[MergeICmp] Fix a bug in MergeICmp that can lead to a block being processed more than once.

[trentxintong]

Summary

Fix a bug in MergeICmp that can lead to a BCECmp block being processed more than once and eventually lead to a broken LLVM module.
The problem is that if the non-constant value is not produced by the last block, the producer will be processed once when the its parent block
is processed and second time when the last block is processed.

We end up having 2 same BCECmpBlock in the merge queue. And eventually lead to a broken LLVM module.

[courbet]

Thanks for the fix !

lib/Transforms/Scalar/MergeICmps.cpp

581 ↗

(On Diff #136091)

Can you add a debug line:

DEBUG(dbgs() << "skip: non-constant value not from cmp\n");

llvm/trunk/lib/Transforms/Scalar/MergeICmps.cpp

bool processPhi(PHINode &Phi, const TargetLibraryInfo *const TLI) {
   BasicBlock *LastBlock = nullptr;
   for (unsigned I = 0; I < Phi.getNumIncomingValues(); ++I) {
     if (isa<ConstantInt>(Phi.getIncomingValue(I))) continue;
     if (LastBlock) {
       // There are several non-constant values.
       DEBUG(dbgs() << "skip: several non-constant values\n");
       return false;
     }
+    if (!isa<ICmpInst>(Phi.getIncomingValue(I)) ||
+        cast<ICmpInst>(Phi.getIncomingValue(I))->getParent() !=
+            Phi.getIncomingBlock(I)) {
+      // Non-constant incoming value is not from a cmp instruction or not
+      // produced by the last block. We could end up processing the value
+      // producing block more than once.
+      //
+      // This is an uncommon case, so we bail.
+      DEBUG(
+          dbgs()
+          << "skip: non-constant value not from cmp or not from last block.\n");
+      return false;
+    }
     LastBlock = Phi.getIncomingBlock(I);
   }
   if (!LastBlock) {
     // There is no non-constant block.
     DEBUG(dbgs() << "skip: no non-constant block\n");
     return false;
   }
   if (LastBlock->getSingleSuccessor() != Phi.getParent()) {

llvm/trunk/test/Transforms/MergeICmps/X86/last-block-produce-no-value.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -mergeicmps -mtriple=x86_64-unknown-unknown -S | FileCheck %s
+
+%"struct.std::pair" = type { i32, i32, i32 }
+
+; Last block does not produce the non-constant value into the phi.
+; We could handle this case, but an easier way would be to allow other transformations such as
+; SimplifyCFG to remove %land.rhs.i.2 and turn the terminator of %land.rhs.i into an unconditional
+; branch.
+
+define zeroext i1 @opeq1(
+; CHECK-LABEL: @opeq1(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[FIRST_I:%.*]] = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* [[A:%.*]], i64 0, i32 0
+; CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* [[FIRST_I]], align 4
+; CHECK-NEXT:    [[FIRST1_I:%.*]] = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* [[B:%.*]], i64 0, i32 0
+; CHECK-NEXT:    [[TMP1:%.*]] = load i32, i32* [[FIRST1_I]], align 4
+; CHECK-NEXT:    [[CMP_I:%.*]] = icmp eq i32 [[TMP0]], [[TMP1]]
+; CHECK-NEXT:    br i1 [[CMP_I]], label [[LAND_RHS_I:%.*]], label [[OPEQ1_EXIT:%.*]]
+; CHECK:       land.rhs.i:
+; CHECK-NEXT:    [[SECOND_I:%.*]] = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* [[A]], i64 0, i32 1
+; CHECK-NEXT:    [[TMP2:%.*]] = load i32, i32* [[SECOND_I]], align 4
+; CHECK-NEXT:    [[SECOND2_I:%.*]] = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* [[B]], i64 0, i32 1
+; CHECK-NEXT:    [[TMP3:%.*]] = load i32, i32* [[SECOND2_I]], align 4
+; CHECK-NEXT:    [[CMP3_I:%.*]] = icmp eq i32 [[TMP2]], [[TMP3]]
+; CHECK-NEXT:    br i1 [[CMP3_I]], label [[LAND_RHS_I:%.*]], label [[OPEQ1_EXIT:%.*]]
+; CHECK:       land.rhs.i.2:
+; CHECK-NEXT:    br label [[OPEQ1_EXIT]]
+; CHECK:       opeq1.exit:
+; CHECK-NEXT:    [[TMP4:%.*]] = phi i1 [ false, [[ENTRY:%.*]] ], [ [[CMP3_I]], [[LAND_RHS_I]] ]
+; CHECK-NEXT:    ret i1 [[TMP4]]
+;
+  %"struct.std::pair"* nocapture readonly dereferenceable(12) %a,
+  %"struct.std::pair"* nocapture readonly dereferenceable(12) %b) local_unnamed_addr #0 {
+entry:
+  %first.i = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* %a, i64 0, i32 0
+  %0 = load i32, i32* %first.i, align 4
+  %first1.i = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* %b, i64 0, i32 0
+  %1 = load i32, i32* %first1.i, align 4
+  %cmp.i = icmp eq i32 %0, %1
+  br i1 %cmp.i, label %land.rhs.i, label %opeq1.exit
+
+land.rhs.i:
+  %second.i = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* %a, i64 0, i32 1
+  %2 = load i32, i32* %second.i, align 4
+  %second2.i = getelementptr inbounds %"struct.std::pair", %"struct.std::pair"* %b, i64 0, i32 1
+  %3 = load i32, i32* %second2.i, align 4
+  %cmp3.i = icmp eq i32 %2, %3
+  br i1 %cmp3.i, label %land.rhs.i.2, label %opeq1.exit
+
+land.rhs.i.2:
+  br label %opeq1.exit
+
+opeq1.exit:
+  %4 = phi i1 [ false, %entry ], [ false,  %land.rhs.i], [ %cmp3.i, %land.rhs.i.2 ]
+  ret i1 %4
+}