[SROA] Remove UB-implying metadata when promoting speculative instruction.

[DianQK]

Summary

After D138238 introduced the then/else blocks, we should remove UB-implying metadata for the promoted speculative instruction.

This patch fixes the LTO=thin appears broken on nightly rustc? #110256.

[StephenFan]

It seems that !noundef means that the loaded value is not an undefined value instead of loading a value from an under ptr is UB.

[DianQK]

In D148456#4271868, @StephenFan wrote:

It seems that !noundef means that the loaded value is not an undefined value instead of loading a value from an under ptr is UB.

Thanks, I tried to correct the summary.

[nikic]

You are looking for dropUBImplyingAttrsAndUnknownMetadata().

[nikic]

Well, actually you are looking for dropUBImplyingAttrsAndMetadata() from D146629, but for now you can just drop all unknown metadata, and we can add the UB implying distinction later.

[DianQK]

Promote the load instruction by calling dropUBImplyingAttrsAndMetadata.

Thanks for the reminder that dropUBImplyingAttrsAndMetadata()/dropUBImplyingAttrsAndUnknownMetadata() is the method I was looking for.

[nikic]

LG

llvm/test/Transforms/SROA/select-load-with-nonnull.ll

2

I'd suggest adding this test to llvm/test/Transforms/SROA/select-load.ll instead.

[DianQK]

Move the test to llvm/test/Transforms/SROA/select-load.ll.

Maybe we should also cherry-pick to 16.0.2?

llvm/lib/Analysis/Loads.cpp

bool llvm::isSafeToLoadUnconditionally(Value *V, Type *Ty, Align Alignment,
                                        const DataLayout &DL,
                                        Instruction *ScanFrom,
                                        AssumptionCache *AC,
                                        const DominatorTree *DT,
                                        const TargetLibraryInfo *TLI) {
   TypeSize TySize = DL.getTypeStoreSize(Ty);
   if (TySize.isScalable())
     return false;
+  // When the load has nonnull and noundef metadata, we cannot easily conclude
+  // that it is safe.
+  if (auto *LI = dyn_cast<LoadInst>(ScanFrom)) {
+    if (LI->getType()->isPointerTy() &&
+        LI->getMetadata(LLVMContext::MD_nonnull) &&
+        LI->getMetadata(LLVMContext::MD_noundef)) {
+      return false;
+    }
+  }
   APInt Size(DL.getIndexTypeSizeInBits(V->getType()), TySize.getFixedValue());
   return isSafeToLoadUnconditionally(V, Alignment, Size, DL, ScanFrom, AC, DT,
                                      TLI);
 }
 
 /// DefMaxInstsToScan - the default number of maximum instructions
 /// to scan in the block, used by FindAvailableLoadedValue().
 /// FindAvailableLoadedValue() was introduced in r60148, to improve jump

llvm/test/Transforms/SROA/select-load-with-nonnull.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes='sroa<modify-cfg>' -S | FileCheck %s
+
+define void @load_of_select_with_noundef_nonnull(ptr %buffer, i1 %b) {
+; CHECK-LABEL: @load_of_select_with_noundef_nonnull(
+; CHECK-NEXT:    br i1 [[B:%.*]], label [[DOTTHEN:%.*]], label [[DOTELSE:%.*]]
+; CHECK:       .then:
+; CHECK-NEXT:    [[LOAD_PTR_THEN_VAL:%.*]] = load ptr, ptr [[BUFFER:%.*]], align 8, !nonnull !0, !noundef !0
+; CHECK-NEXT:    br label [[DOTCONT:%.*]]
+; CHECK:       .else:
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ne ptr undef, null
+; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP1]])
+; CHECK-NEXT:    br label [[DOTCONT]]
+; CHECK:       .cont:
+; CHECK-NEXT:    [[LOAD_PTR:%.*]] = phi ptr [ [[LOAD_PTR_THEN_VAL]], [[DOTTHEN]] ], [ undef, [[DOTELSE]] ]
+; CHECK-NEXT:    ret void
+;
+  %ub_ptr = alloca ptr
+  %select_ptr = select i1 %b, ptr %buffer, ptr %ub_ptr
+  %load_ptr = load ptr, ptr %select_ptr, !nonnull !1, !noundef !1
+  ret void
+}
+
+define void @load_of_select_with_noundef(ptr %buffer, i1 %b) {
+; CHECK-LABEL: @load_of_select_with_noundef(
+; CHECK-NEXT:    br i1 [[B:%.*]], label [[DOTTHEN:%.*]], label [[DOTCONT:%.*]]
+; CHECK:       .then:
+; CHECK-NEXT:    [[LOAD_PTR_THEN_VAL:%.*]] = load ptr, ptr [[BUFFER:%.*]], align 8, !noundef !0
+; CHECK-NEXT:    br label [[DOTCONT]]
+; CHECK:       .cont:
+; CHECK-NEXT:    [[LOAD_PTR:%.*]] = phi ptr [ [[LOAD_PTR_THEN_VAL]], [[DOTTHEN]] ], [ undef, [[TMP0:%.*]] ]
+; CHECK-NEXT:    ret void
+;
+  %ub_ptr = alloca ptr
+  %select_ptr = select i1 %b, ptr %buffer, ptr %ub_ptr
+  %load_ptr = load ptr, ptr %select_ptr, !noundef !1
+  ret void
+}
+
+define void @load_of_select(ptr %buffer, i1 %b) {
+; CHECK-LABEL: @load_of_select(
+; CHECK-NEXT:    br i1 [[B:%.*]], label [[DOTTHEN:%.*]], label [[DOTCONT:%.*]]
+; CHECK:       .then:
+; CHECK-NEXT:    [[LOAD_PTR_THEN_VAL:%.*]] = load ptr, ptr [[BUFFER:%.*]], align 8
+; CHECK-NEXT:    br label [[DOTCONT]]
+; CHECK:       .cont:
+; CHECK-NEXT:    [[LOAD_PTR:%.*]] = phi ptr [ [[LOAD_PTR_THEN_VAL]], [[DOTTHEN]] ], [ undef, [[TMP0:%.*]] ]
+; CHECK-NEXT:    ret void
+;
+  %ub_ptr = alloca ptr
+  %select_ptr = select i1 %b, ptr %buffer, ptr %ub_ptr
+  %load_ptr = load ptr, ptr %select_ptr
+  ret void
+}
+
+!1 = !{}