Support: Add BranchProbability::scale() and ::scaleByInverse()

Add API to `BranchProbability` for scaling big integers.  Next job is to
rip the logic out of `BlockMass` and `BlockFrequency`.

llvm-svn: 207544

1 files changed, 47 insertions, 0 deletions

diff --git a/llvm/lib/Support/BranchProbability.cpp b/llvm/lib/Support/BranchProbability.cpp
index e8b83e59802..c541fca3fb4 100644
--- a/llvm/lib/Support/BranchProbability.cpp
+++ b/llvm/lib/Support/BranchProbability.cpp
@@ -26,6 +26,53 @@ void BranchProbability::dump() const {
   dbgs() << *this << '\n';
 }
 
+static uint64_t scale(uint64_t Num, uint32_t N, uint32_t D) {
+  assert(D && "divide by 0");
+
+  // Fast path for multiplying by 1.0.
+  if (!Num || D == N)
+    return Num;
+
+  // Split Num into upper and lower parts to multiply, then recombine.
+  uint64_t ProductHigh = (Num >> 32) * N;
+  uint64_t ProductLow = (Num & UINT32_MAX) * N;
+
+  // Split into 32-bit digits.
+  uint32_t Upper32 = ProductHigh >> 32;
+  uint32_t Lower32 = ProductLow & UINT32_MAX;
+  uint32_t Mid32Partial = ProductHigh & UINT32_MAX;
+  uint32_t Mid32 = Mid32Partial + (ProductLow >> 32);
+
+  // Carry.
+  Upper32 += Mid32 < Mid32Partial;
+
+  // Check for overflow.
+  if (Upper32 >= D)
+    return UINT64_MAX;
+
+  uint64_t Rem = (uint64_t(Upper32) << 32) | Mid32;
+  uint64_t UpperQ = Rem / D;
+
+  // Check for overflow.
+  if (UpperQ > UINT32_MAX)
+    return UINT64_MAX;
+
+  Rem = ((Rem % D) << 32) | Lower32;
+  uint64_t LowerQ = Rem / D;
+  uint64_t Q = (UpperQ << 32) + LowerQ;
+
+  // Check for overflow.
+  return Q < LowerQ ? UINT64_MAX : Q;
+}
+
+uint64_t BranchProbability::scale(uint64_t Num) const {
+  return ::scale(Num, N, D);
+}
+
+uint64_t BranchProbability::scaleByInverse(uint64_t Num) const {
+  return ::scale(Num, D, N);
+}
+
 namespace llvm {
 
 raw_ostream &operator<<(raw_ostream &OS, const BranchProbability &Prob) {