[PATCH] D74155: [X86CmovConversion] Make heuristic for optimized cmov depth more conservative (PR44539)

[Nikita Popov via Phabricator via llvm-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rG5eb19bf4a2b0: [X86CmovConversion] Make heuristic for optimized cmov depth more conservative… (authored by nikic).

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D74155/new/

https://reviews.llvm.org/D74155

Files:
  llvm/lib/Target/X86/X86CmovConversion.cpp


Index: llvm/lib/Target/X86/X86CmovConversion.cpp
===================================================================
--- llvm/lib/Target/X86/X86CmovConversion.cpp
+++ llvm/lib/Target/X86/X86CmovConversion.cpp
@@ -364,12 +364,13 @@
 /// \param TrueOpDepth depth cost of CMOV true value operand.
 /// \param FalseOpDepth depth cost of CMOV false value operand.
 static unsigned getDepthOfOptCmov(unsigned TrueOpDepth, unsigned FalseOpDepth) {
-  //===--------------------------------------------------------------------===//
-  // With no info about branch weight, we assume 50% for each value operand.
-  // Thus, depth of optimized CMOV instruction is the rounded up average of
-  // its True-Operand-Value-Depth and False-Operand-Value-Depth.
-  //===--------------------------------------------------------------------===//
-  return (TrueOpDepth + FalseOpDepth + 1) / 2;
+  // The depth of the result after branch conversion is
+  // TrueOpDepth * TrueOpProbability + FalseOpDepth * FalseOpProbability.
+  // As we have no info about branch weight, we assume 75% for one and 25% for
+  // the other, and pick the result with the largest resulting depth.
+  return std::max(
+      divideCeil(TrueOpDepth * 3 + FalseOpDepth, 4),
+      divideCeil(FalseOpDepth * 3 + TrueOpDepth, 4));
 }
 
 bool X86CmovConverterPass::checkForProfitableCmovCandidates(


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