Add an experimental early if-conversion pass, off by default.

This pass performs if-conversion on SSA form machine code by
speculatively executing both sides of the branch and using a cmov
instruction to select the result. This can help lower the number of
branch mispredictions on architectures like x86 that don't have
predicable instructions.

The current implementation is very aggressive, and causes regressions on
mosts tests. It needs good heuristics that have yet to be implemented.

llvm-svn: 159694

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/Passes.cpp b/llvm/lib/CodeGen/Passes.cpp
index 9693780bdaa..e4ad6c17bc2 100644
--- a/llvm/lib/CodeGen/Passes.cpp
+++ b/llvm/lib/CodeGen/Passes.cpp
@@ -49,6 +49,8 @@ static cl::opt<bool> DisableSSC("disable-ssc", cl::Hidden,
     cl::desc("Disable Stack Slot Coloring"));
 static cl::opt<bool> DisableMachineDCE("disable-machine-dce", cl::Hidden,
     cl::desc("Disable Machine Dead Code Elimination"));
+static cl::opt<bool> EnableEarlyIfConversion("enable-early-ifcvt", cl::Hidden,
+    cl::desc("Enable Early If-conversion"));
 static cl::opt<bool> DisableMachineLICM("disable-machine-licm", cl::Hidden,
     cl::desc("Disable Machine LICM"));
 static cl::opt<bool> DisableMachineCSE("disable-machine-cse", cl::Hidden,
@@ -154,6 +156,9 @@ static AnalysisID overridePass(AnalysisID StandardID, AnalysisID TargetID) {
   if (StandardID == &DeadMachineInstructionElimID)
     return applyDisable(TargetID, DisableMachineDCE);
 
+  if (StandardID == &EarlyIfConverterID)
+    return applyDisable(TargetID, !EnableEarlyIfConversion);
+
   if (StandardID == &MachineLICMID)
     return applyDisable(TargetID, DisableMachineLICM);
 
@@ -228,6 +233,9 @@ TargetPassConfig::TargetPassConfig(TargetMachine *tm, PassManagerBase &pm)
   substitutePass(&EarlyTailDuplicateID, &TailDuplicateID);
   substitutePass(&PostRAMachineLICMID, &MachineLICMID);
 
+  // Disable early if-conversion. Targets that are ready can enable it.
+  disablePass(&EarlyIfConverterID);
+
   // Temporarily disable experimental passes.
   substitutePass(&MachineSchedulerID, 0);
 }
@@ -527,6 +535,7 @@ void TargetPassConfig::addMachineSSAOptimization() {
   addPass(&DeadMachineInstructionElimID);
   printAndVerify("After codegen DCE pass");
 
+  addPass(&EarlyIfConverterID);
   addPass(&MachineLICMID);
   addPass(&MachineCSEID);
   addPass(&MachineSinkingID);