[LoopUtils][NFC] Minor refactoring in getLoopEstimatedTripCount.

[ebrevnov]

Summary

Split out of https://reviews.llvm.org/D71990. Minor stuff like renaming and introduction of divideNearest utility.

[Ayal]

llvm/lib/Transforms/Utils/LoopUtils.cpp

718

Can we agree on BackedgeTakenWeight and LatchExitWeight? They directly correspond to "the number of times the backedge was taken" and "the number of times we exited the loop" documented above, and the comment removed below.

726

(Above early exit can precede the swap, but agree it's better to first set the variables to their correct meaning.)

727

Original "// Divide the count ..." comment removed deliberately?

[ebrevnov]

llvm/lib/Transforms/Utils/LoopUtils.cpp

718

Works for me.

727

Restored.

[ebrevnov]

Comments addressed.

[Ayal]

This looks good to me, thanks!

[ebrevnov]

ebrevnov committed rGf0abe820eebf: [LoopUtils][NFC] Minor refactoring in getLoopEstimatedTripCount. (authored by ebrevnov).

llvm/include/llvm/Support/MathExtras.h

template <uint64_t Align> constexpr inline uint64_t alignTo(uint64_t Value) {
   return (Value + Align - 1) / Align * Align;
 }
 
 /// Returns the integer ceil(Numerator / Denominator).
 inline uint64_t divideCeil(uint64_t Numerator, uint64_t Denominator) {
   return alignTo(Numerator, Denominator) / Denominator;
 }
 
+/// Returns the integer nearest(Numerator / Denominator).
+inline uint64_t divideNearest(uint64_t Numerator, uint64_t Denominator) {
+  return (Numerator + (Denominator / 2)) / Denominator;
+}
+
 /// Returns the largest uint64_t less than or equal to \p Value and is
 /// \p Skew mod \p Align. \p Align must be non-zero
 inline uint64_t alignDown(uint64_t Value, uint64_t Align, uint64_t Skew = 0) {
   assert(Align != 0u && "Align can't be 0.");
   Skew %= Align;
   return (Value - Skew) / Align * Align + Skew;
 }
 

llvm/lib/Transforms/Utils/LoopUtils.cpp

Optional<unsigned> llvm::getLoopEstimatedTripCount(Loop *L) {
   if (any_of(ExitBlocks, [](const BasicBlock *EB) {
         return !EB->getTerminatingDeoptimizeCall();
       }))
     return None;
 
   // To estimate the number of times the loop body was executed, we want to
   // know the number of times the backedge was taken, vs. the number of times
   // we exited the loop.
-  uint64_t TrueVal, FalseVal;
-  if (!LatchBR->extractProfMetadata(TrueVal, FalseVal))
+  uint64_t LatchCycleWeight, LatchExitWeight;
+  if (!LatchBR->extractProfMetadata(LatchCycleWeight, LatchExitWeight))
     return None;
 
-  if (!TrueVal || !FalseVal)
+  if (LatchBR->getSuccessor(0) != L->getHeader())
+    std::swap(LatchCycleWeight, LatchExitWeight);
+
+  if (!LatchCycleWeight || !LatchExitWeight)
     return 0;
 
-  // Divide the count of the backedge by the count of the edge exiting the loop,
-  // rounding to nearest.
-  if (LatchBR->getSuccessor(0) == L->getHeader())
-    return (TrueVal + (FalseVal / 2)) / FalseVal;
-  else
-    return (FalseVal + (TrueVal / 2)) / TrueVal;
+  return llvm::divideNearest(LatchCycleWeight, LatchExitWeight);
 }
 
 bool llvm::hasIterationCountInvariantInParent(Loop *InnerLoop,
                                               ScalarEvolution &SE) {
   Loop *OuterL = InnerLoop->getParentLoop();
   if (!OuterL)
     return true;