LICM

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1

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

test.bc (624 Bytes)

2

2009/10/27 Marc Brünink <marc@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

3

This is expected behavior, you need to run loop rotate before licm to get this. This is completely eliminate at -O3.

-Chris

4

So, while this is true, Nick has a good point in PR5319 that we’re not catching this at O2 with a slightly more complex testcase. LICM can definitely be improved here.

-Chris