[llvm-dev] Vectorization in LLVM x86 backend (original) (raw)

Charith Mendis via llvm-dev llvm-dev at lists.llvm.org
Mon Aug 21 09:23:30 PDT 2017

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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170821/7ac8b1f7/attachment.html>

More information about the llvm-dev mailing list