IndVarSimplify: do not recompute an IV value outside of the loop if :

- it is trivially known to be used inside the loop in a way that can not be optimized away
- there is no use outside of the loop which can take advantage of the computation hoisting

llvm-svn: 177432

1 files changed, 39 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
index 97fff7e7826..8e76c78f5ac 100644
--- a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -535,6 +535,45 @@ void IndVarSimplify::RewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter) {
         if (!SE->isLoopInvariant(ExitValue, L))
           continue;
 
+        // Computing the value outside of the loop brings no benefit if :
+        //  - it is definitely used inside the loop in a way which can not be
+        //    optimized away.
+        //  - no use outside of the loop can take advantage of hoisting the
+        //    computation out of the loop
+        if (ExitValue->getSCEVType()>=scMulExpr) {
+          unsigned NumHardInternalUses = 0;
+          unsigned NumSoftExternalUses = 0;
+          unsigned NumUses = 0;
+          for (Value::use_iterator IB=Inst->use_begin(), IE=Inst->use_end();
+               IB!=IE && NumUses<=6 ; ++IB) {
+            Instruction *UseInstr = cast<Instruction>(*IB);
+            unsigned Opc = UseInstr->getOpcode();
+            NumUses++;
+            if (L->contains(UseInstr)) {
+              if (Opc == Instruction::Call || Opc == Instruction::Ret)
+                NumHardInternalUses++;
+            } else {
+              if (Opc == Instruction::PHI) {
+                // Do not count the Phi as a use. LCSSA may have inserted
+                // plenty of trivial ones.
+                NumUses--;
+                for (Value::use_iterator PB=UseInstr->use_begin(),
+                                         PE=UseInstr->use_end();
+                     PB!=PE && NumUses<=6 ; ++PB, ++NumUses) {
+                  unsigned PhiOpc = cast<Instruction>(*PB)->getOpcode();
+                  if (PhiOpc != Instruction::Call && PhiOpc != Instruction::Ret)
+                    NumSoftExternalUses++;
+                }
+                continue;
+              }
+              if (Opc != Instruction::Call && Opc != Instruction::Ret)
+                NumSoftExternalUses++;
+            }
+          }
+          if (NumUses <= 6 && NumHardInternalUses && !NumSoftExternalUses)
+            continue;
+        }
+
         Value *ExitVal = Rewriter.expandCodeFor(ExitValue, PN->getType(), Inst);
 
         DEBUG(dbgs() << "INDVARS: RLEV: AfterLoopVal = " << *ExitVal << '\n'