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BlockFrequency: Use a smarter overflow check.

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This trades one 64 bit div for one 64 bit mul and some arithmetic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143106 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer 2011-10-27 16:38:50 +00:00
parent 090697321b
commit bc1430a0a0
1 changed files with 8 additions and 4 deletions
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12
lib/Support/BlockFrequency.cpp
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@ -70,8 +70,13 @@ BlockFrequency &BlockFrequency::operator*=(const BranchProbability &Prob) {
assert(n <= d && "Probability must be less or equal to 1.");
// If we can overflow use 96-bit operations.
if (n > 0 && Frequency > UINT64_MAX / n) {
// Calculate Frequency * n.
uint64_t mulLo = (Frequency & UINT32_MAX) * n;
uint64_t mulHi = (Frequency >> 32) * n;
uint64_t mulRes = (mulHi << 32) + mulLo;
// If there was overflow use 96-bit operations.
if (mulHi > UINT32_MAX || mulRes < mulLo) {
// 96-bit value represented as W[1]:W[0].
uint64_t W[2];
@ -82,8 +87,7 @@ BlockFrequency &BlockFrequency::operator*=(const BranchProbability &Prob) {
return *this;
}
Frequency *= n;
Frequency /= d;
Frequency = mulRes / d;
return *this;
}