[llvm-commits] [llvm] r122326 - in /llvm/trunk: lib/Analysis/InstructionSimplify.cpp test/Transforms/InstSimplify/ test/Transforms/InstSimplify/2010-12-20-Reassociate.ll test/Transforms/InstSimplify/dg.exp

[Chris Lattner]

On Dec 22, 2010, at 1:33 AM, Duncan Sands wrote:

> Hi Chris,
> 
>>> Add generic simplification of associative operations, generalizing
>>> a couple of existing transforms.  This fires surprisingly often, for
>>> example when compiling gcc "(X+(-1))+1->X" fires quite a lot as well
>>> as various "and" simplifications (usually with a phi node operand).
>>> Most of the time this doesn't make a real difference since the same
>>> thing would have been done elsewhere anyway, eg: by instcombine, but
>>> there are a few places where this results in simplifications that we
>>> were not doing before.
>> 
>> Nice.  Should the related logic get ripped out of instcombine?  It is probably a little more general, but if it doesn't matter in practice, we should remove it.
> 
> when running std-compile-opts on gcc-as-one-big-file, this logic in
> InstructionSimplify succeeds 171 times, while the corresponding logic in
> InstCombine succeeds 4095 times.  As a point of comparison, InstCombine does
> 3536 constant folds on this testcase.  It is not surprising that instcombine
> succeeds so much more often, as instsimplify is hugely constrained by the
> requirement that it is only allowed to simplify to existing instructions, it is
> not allowed to create any new instructions.  That said, if instsimplify was
> allowed to modify instructions in place then I think for reassociation it would
> catch everything.

Ok, makes sense.  Thanks,

-Chris