[LLVMdev] logic function optimization: IAR 1 - LLVM 0 ?

[Bill Wendling]

Have you tried armv7?

-bw

On Feb 2, 2013, at 3:50 PM, matic at nimp.co.uk wrote:

> I gave the following function to IAR compiler (targeting CortexM0) and to clang/LLVM 3.2 (clang -O3 -target thumbv6-eabi -emit-llvm)
> 
> int calleeSave8(int in[]){
>     int out=0;
>     int i;
>     for(i=0;i<8;i++){
>         out ^= in[i] & in[(i+1)%8];
>     }//expand to out = (in[0]&in[1])^(in[1]&in[2])^(in[2]&in[3])^(in[3]&in[4])^(in[4]&in[5])^(in[5]&in[6])^(in[6]&in[7])^(in[7]&in[0])    
>     return out;
> }
> 
> In such case, IAR compiler is able to factor out half of the and operations so it performs 4 and + 7 xor, LLVM factored only one and operation so it performs 7 and + 7 xor. (I looked at IR code and assembly output)
> 
> Did I miss some option that would improve the result ?
> 
> My understanding is that this kind of optimization should be done by the target independent part of the code generator, backends should not have to implement such optimization right ?
> 
> Cheers,
> 
> Sebastien
> 
>  
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