Fix loop rerolling pass failure with non-consant loop lower bound

The loop rerolling pass was failing with an assertion failure from a
failed cast on loops like this:

  void foo(int *A, int *B, int m, int n) {
    for (int i = m; i < n; i+=4) {
      A[i+0] = B[i+0] * 4;
      A[i+1] = B[i+1] * 4;
      A[i+2] = B[i+2] * 4;
      A[i+3] = B[i+3] * 4;
    }
  }

The code was casting the SCEV-expanded code for the new
induction variable to a phi-node. When the loop had a non-constant
lower bound, the SCEV expander would end the code expansion with an
add insted of a phi node and the cast would fail.

It looks like the cast to a phi node was only needed to get the
induction variable value coming from the backedge to compute the end
of loop condition. This patch changes the loop reroller to compare
the induction variable to the number of times the backedge is taken
instead of the iteration count of the loop. In other words, we stop
the loop when the current value of the induction variable ==
IterationCount-1. Previously, the comparison was comparing the
induction variable value from the next iteration == IterationCount.

This problem only seems to occur on 32-bit targets. For some reason,
the loop is not rerolled on 64-bit targets.

PR18290


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198425 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 12 insertions, 10 deletions

diff --git a/lib/Transforms/Scalar/LoopRerollPass.cpp b/lib/Transforms/Scalar/LoopRerollPass.cpp
index db0c765cef..7186e8b6fd 100644
--- a/lib/Transforms/Scalar/LoopRerollPass.cpp
+++ b/lib/Transforms/Scalar/LoopRerollPass.cpp
@@ -1089,9 +1089,8 @@ bool LoopReroll::reroll(Instruction *IV, Loop *L, BasicBlock *Header,
                            L, SCEV::FlagAnyWrap));
   { // Limit the lifetime of SCEVExpander.
     SCEVExpander Expander(*SE, "reroll");
-    PHINode *NewIV =
-      cast<PHINode>(Expander.expandCodeFor(H, IV->getType(),
-                                           Header->begin()));
+    Value *NewIV = Expander.expandCodeFor(H, IV->getType(), Header->begin());
+
     for (DenseSet<Instruction *>::iterator J = BaseUseSet.begin(),
          JE = BaseUseSet.end(); J != JE; ++J)
       (*J)->replaceUsesOfWith(IV, NewIV);
@@ -1102,20 +1101,23 @@ bool LoopReroll::reroll(Instruction *IV, Loop *L, BasicBlock *Header,
         if (Inc == 1)
           ICSCEV =
             SE->getMulExpr(ICSCEV, SE->getConstant(ICSCEV->getType(), Scale));
-        Value *IC;
-        if (isa<SCEVConstant>(ICSCEV)) {
-          IC = Expander.expandCodeFor(ICSCEV, NewIV->getType(), BI);
+        // Iteration count SCEV minus 1
+        const SCEV *ICMinus1SCEV =
+          SE->getMinusSCEV(ICSCEV, SE->getConstant(ICSCEV->getType(), 1));
+
+        Value *ICMinus1; // Iteration count minus 1
+        if (isa<SCEVConstant>(ICMinus1SCEV)) {
+          ICMinus1 = Expander.expandCodeFor(ICMinus1SCEV, NewIV->getType(), BI);
         } else {
           BasicBlock *Preheader = L->getLoopPreheader();
           if (!Preheader)
             Preheader = InsertPreheaderForLoop(L, this);
 
-          IC = Expander.expandCodeFor(ICSCEV, NewIV->getType(),
-                                      Preheader->getTerminator());
+          ICMinus1 = Expander.expandCodeFor(ICMinus1SCEV, NewIV->getType(),
+                                            Preheader->getTerminator());
         }
  
-        Value *NewIVNext = NewIV->getIncomingValueForBlock(Header); 
-        Value *Cond = new ICmpInst(BI, CmpInst::ICMP_EQ, NewIVNext, IC,
+        Value *Cond = new ICmpInst(BI, CmpInst::ICMP_EQ, NewIV, ICMinus1,
                                    "exitcond");
         BI->setCondition(Cond);