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BlockFrequency: Saturate at 1 instead of 0 when multiplying a frequency with a branch probability.

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Zero is used by BlockFrequencyInfo as a special "don't know" value. It also
causes a sink for frequencies as you can't ever get off a zero frequency with
more multiplies.

This recovers a 10% regression on MultiSource/Benchmarks/7zip. A zero frequency
was propagated into an inner loop causing excessive spilling.

PR16402.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184584 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer
2013-06-21 19:30:05 +00:00
parent 65af4b5333
commit 93702a3b07
4 changed files with 95 additions and 3 deletions
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12
lib/Support/BlockFrequency.cpp
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@@ -65,6 +65,9 @@ uint64_t div96bit(uint64_t W[2], uint32_t D) {
BlockFrequency &BlockFrequency::operator*=(const BranchProbability &Prob) {
if (Frequency == 0)
return *this;
uint32_t n = Prob.getNumerator();
uint32_t d = Prob.getDenominator();
@@ -84,10 +87,15 @@ BlockFrequency &BlockFrequency::operator*=(const BranchProbability &Prob) {
// 64-bit.
mult96bit(Frequency, n, W);
Frequency = div96bit(W, d);
return *this;
} else {
// Fast case.
Frequency = mulRes / d;
}
Frequency = mulRes / d;
// Limit the result to 1; 0 is a sentinel value. This keeps BlockFrequencyInfo
// from getting stuck at zero frequencies just because a value became too
// small to be represented as a BlockFrequency.
Frequency = (n == 0 || Frequency != 0) ? Frequency : 1;
return *this;
}