llvm dev - Oct 2009 - [LLVMdev] LICM

Hi all,

I just noticed that LICM does not hoist/sink the following store out of 
the loop:

         int array[20];
         int i;
         for (i = 0; i<100; i++) {
                 array [0] = 0;
         }

The getElementPtr instruction is hoisted out of the loop; the store is 
not. Did I miss something obvious? Bitcode file attached.
Generated using LLVM 2.5 and
llvm-gcc -c -emit-llvm test_loop.c  -o - | opt  -licm -o test.bc -f

Thanks
Marc
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2009/10/27 Marc Brünink <marc at bruenink.de>
> Hi all,
>
> I just noticed that LICM does not hoist/sink the following store out of the
> loop:
>
>        int array[20];
>        int i;
>        for (i = 0; i<100; i++) {
>                array [0] = 0;
>        }
>
> The getElementPtr instruction is hoisted out of the loop; the store is not.
> Did I miss something obvious? Bitcode file attached.
> Generated using LLVM 2.5 and
> llvm-gcc -c -emit-llvm test_loop.c  -o - | opt  -licm -o test.bc -f
>
Thanks for the report. Since this still happens with LLVM at the top of SVN,
I filed it as http://llvm.org/PR5319 . Please add yourself to the cc list
there.

Nick Lewycky
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On Oct 27, 2009, at 9:26 AM, Nick Lewycky wrote:
> 2009/10/27 Marc Brünink <marc at bruenink.de>
> Hi all,
>
> I just noticed that LICM does not hoist/sink the following store out  
> of the loop:
>
>        int array[20];
>        int i;
>        for (i = 0; i<100; i++) {
>                array [0] = 0;
>        }
>
> The getElementPtr instruction is hoisted out of the loop; the store  
> is not. Did I miss something obvious? Bitcode file attached.
> Generated using LLVM 2.5 and
> llvm-gcc -c -emit-llvm test_loop.c  -o - | opt  -licm -o test.bc -f
>
> Thanks for the report. Since this still happens with LLVM at the top  
> of SVN, I filed it as http://llvm.org/PR5319 . Please add yourself  
> to the cc list there.
This is expected behavior, you need to run loop rotate before licm to  
get this.   This is completely eliminate at -O3.

-Chris
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