Please review: Optimize vector multiply on X86

[Demikhovsky, Elena]

I just don't know what happens on other targets. If vector multiply exists and shift-left does not, it may cause scalarization.
If you state that all targets will benefit from this, I'll put the code in under visitMUL() in DAGCombiner.cpp.

-  Elena


-----Original Message-----
From: Benjamin Kramer [mailto:benny.kra at gmail.com] 
Sent: Sunday, June 16, 2013 16:32
To: Demikhovsky, Elena
Cc: llvm-commits at cs.uiuc.edu
Subject: Re: Please review: Optimize vector multiply on X86


On 16.06.2013, at 14:22, "Demikhovsky, Elena" <elena.demikhovsky at intel.com> wrote:

> I added an optimization that converts vector operation multiply by const to SHIFT.
> I did this optimization for X86 only.
> I'm wondering why it was not implemented for all targets.  I saw a proposal sent by Andrea about month ago, but it is, probably, was rejected.

Andrea's patch went into trunk as r183005. It's a mid-level optimization though (in instcombine) so you'll see the effect with opt but not with llc.

I think shl is the canonical version of a mul and I don't see why adding this as an x86-specific optimization provides any additional value. If these occur in places where instcombine can't catch them it may make sense to do it in the target-independent part of DAGCombiner.

- Ben
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