[llvm-dev] Vectorization in LLVM x86 backend

[Charith Mendis via llvm-dev]

Oh yes, my bad just forgot that floating point instructions use the xmms
too! Not the old X87 stack!

Thanks for the help!

On Mon, Aug 21, 2017 at 12:18 PM, Craig Topper <craig.topper at gmail.com>
wrote:

> Almost all of those instructions end in "sdl" which are the scalar
> floating point instructions using the lower 32-bits of the xmm registers.
> The only one that ends in "ps" is an xor of a register with itself which is
> the idiom for zeroing a register.
>
> ~Craig
>
> On Mon, Aug 21, 2017 at 9:14 AM, Charith Mendis <char.mendis1989 at gmail.com
> > wrote:
>
>> I isolated the LLVM IR and the X86 instructions emitted for the function
>> and are attached herewith and it is clearly emitting vector instructions. I
>> am having a hard time figuring out where the vector instructions are
>> formulated. For sure SLP and Loop vectorizer is not doing anything.
>>
>> On Mon, Aug 21, 2017 at 11:56 AM, Craig Topper <craig.topper at gmail.com>
>> wrote:
>>
>>> The X86 backend shouldn't be doing any additional vectorization. If
>>> there are no vector types in IR, I don't think the X86 backend will create
>>> any.
>>>
>>> ~Craig
>>>
>>> On Mon, Aug 21, 2017 at 8:49 AM, Charith Mendis via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>>
>>>> Hi all,
>>>>
>>>> Recently I compiled the attached .c file using Clang with "-mavx2 -mfma
>>>> -m32 -O3" optimization flags.
>>>>
>>>> First I used -emit-llvm and inspected the LLVM IR and there are no
>>>> vector instructions. Then I got the assembly output of the file in it I can
>>>> clearly see vector instructions in it.
>>>>
>>>> Neither the SLPVectorizer or the LoopVectorizer is however doing any
>>>> vectorization (also checked it using -debug-only flag) as witnessed by the
>>>> LLVM IR dump.
>>>>
>>>> Therefore, the vectorization should happen in the backend(?).
>>>>
>>>> Can I know whether the x86 backend does additional vectorization of
>>>> scalar code and if so in which passes?
>>>>
>>>> NB - I posed the same question with the source files in a previous
>>>> email, but the limit of 100kB was reached.
>>>>
>>>> Thank You.
>>>> --
>>>> Kind regards,
>>>> Charith Mendis
>>>>
>>>> Graduate Student,
>>>> CSAIL,
>>>> Massachusetts Institute of Technology
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>
>>
>>
>> --
>> Kind regards,
>> Charith Mendis
>>
>> Graduate Student,
>> CSAIL,
>> Massachusetts Institute of Technology
>>
>
>


-- 
Kind regards,
Charith Mendis

Graduate Student,
CSAIL,
Massachusetts Institute of Technology
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