[LLVMdev] X86TargetLowering::LowerToBT

[Chris Sears]

tst64.ll is attached from clang -S -emit-llvm tst64.c

On Sat, Jan 24, 2015 at 11:56 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:

> 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.
> > <pat><tst64.c>_______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>


-- 
Ite Ursi
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