[GVN] Don't explicitly materialize undefs from dead blocks

When materializing an available load value, do not explicitly
materialize the undef values from dead blocks. Doing so will
will force creation of a phi with an undef operand, even if there
is a dominating definition. The phi will be folded away on
subsequent GVN iterations, but by then we may have already
poisoned MDA cache slots.

Simply don't register these values in the first place, and let
SSAUpdater do its thing.

Author: nikic

llvm/lib/Transforms/Scalar/GVN.cpp

@@ -855,6 +855,9 @@ static Value *ConstructSSAForLoadSet(LoadInst *LI,
   for (const AvailableValueInBlock &AV : ValuesPerBlock) {
     BasicBlock *BB = AV.BB;
 
+    if (AV.AV.isUndefValue())
+      continue;
+
     if (SSAUpdate.HasValueForBlock(BB))
       continue;
 
@@ -915,9 +918,7 @@ Value *AvailableValue::MaterializeAdjustedValue(LoadInst *LI,
                       << *Res << '\n'
                       << "\n\n\n");
   } else {
-    assert(isUndefValue() && "Should be UndefVal");
-    LLVM_DEBUG(dbgs() << "GVN COERCED NONLOCAL Undef:\n";);
-    return UndefValue::get(LoadTy);
+    llvm_unreachable("Should not materialize value from dead block");
   }
   assert(Res && "failed to materialize?");
   return Res;

llvm/test/Transforms/GVN/load-dead-block.ll

@@ -13,8 +13,7 @@ define i64 @test(i64** noalias %p, i64* noalias %q) {
 ; CHECK-NEXT:    store i64 1, i64* [[Q]], align 4
 ; CHECK-NEXT:    [[Q3:%.*]] = getelementptr i64, i64* [[Q]], i64 1
 ; CHECK-NEXT:    store i64 2, i64* [[Q3]], align 4
-; CHECK-NEXT:    [[V2:%.*]] = load i64, i64* [[Q]], align 4
-; CHECK-NEXT:    ret i64 [[V2]]
+; CHECK-NEXT:    ret i64 1
 ;
 entry:
   store i64* %q, i64** %p