[llvm] r319583 - [IndVars] Fix a bug introduced in r317012

[Philip Reames via llvm-commits]

Author: reames
Date: Fri Dec  1 12:57:19 2017
New Revision: 319583

URL: http://llvm.org/viewvc/llvm-project?rev=319583&view=rev
Log:
[IndVars] Fix a bug introduced in r317012

Turns out we can have comparisons which are indirect users of the induction variable that we can make invariant.  In this case, there is no loop invariant value contributing and we'd fail an assert.

The test case was found by a java fuzzer and reduced.  It's a real cornercase.  You have to have a static loop which we've already proven only executes once, but haven't broken the backedge on, and an inner phi whose result can be constant folded by SCEV using exit count reasoning but not proven by isKnownPredicate.  To my knowledge, only the fuzzer has hit this case.


Modified:
    llvm/trunk/lib/Transforms/Utils/SimplifyIndVar.cpp
    llvm/trunk/test/Transforms/IndVarSimplify/loop-invariant-conditions.ll

Modified: llvm/trunk/lib/Transforms/Utils/SimplifyIndVar.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyIndVar.cpp?rev=319583&r1=319582&r2=319583&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/SimplifyIndVar.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/SimplifyIndVar.cpp Fri Dec  1 12:57:19 2017
@@ -200,13 +200,23 @@ bool SimplifyIndvar::makeIVComparisonInv
   // TODO: Support multiple entry loops?  (We currently bail out of these in
   // the IndVarSimplify pass)
   if (auto *BB = L->getLoopPredecessor()) {
-    Value *Incoming = PN->getIncomingValueForBlock(BB);
-    const SCEV *IncomingS = SE->getSCEV(Incoming);
-    CheapExpansions[IncomingS] = Incoming;
+    const int Idx = PN->getBasicBlockIndex(BB);
+    if (Idx >= 0) {
+      Value *Incoming = PN->getIncomingValue(Idx);
+      const SCEV *IncomingS = SE->getSCEV(Incoming);
+      CheapExpansions[IncomingS] = Incoming;
+    }
   }
   Value *NewLHS = CheapExpansions[InvariantLHS];
   Value *NewRHS = CheapExpansions[InvariantRHS];
 
+  if (!NewLHS)
+    if (auto *ConstLHS = dyn_cast<SCEVConstant>(InvariantLHS))
+      NewLHS = ConstLHS->getValue();
+  if (!NewRHS)
+    if (auto *ConstRHS = dyn_cast<SCEVConstant>(InvariantRHS))
+      NewRHS = ConstRHS->getValue();
+
   if (!NewLHS || !NewRHS)
     // We could not find an existing value to replace either LHS or RHS.
     // Generating new instructions has subtler tradeoffs, so avoid doing that

Modified: llvm/trunk/test/Transforms/IndVarSimplify/loop-invariant-conditions.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IndVarSimplify/loop-invariant-conditions.ll?rev=319583&r1=319582&r2=319583&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/IndVarSimplify/loop-invariant-conditions.ll (original)
+++ llvm/trunk/test/Transforms/IndVarSimplify/loop-invariant-conditions.ll Fri Dec  1 12:57:19 2017
@@ -295,6 +295,36 @@ for.end:
   ret void
 }
 
+; check that we handle conditions with loop invariant operands which
+; *aren't* in the header - this is a very rare and fragile case where
+; we have a "loop" which is known to run exactly one iteration but
+; haven't yet simplified the uses of the IV
+define void @test10() {
+; CHECK-LABEL: @test10
+entry:
+  br label %loop
+
+loop:
+  %phi1 = phi i32 [ %phi2, %latch ], [ 0, %entry ]
+  %dec = add i32 %phi1, -1
+  br i1 false, label %left, label %right
+
+left:
+  br label %latch
+
+right:
+  br label %latch
+
+latch:
+  %phi2 = phi i32 [ %phi1, %left ], [ %dec, %right ]
+  ; CHECK: %cmp = icmp slt i32 -1, undef
+  %cmp = icmp slt i32 %phi2, undef
+  br i1 true, label %exit, label %loop
+
+exit:
+  ret void
+}
+
 !1 = !{i64 -1, i64 100}