Add a division operator to BlockFrequency.
Allow a BlockFrequency to be divided by a non-zero BranchProbability
with saturating arithmetic. This will be used to compute the frequency
of a loop header given the probability of leaving the loop.
Our long division algorithm already saturates on overflow, so that was a
freebie.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185184 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/BlockFrequency.cpp b/lib/Support/BlockFrequency.cpp
index 572dbf5..08fa620 100644
--- a/lib/Support/BlockFrequency.cpp
+++ b/lib/Support/BlockFrequency.cpp
@@ -42,12 +42,14 @@
}
-/// div96bit - Divide 96-bit value stored in W array by D. Return 64-bit frequency.
+/// div96bit - Divide 96-bit value stored in W array by D.
+/// Return 64-bit quotient, saturated to UINT64_MAX on overflow.
uint64_t div96bit(uint64_t W[2], uint32_t D) {
uint64_t y = W[0];
uint64_t x = W[1];
int i;
+ // This long division algorithm automatically saturates on overflow.
for (i = 1; i <= 64 && x; ++i) {
uint32_t t = (int)x >> 31;
x = (x << 1) | (y >> 63);
@@ -63,31 +65,30 @@
}
+void BlockFrequency::scale(uint32_t N, uint32_t D) {
+ assert(D != 0 && "Division by zero");
-BlockFrequency &BlockFrequency::operator*=(const BranchProbability &Prob) {
- uint32_t n = Prob.getNumerator();
- uint32_t d = Prob.getDenominator();
+ // Calculate Frequency * N.
+ uint64_t MulLo = (Frequency & UINT32_MAX) * N;
+ uint64_t MulHi = (Frequency >> 32) * N;
+ uint64_t MulRes = (MulHi << 32) + MulLo;
- assert(n <= d && "Probability must be less or equal to 1.");
-
- // 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];
-
- // Probability is less or equal to 1 which means that results must fit
- // 64-bit.
- mult96bit(Frequency, n, W);
- Frequency = div96bit(W, d);
- return *this;
+ // If the product fits in 64 bits, just use built-in division.
+ if (MulHi <= UINT32_MAX && MulRes <= MulLo) {
+ Frequency = MulRes / D;
+ return;
}
- Frequency = mulRes / d;
+ // Product overflowed, use 96-bit operations.
+ // 96-bit value represented as W[1]:W[0].
+ uint64_t W[2];
+ mult96bit(Frequency, N, W);
+ Frequency = div96bit(W, D);
+ return;
+}
+
+BlockFrequency &BlockFrequency::operator*=(const BranchProbability &Prob) {
+ scale(Prob.getNumerator(), Prob.getDenominator());
return *this;
}
@@ -98,6 +99,17 @@
return Freq;
}
+BlockFrequency &BlockFrequency::operator/=(const BranchProbability &Prob) {
+ scale(Prob.getDenominator(), Prob.getNumerator());
+ return *this;
+}
+
+BlockFrequency BlockFrequency::operator/(const BranchProbability &Prob) const {
+ BlockFrequency Freq(Frequency);
+ Freq /= Prob;
+ return Freq;
+}
+
BlockFrequency &BlockFrequency::operator+=(const BlockFrequency &Freq) {
uint64_t Before = Freq.Frequency;
Frequency += Freq.Frequency;