[LLVMdev] X86TargetLowering::LowerToBT

[Stephen Canon]

I suspect that this is because the mask in your example is the result of a variable shift, which (a) has it’s own performance and flags hazards pre-SHLX and (b) requires additional µops to do with TEST.  I expect that ICC is putting a dummy TEST or XOR ahead of the BT to break the false flags dependency, as well.

If the mask were constant, I expect ICC would generate TEST instead (but I don’t have it handy to check).

– Steve

> On Jan 23, 2015, at 11:32 AM, Sanjay Patel <spatel at rotateright.com> wrote:
> 
> If 'bt' is a perf sin, icc doesn't seem to know it:
> 
> $ icc -v 
> icc version 15.0.1 (gcc version 4.9.0 compatibility)
> 
> $ cat bt.c
> unsigned long long IsBitSetB_64(unsigned long long val, int index) { return (val & (1ULL<<index)) != 0ULL; } 
> unsigned int IsBitSetB_32(unsigned int val, int index) { return (val & (1U<<index)) != 0U; } 
> 
> $ icc -O3 -S bt.c -o - | grep bt
>     .file "bt.c"
>         btq       %rsi, %rdi
>         btl       %esi, %edi
> 
> Does anyone at Intel have guidance for us?
> 
> 
> On Thu, Jan 22, 2015 at 4:34 PM, Eric Christopher <echristo at gmail.com <mailto:echristo at gmail.com>> wrote:
> 
> 
> On Thu Jan 22 2015 at 3:32:53 PM Chris Sears <chris.sears at gmail.com <mailto:chris.sears at gmail.com>> wrote:
> The status quo is:
> 
> a) 40b REX+BT instruction for the 64b case
> b) 48b TEST for the 32b case
> c) unless it's small TEST
> 
> You are currently paying a 16b penalty for TEST vs BT in the 32b case.
> That may be worth testing the -Os flag.
> 
> You'll want -Oz here, Os isn't supposed to affect the runtime as much as this is going to.
> 
> -eric 
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu>         http://llvm.cs.uiuc.edu <http://llvm.cs.uiuc.edu/>
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150123/8e2572c2/attachment.html>