I think this is a missed optimization, but maybe I'm missing some
significant
piece of knowledge(!) as to why this might not be optimizable :) Test
case...
int A; // some global
int B; // some global
void Test(int *Out)
{
*Out = A; // Can't this be optimized away?
*Out = B;
};
The LLVM backend (tested 3.1 and 3.0 online demo) doesn't optimize away the
first store, even with O3 level compiling in clang. Is there some valid
reason for this?
Any insight appreciated. Thanks.
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Hi, If the function was called Test(&B) then the first store would affect the second load. Tim. On Fri, Nov 16, 2012 at 11:58 AM, AnonW <wayne.phillips at gmail.com> wrote:> I think this is a missed optimization, but maybe I'm missing some significant > piece of knowledge(!) as to why this might not be optimizable :) Test > case... > > int A; // some global > int B; // some global > > void Test(int *Out) > { > *Out = A; // Can't this be optimized away? > *Out = B; > }; > > The LLVM backend (tested 3.1 and 3.0 online demo) doesn't optimize away the > first store, even with O3 level compiling in clang. Is there some valid > reason for this? > > Any insight appreciated. Thanks. > > > > -- > View this message in context: http://llvm.1065342.n5.nabble.com/Is-this-a-missed-simple-optimization-tp51361.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
Argh.... quite right! I knew I was missing something obvious... Many thanks. -- View this message in context: http://llvm.1065342.n5.nabble.com/Is-this-a-missed-simple-optimization-tp51361p51363.html Sent from the LLVM - Dev mailing list archive at Nabble.com.
> If the function was called Test(&B) then the first store would affect > the second load.You can use the "restrict" qualifier ("noalias" in LLVM) to tell the optimizers that this kind of aliasing doesn't occur. Ciao, Duncan.> > Tim. > > On Fri, Nov 16, 2012 at 11:58 AM, AnonW <wayne.phillips at gmail.com> wrote: >> I think this is a missed optimization, but maybe I'm missing some significant >> piece of knowledge(!) as to why this might not be optimizable :) Test >> case... >> >> int A; // some global >> int B; // some global >> >> void Test(int *Out) >> { >> *Out = A; // Can't this be optimized away? >> *Out = B; >> }; >> >> The LLVM backend (tested 3.1 and 3.0 online demo) doesn't optimize away the >> first store, even with O3 level compiling in clang. Is there some valid >> reason for this? >> >> Any insight appreciated. Thanks. >> >> >> >> -- >> View this message in context: http://llvm.1065342.n5.nabble.com/Is-this-a-missed-simple-optimization-tp51361.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 > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >