https://bugs.llvm.org/show_bug.cgi?id=33954
Bug ID: 33954
Summary: Performance regression with rrL308142
Product: tools
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: llc
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Created attachment 18851
--> https://bugs.llvm.org/attachment.cgi?id=18851&action=edit
IR file to run with llc
With rL308142, there was an optimization introduced to convert cmov to branches
when profitable.
We noticed couple of regressions (around 2-5%) on skylake hardware on internal
benchmarks. The performance was back to normal when -x86-cmov-converter=false
was supplied.
I've tried to reduce the IR as much as possible, and added a main method along
with it. However, the regression (using time command) is not quite visible,
seems to be attributed to noise.
I've attached the IR and how the assembly is generated. Perhaps something may
jump out wrt the heuristics, which seems to be having a performance cliff: we
convert a cmov to branch when the gain is greater than 25% of misprediction
penalty.
Reproduce as:
llc -mcpu=skylake
-mattr=+sse2,+cx16,+prfchw,+bmi2,+xsavec,+fsgsbase,+popcnt,+aes,+xsaves,+mmx,+rdseed,+clflushopt,+xsave,+avx,+rtm,+fma,+bmi,+rdrnd,+sse4.1,+sse4.2,+avx2,+sse,+lzcnt,+pclmul,+f16c,+ssse3,+sgx,+cmov,+movbe,+xsaveopt,+adx,+sse3,
-O3 -x86-cmov-converter=false test.ll ; mv test.s test.falsenative.s; gcc
test.falsenative.s -o test.falsenative
llc -mcpu=skylake
-mattr=+sse2,+cx16,+prfchw,+bmi2,+xsavec,+fsgsbase,+popcnt,+aes,+xsaves,+mmx,+rdseed,+clflushopt,+xsave,+avx,+rtm,+fma,+bmi,+rdrnd,+sse4.1,+sse4.2,+avx2,+sse,+lzcnt,+pclmul,+f16c,+ssse3,+sgx,+cmov,+movbe,+xsaveopt,+adx,+sse3,
-O3 -x86-cmov-converter=true test.ll ; mv test.s test.truenative.s; gcc
test.truenative.s -o test.truenative
echo "time for x86-cmov-converter=true"
time ./test.truenative > chk2
echo "time for x86-cmov-converter=false"
time ./test.falsenative > chk
time for x86-cmov-converter=true
real 0m0.155s
user 0m0.065s
sys 0m0.007s
time for x86-cmov-converter=false
real 0m0.154s
user 0m0.064s
sys 0m0.008s
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