Tweak ConstantHoisting on X86

[Michael Zolotukhin]

On Apr 30, 2014, at 5:17 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- 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.
Sure.

> -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?
I’ve found a simple way to fix the test - as the current change prevents constants from hoisting, I did it in the test manually. Now it checks what it checked before. Also to make sure it’s still relevant I checked that it fails if r206060 is reverted.

Here is an updated patch:
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Is it ok to commit?

Thanks,
Michael

>> 
>> Thanks,
>> Michael
>> 
>> 
>> 
>> 
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> 
> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory