X86TargetLowering::LowerToBT

[Mehdi Amini]

Can you transform you C file into a LLVM IR lit test?

See example in test/CodeGen/X86/*.ll

Thanks,

Mehdi

> On Jan 24, 2015, at 11:47 AM, Chris Sears <chris.sears at gmail.com> wrote:
> 
> This is a patch to X86TargetLowering::LowerToBT() which was hashed over on the Developers list with Intel concurring.
> 
> It checks whether the -Oz (optimize for size) flag is set or whether the containing function's PGO cold attribute is set. If either are true it emits BT for tests of bits 8-31 instead of TEST. Previously, TEST was always used for bits 0-31 and BT was always used for bits 32-63.
> 
> Since the BT instruction is 16b smaller than TEST for the bits 8-31 case, 32b vs 48b, and not irredeemably slower, it makes sense to use BT in cases where size matters.
> 
> Similar logic is possible for BTC and BTS. However, LowerToBTC and LowerToBTS would need to be written and used and that's a larger patch.
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