[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

1 ↗

(On Diff #513993)

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/Transforms/Scalar/SROA.cpp

static void rewriteMemOpOfSelect(SelectInst &SI, T &I,
   Tail->setName(Head->getName() + ".cont");
   PHINode *PN;
   if (isa<LoadInst>(I))
     PN = PHINode::Create(I.getType(), 2, "", &I);
   for (BasicBlock *SuccBB : successors(Head)) {
     bool IsThen = SuccBB == HeadBI->getSuccessor(0);
     int SuccIdx = IsThen ? 0 : 1;
     auto *NewMemOpBB = SuccBB == Tail ? Head : SuccBB;
+    auto &CondMemOp = cast<T>(*I.clone());
     if (NewMemOpBB != Head) {
       NewMemOpBB->setName(Head->getName() + (IsThen ? ".then" : ".else"));
       if (isa<LoadInst>(I))
         ++NumLoadsPredicated;
       else
         ++NumStoresPredicated;
-    } else
+    } else {
+      CondMemOp.dropUBImplyingAttrsAndUnknownMetadata();
       ++NumLoadsSpeculated;
-    auto &CondMemOp = cast<T>(*I.clone());
+    }
     CondMemOp.insertBefore(NewMemOpBB->getTerminator());
     Value *Ptr = SI.getOperand(1 + SuccIdx);
     if (auto *PtrTy = Ptr->getType();
         !PtrTy->isOpaquePointerTy() &&
         PtrTy != CondMemOp.getPointerOperandType())
       Ptr = BitCastInst::CreatePointerBitCastOrAddrSpaceCast(
           Ptr, CondMemOp.getPointerOperandType(), "", &CondMemOp);
     CondMemOp.setOperand(I.getPointerOperandIndex(), Ptr);

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

entry:
   %min = alloca i32, align 4
   store i32 0, ptr %min, align 4
   %min.addr.data = select i1 %cond_inner, ptr %min_then, ptr %min, !prof !0
   %addr = select i1 %cond_outer, ptr %min.addr.data, ptr %data_else, !prof !0
   %r = load i32, ptr %addr, align 4
   ret i32 %r
 }
 
+; When promoting speculative instruction, metadata that may trigger immediate UB should be dropped.
+define void @load_of_select_with_noundef_nonnull(ptr %buffer, i1 %b) {
+; CHECK-PRESERVE-CFG-LABEL: @load_of_select_with_noundef_nonnull(
+; CHECK-PRESERVE-CFG-NEXT:    [[UB_PTR:%.*]] = alloca ptr, align 8
+; CHECK-PRESERVE-CFG-NEXT:    [[SELECT_PTR:%.*]] = select i1 [[B:%.*]], ptr [[BUFFER:%.*]], ptr [[UB_PTR]]
+; CHECK-PRESERVE-CFG-NEXT:    [[LOAD_PTR:%.*]] = load ptr, ptr [[SELECT_PTR]], align 8, !nonnull !1, !noundef !1
+; CHECK-PRESERVE-CFG-NEXT:    ret void
+;
+; CHECK-MODIFY-CFG-LABEL: @load_of_select_with_noundef_nonnull(
+; CHECK-MODIFY-CFG-NEXT:    br i1 [[B:%.*]], label [[DOTTHEN:%.*]], label [[DOTCONT:%.*]]
+; CHECK-MODIFY-CFG:       .then:
+; CHECK-MODIFY-CFG-NEXT:    [[LOAD_PTR_THEN_VAL:%.*]] = load ptr, ptr [[BUFFER:%.*]], align 8, !nonnull !2, !noundef !2
+; CHECK-MODIFY-CFG-NEXT:    br label [[DOTCONT]]
+; CHECK-MODIFY-CFG:       .cont:
+; CHECK-MODIFY-CFG-NEXT:    [[LOAD_PTR:%.*]] = phi ptr [ [[LOAD_PTR_THEN_VAL]], [[DOTTHEN]] ], [ undef, [[TMP0:%.*]] ]
+; CHECK-MODIFY-CFG-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
+}
+
 !0  = !{!"branch_weights", i32 1,  i32 99}
+!1 = !{}
 
 ; Ensure that the branch metadata is reversed to match the reversals above.
 
 declare void @llvm.lifetime.start.p0(i64, ptr )
 declare void @llvm.lifetime.end.p0(i64, ptr)
 declare i32 @llvm.smax.i32(i32, i32)