[PATCH] D131776: [GVN] Fix miscompile shown in github issue #57025

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope created this revision.
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Make sure that we are conservative in AnalyzeLoadAvailability when
finding an Unknown memdep, also when the dependent BB is dead.

Fixes: https://github.com/llvm/llvm-project/issues/57025


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D131776

Files:
  llvm/lib/Transforms/Scalar/GVN.cpp
  llvm/test/Transforms/GVN/memdep-unknown-deadblocks.ll


Index: llvm/test/Transforms/GVN/memdep-unknown-deadblocks.ll
===================================================================
--- llvm/test/Transforms/GVN/memdep-unknown-deadblocks.ll
+++ llvm/test/Transforms/GVN/memdep-unknown-deadblocks.ll
@@ -34,8 +34,7 @@
 ; CHECK-GVN-NEXT:    [[ARR:%.*]] = alloca [4 x i16], align 1
 ; CHECK-GVN-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK-GVN:       for.body:
-; CHECK-GVN-NEXT:    [[VALUE2:%.*]] = phi i16 [ undef, [[ENTRY:%.*]] ], [ [[DEAD:%.*]], [[WHILE_END:%.*]] ]
-; CHECK-GVN-NEXT:    [[I:%.*]] = phi i16 [ 0, [[ENTRY]] ], [ [[NEXT_I:%.*]], [[WHILE_END]] ]
+; CHECK-GVN-NEXT:    [[I:%.*]] = phi i16 [ 0, [[ENTRY:%.*]] ], [ [[NEXT_I:%.*]], [[WHILE_END:%.*]] ]
 ; CHECK-GVN-NEXT:    [[CMP0:%.*]] = icmp eq i16 [[I]], 0
 ; CHECK-GVN-NEXT:    br i1 [[CMP0]], label [[STORE_IDX_0:%.*]], label [[STORE_IDX_I:%.*]]
 ; CHECK-GVN:       store.idx.0:
@@ -46,7 +45,6 @@
 ; CHECK-GVN-NEXT:    store i16 42, ptr [[ARR_I]], align 1
 ; CHECK-GVN-NEXT:    br label [[STORE_DONE]]
 ; CHECK-GVN:       store.done:
-; CHECK-GVN-NEXT:    [[VALUE:%.*]] = phi i16 [ 42, [[STORE_IDX_I]] ], [ [[VALUE2]], [[STORE_IDX_0]] ]
 ; CHECK-GVN-NEXT:    br label [[WHILE_BODY:%.*]]
 ; CHECK-GVN:       while.body:
 ; CHECK-GVN-NEXT:    br i1 false, label [[WHILE_BODY_WHILE_BODY_CRIT_EDGE:%.*]], label [[WHILE_END]]
@@ -54,10 +52,10 @@
 ; CHECK-GVN-NEXT:    br label [[WHILE_BODY]]
 ; CHECK-GVN:       while.end:
 ; CHECK-GVN-NEXT:    [[ARR_J:%.*]] = getelementptr [4 x i16], ptr [[ARR]], i16 0, i16 [[I]]
+; CHECK-GVN-NEXT:    [[VALUE:%.*]] = load i16, ptr [[ARR_J]], align 1
 ; CHECK-GVN-NEXT:    tail call void @verify(i16 [[VALUE]])
 ; CHECK-GVN-NEXT:    [[NEXT_I]] = add i16 [[I]], 1
 ; CHECK-GVN-NEXT:    [[ARR_NEXT_I:%.*]] = getelementptr [4 x i16], ptr [[ARR]], i16 0, i16 [[NEXT_I]]
-; CHECK-GVN-NEXT:    [[DEAD]] = load i16, ptr [[ARR_NEXT_I]], align 1
 ; CHECK-GVN-NEXT:    [[CMP4:%.*]] = icmp slt i16 [[NEXT_I]], 3
 ; CHECK-GVN-NEXT:    br i1 [[CMP4]], label [[FOR_BODY]], label [[FOR_END:%.*]]
 ; CHECK-GVN:       for.end:
@@ -65,7 +63,7 @@
 ;
 ; CHECK-GVN-O1-LABEL: @test(
 ; CHECK-GVN-O1-NEXT:  entry:
-; CHECK-GVN-O1-NEXT:    tail call void @verify(i16 42)
+; CHECK-GVN-O1-NEXT:    tail call void @verify(i16 7)
 ; CHECK-GVN-O1-NEXT:    tail call void @verify(i16 42)
 ; CHECK-GVN-O1-NEXT:    tail call void @verify(i16 42)
 ; CHECK-GVN-O1-NEXT:    ret void
Index: llvm/lib/Transforms/Scalar/GVN.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/GVN.cpp
+++ llvm/lib/Transforms/Scalar/GVN.cpp
@@ -1290,6 +1290,14 @@
     MemDepResult DepInfo = Deps[i].getResult();
 
     if (DeadBlocks.count(DepBB)) {
+      // A non-local dependency means that the analysis might have given up
+      // e.g. based on phi translation failure (at least if the dependency is
+      // Unknown). Need to be conservative and say that the value isn't
+      // available.
+      if (DepInfo.isUnknown()) {
+        UnavailableBlocks.push_back(DepBB);
+        continue;
+      }
       // Dead dependent mem-op disguise as a load evaluating the same value
       // as the load in question.
       ValuesPerBlock.push_back(AvailableValueInBlock::getUndef(DepBB));


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