[PATCH] D51801: [MemorySSAUpdater] Avoid creating self-referencing MemoryDefs

[Alexandros Lamprineas via Phabricator via llvm-commits]

labrinea created this revision.
labrinea added reviewers: llvm-commits, sebpop.
Herald added subscribers: george.burgess.iv, Prazek.

Fix for https://bugs.llvm.org/show_bug.cgi?id=38807, which occurred while compiling SemaTemplateInstantiate.cpp with clang and GVNHoist enabled.
In the following example:

        1=def(entry)
        /        \
  2=def(1)       4=def(1)
  3=def(2)       5=def(4)

When removing the MemoryDef `2=def(1)` from its basic block, and just before adding it to the end of the parent basic block, we first replace all its uses with the defining memory access:

  3=def(2) -> 3=def(1)

Then we call insertDef for adding `2=def(1)` to the parent basic block, where we replace the uses of `1=def(entry)` with `2=def(1)`. Doing so we create a self reference:

  2=def(1) -> 2=def(2)  (bad)
  3=def(1) -> 3=def(2)  (ok)
  4=def(1) -> 4=def(2)  (ok)


https://reviews.llvm.org/D51801

Files:
  lib/Analysis/MemorySSAUpdater.cpp
  test/Transforms/GVNHoist/pr38807.ll


Index: test/Transforms/GVNHoist/pr38807.ll
===================================================================
--- /dev/null
+++ test/Transforms/GVNHoist/pr38807.ll
@@ -0,0 +1,50 @@
+; RUN: opt < %s -early-cse-memssa -gvn-hoist -S | FileCheck %s
+
+; Make sure opt doesn't crash. On top of that, the instructions
+; of the side blocks should be hoisted to the entry block.
+
+%s = type { i32, i64 }
+%S = type { %s, i32 }
+
+;CHECK-LABEL: @foo
+
+define void @foo(i32* %arg) {
+bb0:
+  %0 = bitcast i32* %arg to %S*
+  %call.idx.i = getelementptr %S, %S* %0, i64 0, i32 0, i32 0
+  %call.idx.val.i = load i32, i32* %call.idx.i
+  br label %bb1
+
+;CHECK: bb1:
+;CHECK:   %call264 = call zeroext i1 @bar
+;CHECK:   store i32 %call.idx.val.i, i32* %call.idx.i
+;CHECK:   %1 = getelementptr inbounds %S, %S* %0, i64 0, i32 0, i32 1
+;CHECK:   store i64 undef, i64* %1
+;CHECK:   br i1 %call264, label %bb2, label %bb3
+
+bb1:
+  %call264 = call zeroext i1 @bar()
+  br i1 %call264, label %bb2, label %bb3
+
+;CHECK:     bb2:
+;CHECK-NOT:   store i32 %call.idx.val.i, i32* %call.idx.i
+;CHECK-NOT:   store i64 undef, i64* %{.*}
+
+bb2:
+  store i32 %call.idx.val.i, i32* %call.idx.i
+  %1 = getelementptr inbounds %S, %S* %0, i64 0, i32 0, i32 1
+  store i64 undef, i64* %1
+  ret void
+
+;CHECK:     bb3:
+;CHECK-NOT:   store i32 %call.idx.val.i, i32* %call.idx.i
+;CHECK-NOT:   store i64 undef, i64* %{.*}
+
+bb3:
+  store i32 %call.idx.val.i, i32* %call.idx.i
+  %2 = getelementptr inbounds %S, %S* %0, i64 0, i32 0, i32 1
+  store i64 undef, i64* %2
+  ret void
+}
+
+declare zeroext i1 @bar()
Index: lib/Analysis/MemorySSAUpdater.cpp
===================================================================
--- lib/Analysis/MemorySSAUpdater.cpp
+++ lib/Analysis/MemorySSAUpdater.cpp
@@ -264,16 +264,15 @@
     for (auto UI = DefBefore->use_begin(), UE = DefBefore->use_end();
          UI != UE;) {
       Use &U = *UI++;
-      // Leave the uses alone
-      if (isa<MemoryUse>(U.getUser()))
+      // Leave the MemoryUses alone.
+      // Also make sure we skip ourselves to avoid self references.
+      if (isa<MemoryUse>(U.getUser()) || U.getUser() == MD)
         continue;
       U.set(MD);
     }
   }
 
   // and that def is now our defining access.
-  // We change them in this order otherwise we will appear in the use list
-  // above and reset ourselves.
   MD->setDefiningAccess(DefBefore);
 
   SmallVector<WeakVH, 8> FixupList(InsertedPHIs.begin(), InsertedPHIs.end());


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