[LLVMdev] X86TargetLowering::LowerToBT

[Sanjay Patel]

Full icc code sequence (for the 32-bit case):
        xorl      %eax, %eax
        movl      $1, %edx
        btl       %esi, %edi
        cmovc     %edx, %eax
        ret

Chris's code example is actually returning the result, so no 'test' or 'bt'
in the constant mask case:

unsigned int IsBitSetA_32(unsigned int val) { return (val & (1U<<25)) !=
0U; }

        andl      $33554432, %edi
        shrl      $25, %edi
        movl      %edi, %eax
        ret




On Fri, Jan 23, 2015 at 9:45 AM, Stephen Canon <scanon at apple.com> wrote:

> 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>
> wrote:
>
>>
>>
>> On Thu Jan 22 2015 at 3:32:53 PM Chris Sears <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
>>
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