Don't sink gep operators through phi nodes if the result would require

more than one phi, since that leads to higher register pressure on
entry to the phi. This is especially problematic when the phi is in
a loop header, as it increases register pressure throughout the loop.

llvm-svn: 81993

1 files changed, 14 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/InstructionCombining.cpp b/llvm/lib/Transforms/Scalar/InstructionCombining.cpp
index 47a02bd8427..006d67b6034 100644
--- a/llvm/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -326,7 +326,7 @@ namespace {
     // instruction.  Instead, visit methods should return the value returned by
     // this function.
     Instruction *EraseInstFromFunction(Instruction &I) {
-      DEBUG(errs() << "IC: erase " << I);
+      DEBUG(errs() << "IC: erase " << I << '\n');
 
       assert(I.use_empty() && "Cannot erase instruction that is used!");
       // Make sure that we reprocess all operands now that we reduced their
@@ -10401,6 +10401,10 @@ Instruction *InstCombiner::FoldPHIArgGEPIntoPHI(PHINode &PN) {
   // This is true if all GEP bases are allocas and if all indices into them are
   // constants.
   bool AllBasePointersAreAllocas = true;
+
+  // We don't want to replace this phi if the replacement would require
+  // more than one phi.
+  bool NeededPhi = false;
   
   // Scan to see if all operands are the same opcode, all have one use, and all
   // kill their operands (i.e. the operands have one use).
@@ -10432,7 +10436,16 @@ Instruction *InstCombiner::FoldPHIArgGEPIntoPHI(PHINode &PN) {
       
       if (FirstInst->getOperand(op)->getType() !=GEP->getOperand(op)->getType())
         return 0;
+
+      // If we already needed a PHI for an earlier operand, and another operand
+      // also requires a PHI, we'd be introducing more PHIs than we're
+      // eliminating, which increases register pressure on entry to the PHI's
+      // block.
+      if (NeededPhi)
+        return 0;
+
       FixedOperands[op] = 0;  // Needs a PHI.
+      NeededPhi = true;
     }
   }