[Mem2Reg] Generate non-terminator unreachable for !noundef undef (#96639)

When performing a load from uninitialized memory using !noundef, insert
a non-terminator unreachable instruction, which will be converted to a
proper unreachable by SimplifyCFG. This way we retain the fact that UB
occurred on this code path.

Author: nikic

llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp

@@ -453,6 +453,15 @@ static void addAssumeNonNull(AssumptionCache *AC, LoadInst *LI) {
 static void convertMetadataToAssumes(LoadInst *LI, Value *Val,
                                      const DataLayout &DL, AssumptionCache *AC,
                                      const DominatorTree *DT) {
+  if (isa<UndefValue>(Val) && LI->hasMetadata(LLVMContext::MD_noundef)) {
+    // Insert non-terminator unreachable.
+    LLVMContext &Ctx = LI->getContext();
+    new StoreInst(ConstantInt::getTrue(Ctx),
+                  PoisonValue::get(PointerType::getUnqual(Ctx)),
+                  /*isVolatile=*/false, Align(1), LI);
+    return;
+  }
+
   // If the load was marked as nonnull we don't want to lose that information
   // when we erase this Load. So we preserve it with an assume. As !nonnull
   // returns poison while assume violations are immediate undefined behavior,

llvm/test/Transforms/Mem2Reg/preserve-nonnull-load-metadata.ll

@@ -143,6 +143,7 @@ fin:
 define ptr @no_store_single_load_noundef() {
 ; CHECK-LABEL: @no_store_single_load_noundef(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
 ; CHECK-NEXT:    ret ptr undef
 ;
 entry:
@@ -156,8 +157,10 @@ define ptr @no_store_multiple_loads_noundef(i1 %c) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br i1 [[C:%.*]], label [[IF:%.*]], label [[ELSE:%.*]]
 ; CHECK:       if:
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
 ; CHECK-NEXT:    ret ptr undef
 ; CHECK:       else:
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
 ; CHECK-NEXT:    ret ptr undef
 ;
 entry:
@@ -176,8 +179,7 @@ if:
 define ptr @no_store_single_load_nonnull_noundef() {
 ; CHECK-LABEL: @no_store_single_load_nonnull_noundef(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp ne ptr undef, null
-; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP0]])
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
 ; CHECK-NEXT:    ret ptr undef
 ;
 entry:
@@ -191,12 +193,10 @@ define ptr @no_store_multiple_loads_nonnull_noundef(i1 %c) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br i1 [[C:%.*]], label [[IF:%.*]], label [[ELSE:%.*]]
 ; CHECK:       if:
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp ne ptr undef, null
-; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP0]])
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
 ; CHECK-NEXT:    ret ptr undef
 ; CHECK:       else:
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp ne ptr undef, null
-; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP1]])
+; CHECK-NEXT:    store i1 true, ptr poison, align 1
 ; CHECK-NEXT:    ret ptr undef
 ;
 entry: