[llvm-dev] Vectorization in LLVM x86 backend

[Charith Mendis via llvm-dev]

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 (.ll file attached) and
there are no vector instructions in the if.else BB in compressStream
function. Then I got the assembly output of the file in it I can clearly
see vector instructions in the compressStream function (.S file attached)

Neither the SLPVectorizer or the LoopVectorizer is however doing any
vectorization (also checked it using -debug-only flag) as witnessed by the
non-vectorized code in the attached .ll file.

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?

Thank You.

-- 
Kind regards,
Charith Mendis

Graduate Student,
CSAIL,
Massachusetts Institute of Technology
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