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author | Benjamin Kramer <benny.kra@googlemail.com> | 2013-06-21 19:30:05 +0000 |
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committer | Benjamin Kramer <benny.kra@googlemail.com> | 2013-06-21 19:30:05 +0000 |
commit | 93702a3b0754052d926c75082abf7ca156b80c45 (patch) | |
tree | 10b032704125296644321d26cc84d3053da88dc7 /lib | |
parent | 65af4b5333f539b9e572ebb9c4d4e97a897e5130 (diff) | |
download | llvm-93702a3b0754052d926c75082abf7ca156b80c45.tar.gz llvm-93702a3b0754052d926c75082abf7ca156b80c45.tar.bz2 llvm-93702a3b0754052d926c75082abf7ca156b80c45.tar.xz |
BlockFrequency: Saturate at 1 instead of 0 when multiplying a frequency with a branch probability.
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
Diffstat (limited to 'lib')
-rw-r--r-- | lib/Support/BlockFrequency.cpp | 12 |
1 files changed, 10 insertions, 2 deletions
diff --git a/lib/Support/BlockFrequency.cpp b/lib/Support/BlockFrequency.cpp index 84a993e3e5..53bbd8ad12 100644 --- a/lib/Support/BlockFrequency.cpp +++ b/lib/Support/BlockFrequency.cpp @@ -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; } |