http://llvm.org/bugs/show_bug.cgi?id=20058
Bug ID: 20058
Summary: High loop unroll counts can result in long compile
times
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Compilation can take a long time with high loop counts and is looks to be
quadratic with loop count. The addition of loop unroll pragmas
(http://reviews.llvm.org/D4147, not yet submitted) will make it more likely
that users will encounter this bad behavior.
Attached is the IR from the following simple C code:
void loop(int *a)
{
for (int i = 0; i < 1000000; i++) {
a[i]++;
}
}
I measured ~2s to unroll this loop 16K times with release build at head, ~10s
to unroll roll it 32K times, and ~50s for 64K, so it appears to be growing (at
least) quadratically. Profiling suggests time is being taken in somewhere
beneath llvm::UnrollLoop -> FoldBlockIntoPredecessor ->
llvm::ScalarEvolution::forgetLoop.
Run:
opt loop.ll -loop-rotate -loop-unroll -unroll-count 16000 -unroll-allow-partial
-unroll-threshold=10000000
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