvm dev - Apr 2013 - [LLVMdev] llvm 'select' instruction

hello guys:
i am thinking about what kind of C instructions can turn into llvm IR
'select' instruction.
i tried "d=a?b:c" and compiled it using clang, i still didn't get
'select'
is there anybody who knows this?
thank you



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Something like this should do the job (taken from a test file from the
libbeauty decompiler project):

/* A very simple jump table. */
int test58(int var1) {
        int n = 0;
        switch (var1) {
        case 1:
                n = 1;
                break;
        case 2:
                n = 2;
                break;
        case 3:
                n = 3;
                break;
        case 4:
                n = 4;
                break;
        case 5:
                n = 5;
                break;
        }
        return n;



On 14 April 2013 13:17, Dong Chen <jameschennerd at gmail.com> wrote:
> hello guys:
> i am thinking about what kind of C instructions can turn into llvm IR
> 'select' instruction.
> i tried "d=a?b:c" and compiled it using clang, i still didn't
get 'select'
> is there anybody who knows this?
> thank you
>
>
>
> --
> View this message in context:
> http://llvm.1065342.n5.nabble.com/llvm-select-instruction-tp56719.html
> Sent from the LLVM - Dev mailing list archive at Nabble.com.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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Hi Dong,

try turning optimizations on. I get the select then:
> └─[$]› echo 'int min(int a, int b) { return a < b ? a : b; }' |
clang -xc -O1 -S -emit-llvm -o - -
> ; ModuleID = '-'
> 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-unknown-linux-gnu"
>
> define i32 @min(i32 %a, i32 %b) nounwind uwtable readnone {
> entry:
>   %cmp = icmp slt i32 %a, %b
>   %cond = select i1 %cmp, i32 %a, i32 %b
>   ret i32 %cond
> }
>
Cheers,
Clemens


On 4/14/13 2:17 PM, Dong Chen wrote:
> hello guys:
> i am thinking about what kind of C instructions can turn into llvm IR
> 'select' instruction.
> i tried "d=a?b:c" and compiled it using clang, i still didn't
get 'select'
> is there anybody who knows this?
> thank you
>
>
>
> --
> View this message in context:
http://llvm.1065342.n5.nabble.com/llvm-select-instruction-tp56719.html
> Sent from the LLVM - Dev mailing list archive at Nabble.com.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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On 04/14/2013 02:17 PM, Dong Chen wrote:
> hello guys:
> i am thinking about what kind of C instructions can turn into llvm IR
> 'select' instruction.
> i tried "d=a?b:c" and compiled it using clang, i still didn't
get 'select'
> is there anybody who knows this?
> thank you
Did you try to compile this with optimization enabled? (at least -O1)

For me, this gets compiled to a select instruction:

int a;

int main() {
 return a > 0 ? 123: 321;
}

Results in:

@a = common global i32 0, align 4

define i32 @main() nounwind uwtable readonly {
entry:
  %0 = load i32* @a, align 4, !tbaa !0
  %cmp = icmp sgt i32 %0, 0
  %cond = select i1 %cmp, i32 123, i32 321
  ret i32 %cond
}

!0 = metadata !{metadata !"int", metadata !1}
!1 = metadata !{metadata !"omnipotent char", metadata !2}
!2 = metadata !{metadata !"Simple C/C++ TBAA"}


Greetings,
Jan

Hi,

If you run the simplify-cfg pass, select instructions should get generated.

Regards,
Sam

On 14/04/2013 14:17, Dong Chen wrote:
> hello guys:
> i am thinking about what kind of C instructions can turn into llvm IR
> 'select' instruction.
> i tried "d=a?b:c" and compiled it using clang, i still didn't
get 'select'
> is there anybody who knows this?
> thank you
>
>
>
> --
> View this message in context:
http://llvm.1065342.n5.nabble.com/llvm-select-instruction-tp56719.html
> Sent from the LLVM - Dev mailing list archive at Nabble.com.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

hi, james:
thank you for your reply.
it seems i should use the right optimization option.
the code you provided may generate 'select' instruction



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hi Clemens Hammacher:
thank you for your helpful answer.
but i have another question about this 'select' instruction.
i think that in LLVM IR, i can use a branch and two seperate assignments to
replace 'select' instruction(am i right?).
so is there any compiling option to aviod generate 'select' instruction?



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hi Jan: 
thank you for your help. 
but i have another question about this 'select' instruction. 
i think that in LLVM IR, i can use a branch and two seperate assignments to
replace 'select' instruction(am i right?). 
so is there any compiling option to aviod generate 'select' instruction?



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http://llvm.1065342.n5.nabble.com/llvm-select-instruction-tp56719p56727.html
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hi Sam: 
thank you for your reply.
but i have another question about this 'select' instruction. 
i think that in LLVM IR, i can use a branch and two seperate assignments to
replace 'select' instruction(am i right?). 
so is there any compiling option to aviod generate 'select' instruction?



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