[PATCH] D36081: [X86] Improved X86::CMOV to Branch heuristic

[Amjad Aboud via Phabricator via llvm-commits]

aaboud created this revision.

Resolved PR33954 <https://bugs.llvm.org/show_bug.cgi?id=33954>.
This patch contains two more constraint that aim to reduce the noise cases where we convert CMOV into branch for small gain, and spending more cycles due to overhead.


https://reviews.llvm.org/D36081

Files:
  lib/Target/X86/X86CmovConversion.cpp


Index: lib/Target/X86/X86CmovConversion.cpp
===================================================================
--- lib/Target/X86/X86CmovConversion.cpp
+++ lib/Target/X86/X86CmovConversion.cpp
@@ -67,6 +67,11 @@
                         cl::desc("Enable the X86 cmov-to-branch optimization."),
                         cl::init(true), cl::Hidden);
 
+static cl::opt<unsigned>
+    GainCycleThreshold("x86-cmov-converter-threshold",
+                       cl::desc("Minimum gain per loop (in cycles) threshold."),
+                       cl::init(4), cl::Hidden);
+
 /// Converts X86 cmov instructions into branches when profitable.
 class X86CmovConverterPass : public MachineFunctionPass {
 public:
@@ -389,19 +394,27 @@
   //           Critical-path is iteration dependent - there is dependency of
   //           critical-path instructions on critical-path instructions of
   //           previous iteration.
-  //           Thus, it is required to check the gradient of the gain - the
-  //           change in Depth-Diff compared to the change in Loop-Depth between
-  //           1st and 2nd iterations.
+  //           Thus, check the gain percent of the 2nd iteration (similar to the
+  //           previous case), but it is also required to check the gradient of
+  //           the gain - the change in Depth-Diff compared to the change in
+  //           Loop-Depth between 1st and 2nd iterations.
   //           To be conservative, the gradient need to be at least 50%.
   //
+  //   In addition, In order not to optimize loops with very small gain, the
+  //   gain (in cycles) after 2nd iteration should not be less than a given
+  //   threshold. Thus, the check (Diff[1] >= GainCycleThreshold) must apply.
+  //
   // If loop is not worth optimizing, remove all CMOV-group-candidates.
   //===--------------------------------------------------------------------===//
   bool WorthOptLoop = false;
-  if (Diff[1] == Diff[0])
+  if (Diff[1] < GainCycleThreshold)
+    ; // Keep WorthOptLoop false
+  else if (Diff[1] == Diff[0])
     WorthOptLoop = Diff[0] * 8 >= LoopDepth[0].Depth;
   else if (Diff[1] > Diff[0])
     WorthOptLoop =
-        (Diff[1] - Diff[0]) * 2 >= (LoopDepth[1].Depth - LoopDepth[0].Depth);
+        (Diff[1] - Diff[0]) * 2 >= (LoopDepth[1].Depth - LoopDepth[0].Depth) &&
+        (Diff[1] * 8 >= LoopDepth[1].Depth);
 
   if (!WorthOptLoop)
     return false;


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