[LoopUnswitch] Fix SCEV invalidation in unswitching

Max Kazantsev · Max Kazantsev · commit d99f3bacb4b9 · 2018-05-23T10:09:53.000Z
Loop unswitching makes substantial changes to a loop that can also affect cached SCEV info in its outer loops as well, but it only cares to invalidate SCEV cache for the innermost loop in case of full unswitching and does not invalidate anything at all in case of trivial unswitching. As result, we may end up with incorrect data in cache. Differential Revision: https://reviews.llvm.org/D46045 Reviewed By: mzolotukhin llvm-svn: 333072
diff --git a/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
@@ -975,6 +975,10 @@ void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
                     << " blocks] in Function "
                     << L->getHeader()->getParent()->getName()
                     << " on cond: " << *Val << " == " << *Cond << "\n");
+  // We are going to make essential changes to CFG. This may invalidate cached
+  // information for L or one of its parent loops in SCEV.
+  if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>())
+    SEWP->getSE().forgetTopmostLoop(L);
 
   // First step, split the preheader, so that we know that there is a safe place
   // to insert the conditional branch.  We will change loopPreheader to have a
@@ -1201,8 +1205,10 @@ void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
                     << " blocks] in Function " << F->getName() << " when '"
                     << *Val << "' == " << *LIC << "\n");
 
+  // We are going to make essential changes to CFG. This may invalidate cached
+  // information for L or one of its parent loops in SCEV.
   if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>())
-    SEWP->getSE().forgetLoop(L);
+    SEWP->getSE().forgetTopmostLoop(L);
 
   LoopBlocks.clear();
   NewBlocks.clear();
diff --git a/llvm/test/Transforms/LoopUnswitch/invalidate-scev.ll b/llvm/test/Transforms/LoopUnswitch/invalidate-scev.ll
@@ -0,0 +1,33 @@
+; RUN: opt -S -indvars -loop-unswitch < %s | FileCheck %s
+
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @test_01() {
+
+; Make sure we don't fail by SCEV's assertion due to incorrect invalidation.
+; CHECK-LABEL: @test_01
+
+entry:
+  br label %loop
+
+loop:                           ; preds = %backedge, %entry
+  %p_50.addr.0 = phi i16 [ undef, %entry ], [ %add2699, %backedge ]
+  %idxprom2690 = sext i16 %p_50.addr.0 to i32
+  %arrayidx2691 = getelementptr inbounds [5 x i32], [5 x i32]* undef, i32 0, i32 %idxprom2690
+  %0 = load i32, i32* %arrayidx2691, align 1
+  %tobool2692 = icmp ne i32 %0, 0
+  br label %inner_loop
+
+inner_loop:                                     ; preds = %inner_backedge, %loop
+  br i1 %tobool2692, label %backedge, label %inner_backedge
+
+inner_backedge:                                       ; preds = %inner_loop
+  br label %inner_loop
+
+backedge:                                      ; preds = %inner_loop
+  %add2699 = add nsw i16 %p_50.addr.0, 1
+  br i1 false, label %loop, label %exit
+
+exit:               ; preds = %backedge
+  unreachable
+}
