[LoopUnroll] Implement profile-based loop peeling

This implements PGO-driven loop peeling.

The basic idea is that when the average dynamic trip-count of a loop is known,
based on PGO, to be low, we can expect a performance win by peeling off the
first several iterations of that loop.
Unlike unrolling based on a known trip count, or a trip count multiple, this
doesn't save us the conditional check and branch on each iteration. However,
it does allow us to simplify the straight-line code we get (constant-folding,
etc.). This is important given that we know that we will usually only hit this
code, and not the actual loop.

This is currently disabled by default.

Differential Revision: https://reviews.llvm.org/D25963

llvm-svn: 288274

1 files changed, 7 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index d3817ee..fd35cd0 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -1090,16 +1090,16 @@ Optional<unsigned> llvm::getLoopEstimatedTripCount(Loop *L) {
   // from the raw counts to provide a better probability estimate. Remove
   // the adjustment by subtracting 1 from both weights.
   uint64_t TrueVal, FalseVal;
-  if (!LatchBR->extractProfMetadata(TrueVal, FalseVal) || (TrueVal <= 1) ||
-      (FalseVal <= 1))
+  if (!LatchBR->extractProfMetadata(TrueVal, FalseVal))
     return None;
 
-  TrueVal -= 1;
-  FalseVal -= 1;
+  if (!TrueVal || !FalseVal)
+    return 0;
 
-  // Divide the count of the backedge by the count of the edge exiting the loop.
+  // 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;
+    return (TrueVal + (FalseVal / 2)) / FalseVal;
   else
-    return FalseVal / TrueVal;
+    return (FalseVal + (TrueVal / 2)) / TrueVal;
 }