Tweak ConstantHoisting on X86

[Quentin Colombet]

Hi Michael,

On Apr 30, 2014, at 6:11 AM, Michael Zolotukhin <mzolotukhin at apple.com> wrote:

> 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?
> 
> 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’ll apply your patch locally and I’ll have a look.
Unfortunately this kind of bugs needs a lot of conditions to trigger, so it is not that surprising that it is fragile :(.

Thanks,
-Quentin
> 
> <ConstantHoisting-generic.patch><ConstantHoisting-x86.patch>
> 
> <test.ll>
> 
> Thanks,
> Michael
> 
>