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.

llvm-svn: 185184

1 files changed, 35 insertions, 23 deletions

diff --git a/llvm/lib/Support/BlockFrequency.cpp b/llvm/lib/Support/BlockFrequency.cpp
index 572dbf57654..08fa620eb88 100644
--- a/llvm/lib/Support/BlockFrequency.cpp
+++ b/llvm/lib/Support/BlockFrequency.cpp
@@ -42,12 +42,14 @@ void mult96bit(uint64_t freq, uint32_t N, uint64_t W[2]) {
 }
 
 
-/// 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 @@ uint64_t div96bit(uint64_t W[2], uint32_t D) {
 
 }
 
+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();
-
-  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;
+  // Calculate Frequency * N.
+  uint64_t MulLo = (Frequency & UINT32_MAX) * N;
+  uint64_t MulHi = (Frequency >> 32) * N;
+  uint64_t MulRes = (MulHi << 32) + MulLo;
+
+  // 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 @@ BlockFrequency::operator*(const BranchProbability &Prob) const {
   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;