Hi!
As written in the PR, I've been looking why is llvm 3.[34] so much faster
on Scimark2 SOR benchmark and the reason is that it's predictive commoning
or whatever it uses doesn't give up on the inner loop, while our predcom
unnecessarily gives up, because there are reads that could alias the write.
This simple patch improves the benchmark by 42%. We already ignore
unsuitable dependencies for read/read, the patch extends that for unsuitable
dependencies for read/write by just putting the read (and anything in it's
component) into the bad component which is ignored. pcom doesn't optimize
away the writes and will keep the potentially aliasing reads unmodified as
well. Without the patch we'd merge the two components, and as
!determine_offset between the two DRs, it would mean the whole merged
component would be always unsuitable and thus ignored. With the patch
we'll hopefully have some other reads with known offset to the write
and can optimize that, so the patch should always either handle what
it did before or handle perhaps some more cases.
The inner loop from the (public domain) benchmark is added in the two tests,
one runtime test and one test looking whether pcom actually optimized it.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2013-12-31 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/59643
* tree-predcom.c (split_data_refs_to_components): If one dr is
read and one write, determine_offset fails and the write isn't
in the bad component, just put the read into the bad component.
* gcc.dg/pr59643.c: New test.
* gcc.c-torture/execute/pr59643.c: New test.