Tweak ConstantHoisting on X86

[Hal Finkel]

----- Original Message -----
> From: "Michael Zolotukhin" <mzolotukhin at apple.com>
> To: "llvm commits" <llvm-commits at cs.uiuc.edu>
> Sent: Wednesday, April 30, 2014 8:11:16 AM
> Subject: Tweak ConstantHoisting on X86
> 
> 
> 
> Hi,
> 
> ConstantHoisting currently could lead to inefficient code on X86 (the
> test is attached). Recently Juergen has solved a similar issue on
> ARM64, so these changes do the same on X86. The change is to
> disallow hoisting of constants from shift instructions.
> 
> While the original issue that I’m fixing is on X86, the same problem
> could happen on other platforms. So, I prepared two versions of the
> patch: the first one touches only X86, and the other changes
> ConstantHoisting itself. Personally, I’d prefer the X86-only
> version, because I don’t know much about other targets affected. Any
> thoughts on this?

Yes, please use the x86-only version. I would not want exactly this behavior for PowerPC, for example.

 -Hal

> 
> There is one more important detail. With this change test
> CodeGen/X86/remat-invalid-liveness.ll stops to check what it’s
> supposed to check. Right now I just silenced it, but we probably
> want to rewrite and make it less fragile. Quentin, could you please
> take a look and help me with that?
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Thanks,
> Michael
> 
> 
> 
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory