https://llvm.org/bugs/show_bug.cgi?id=30939
Bug ID: 30939
Summary: bubble sort test performance is 2 times worse with
-unroll-runtime-epilog
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: evstu...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 17562
--> https://llvm.org/bugs/attachment.cgi?id=17562&action=edit
test compiled with epilog
The performance difference between prologue and epilogue unroll is unclear for
the case.
performance differs 2 times on X86 when
SingleSource/Benchmarks/Stanford/Bubblesort.c is compiled with
-O2 -march=core-avx2 -mllvm -unroll-runtime-epilog=true (bad case)
and
-O2 -march=core-avx2 -mllvm -unroll-runtime-epilog=false (good case)
Attached assemblies from current compiler:
bs_epil.s
bs_prol.s
and assembly from hottest loop:
bs_epil_loop.s
bs_prol_loop.s
The code looks very similar and with some assembly modifications I was able to
make hottest loops identical keeping the same performance gap (2 times).
Deeper analysis uncovered that hottest loop (99% of execution time) mostly
consist of unpredictable branches stalls:
while ( i<top ) {
if ( sortlist[i] > sortlist[i+1] ) {
j = sortlist[i];
sortlist[i] = sortlist[i+1];
sortlist[i+1] = j;
}
i=i+1;
}
sortlist is randomly filled array. That way comparison in the loop is
completely unpredictable. The distance between branches is very short.
This makes the test very sensitive to code shifts and memory accesses order (as
it influence on branch prediction in the loop).
See related discussions:
https://reviews.llvm.org/D18158
https://reviews.llvm.org/D24593
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