Fix PR25372 - teach replaceCongruentPHIs to handle cases where SE evaluates a PHI...

Summary:
Since now Scalar Evolution can create non-add rec expressions for PHI
nodes, it can also create SCEVConstant expressions. This will confuse
replaceCongruentPHIs, which previously relied on the fact that SCEV
could not produce constants in this case.

We will now replace the node with a constant in these cases - or avoid
processing the Phi in case of a type mismatch.

Reviewers: sanjoy

Subscribers: llvm-commits, majnemer

Differential Revision: http://reviews.llvm.org/D14230

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251938 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/ScalarEvolutionExpander.cpp b/lib/Analysis/ScalarEvolutionExpander.cpp
index 2c2e5828003aea6dd76c7ce27108b0df39d16a1d..b3b04ad148fd39272217ee0156836c1f554650c1 100644 (file)
--- a/lib/Analysis/ScalarEvolutionExpander.cpp
+++ b/lib/Analysis/ScalarEvolutionExpander.cpp
@@ -1735,9 +1735,22 @@ unsigned SCEVExpander::replaceCongruentIVs(Loop *L, const DominatorTree *DT,
          PEnd = Phis.end(); PIter != PEnd; ++PIter) {
     PHINode *Phi = *PIter;
 
+    auto SimplifyPHINode = [&](PHINode *PN) -> Value * {
+      if (Value *V = SimplifyInstruction(PN, DL, &SE.TLI, &SE.DT, &SE.AC))
+        return V;
+      if (!SE.isSCEVable(PN->getType()))
+        return nullptr;
+      auto *Const = dyn_cast<SCEVConstant>(SE.getSCEV(PN));
+      if (!Const)
+        return nullptr;
+      return Const->getValue();
+    };
+
     // Fold constant phis. They may be congruent to other constant phis and
     // would confuse the logic below that expects proper IVs.
-    if (Value *V = SimplifyInstruction(Phi, DL, &SE.TLI, &SE.DT, &SE.AC)) {
+    if (Value *V = SimplifyPHINode(Phi)) {
+      if (V->getType() != Phi->getType())
+        continue;
       Phi->replaceAllUsesWith(V);
       DeadInsts.emplace_back(Phi);
       ++NumElim;

diff --git a/test/Transforms/IndVarSimplify/const_phi.ll b/test/Transforms/IndVarSimplify/const_phi.ll
new file mode 100644 (file)
index 0000000..33dc551
--- /dev/null
+++ b/test/Transforms/IndVarSimplify/const_phi.ll
@@ -0,0 +1,33 @@
+; RUN: opt < %s -indvars -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; PR25372
+; We can compute the expression of %phi0 and that is a SCEV
+; constant. However, instcombine can't deduce this, so we can
+; potentially end up trying to handle a constant when replacing
+; congruent IVs.
+
+; CHECK-LABEL: crash
+define void @crash() {
+entry:
+  br i1 false, label %not_taken, label %pre
+
+not_taken:
+  br label %pre
+
+pre:
+; %phi0.pre and %phi1.pre are evaluated by SCEV to constant 0.
+  %phi0.pre = phi i32 [ 0, %entry ], [ 2, %not_taken ]
+  %phi1.pre = phi i32 [ 0, %entry ], [ 1, %not_taken ]
+  br label %loop
+
+loop:
+; %phi0 and %phi1 are evaluated by SCEV to constant 0.
+  %phi0 = phi i32 [ 0, %loop ], [ %phi0.pre, %pre ]
+  %phi1 = phi i32 [ 0, %loop ], [ %phi1.pre, %pre ]
+  br i1 undef, label %exit, label %loop
+
+exit:
+  ret void
+}