From 0b4a4cc72d81a522230a7270abdfa804aa49d89b Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo@fhahn.com>
Date: Wed, 29 Sep 2021 14:38:35 +0100
Subject: [PATCH] [IndVarSimplify] Forget phi value after changing incoming
 value.

This fixes an issue exposed by D71539, where IndVarSimplify tries
to access an invalid cached SCEV expression after making changes to the
underlying PHI instruction earlier.

When changing the incoming value of a PHI, forget the cached SCEV for
the PHI.
---
 llvm/lib/Transforms/Scalar/IndVarSimplify.cpp      |  1 +
 .../invalidate-modified-lcssa-phi.ll               | 44 ++++++++++++++++++++++
 2 files changed, 45 insertions(+)
 create mode 100644 llvm/test/Transforms/IndVarSimplify/invalidate-modified-lcssa-phi.ll

diff --git a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
index 6d13b3b..0b5979a 100644
--- a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -494,6 +494,7 @@ bool IndVarSimplify::rewriteFirstIterationLoopExitValues(Loop *L) {
           MadeAnyChanges = true;
           PN.setIncomingValue(IncomingValIdx,
                               ExitVal->getIncomingValue(PreheaderIdx));
+          SE->forgetValue(&PN);
         }
       }
     }
diff --git a/llvm/test/Transforms/IndVarSimplify/invalidate-modified-lcssa-phi.ll b/llvm/test/Transforms/IndVarSimplify/invalidate-modified-lcssa-phi.ll
new file mode 100644
index 0000000..fcb35a0
--- /dev/null
+++ b/llvm/test/Transforms/IndVarSimplify/invalidate-modified-lcssa-phi.ll
@@ -0,0 +1,44 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes='loop(indvars,loop-deletion)' -S %s | FileCheck %s
+
+; Make sure indvarsimplify properly forgets the exit value %p.2.lcssa phi after
+; modifying it. Loop deletion is required to show the incorrect use of the cached
+; SCEV value.
+
+define void @test(i1 %c) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[HEADER_1:%.*]]
+; CHECK:       header.1.loopexit:
+; CHECK-NEXT:    br label [[HEADER_1_BACKEDGE:%.*]]
+; CHECK:       header.1:
+; CHECK-NEXT:    br label [[HEADER_2:%.*]]
+; CHECK:       header.2:
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[LATCH_1:%.*]], label [[LATCH_2:%.*]]
+; CHECK:       latch.1:
+; CHECK-NEXT:    br label [[HEADER_1_LOOPEXIT:%.*]]
+; CHECK:       latch.2:
+; CHECK-NEXT:    br label [[HEADER_1_BACKEDGE]]
+; CHECK:       header.1.backedge:
+; CHECK-NEXT:    br label [[HEADER_1]]
+;
+entry:
+  br label %header.1
+
+header.1:
+  %p.1 = phi i32 [ 0, %entry ], [ %p.2.lcssa, %latch.2 ], [ 0, %latch.1 ]
+  br label %header.2
+
+header.2:
+  %p.2 = phi i32 [ %p.1, %header.1 ], [ %p.2.next, %latch.1 ]
+  br i1 %c, label %latch.1, label %latch.2
+
+latch.1:
+  %p.2.next = add i32 %p.2, 1
+  br i1 false, label %header.2, label %header.1
+
+latch.2:
+  %p.2.lcssa = phi i32 [ %p.2, %header.2 ]
+  br label %header.1
+
+}
-- 
2.7.4