[MSSA] Don't require clone creation to succeed (llvm#76819)

Sometimes, we create a MemoryAccess for an instruction, which is later
simplified (e.g. via devirtualization) such that the new instruction has
no memory effects anymore.

If we later clone the instruction (e.g. during unswitching), then MSSA
will not create a MemoryAccess for the new instruction, triggering an
assert.

Disable the assertion (by passing CreationMustSucceed=false) and adjust
getDefiningAccessForClone() to work correctly in that case.

This PR implements the alternative suggestion by alinas from
llvm#76142.

llvm/lib/Analysis/MemorySSAUpdater.cpp

@@ -568,7 +568,6 @@ static MemoryAccess *onlySingleValue(MemoryPhi *MP) {
 static MemoryAccess *getNewDefiningAccessForClone(MemoryAccess *MA,
                                                   const ValueToValueMapTy &VMap,
                                                   PhiToDefMap &MPhiMap,
-                                                  bool CloneWasSimplified,
                                                   MemorySSA *MSSA) {
   MemoryAccess *InsnDefining = MA;
   if (MemoryDef *DefMUD = dyn_cast<MemoryDef>(InsnDefining)) {
@@ -578,18 +577,10 @@ static MemoryAccess *getNewDefiningAccessForClone(MemoryAccess *MA,
       if (Instruction *NewDefMUDI =
               cast_or_null<Instruction>(VMap.lookup(DefMUDI))) {
         InsnDefining = MSSA->getMemoryAccess(NewDefMUDI);
-        if (!CloneWasSimplified)
-          assert(InsnDefining && "Defining instruction cannot be nullptr.");
-        else if (!InsnDefining || isa<MemoryUse>(InsnDefining)) {
+        if (!InsnDefining || isa<MemoryUse>(InsnDefining)) {
           // The clone was simplified, it's no longer a MemoryDef, look up.
-          auto DefIt = DefMUD->getDefsIterator();
-          // Since simplified clones only occur in single block cloning, a
-          // previous definition must exist, otherwise NewDefMUDI would not
-          // have been found in VMap.
-          assert(DefIt != MSSA->getBlockDefs(DefMUD->getBlock())->begin() &&
-                 "Previous def must exist");
           InsnDefining = getNewDefiningAccessForClone(
-              &*(--DefIt), VMap, MPhiMap, CloneWasSimplified, MSSA);
+              DefMUD->getDefiningAccess(), VMap, MPhiMap, MSSA);
         }
       }
     }
@@ -624,9 +615,9 @@ void MemorySSAUpdater::cloneUsesAndDefs(BasicBlock *BB, BasicBlock *NewBB,
         MemoryAccess *NewUseOrDef = MSSA->createDefinedAccess(
             NewInsn,
             getNewDefiningAccessForClone(MUD->getDefiningAccess(), VMap,
-                                         MPhiMap, CloneWasSimplified, MSSA),
+                                         MPhiMap, MSSA),
             /*Template=*/CloneWasSimplified ? nullptr : MUD,
-            /*CreationMustSucceed=*/CloneWasSimplified ? false : true);
+            /*CreationMustSucceed=*/false);
         if (NewUseOrDef)
           MSSA->insertIntoListsForBlock(NewUseOrDef, NewBB, MemorySSA::End);
       }

llvm/test/Transforms/SimpleLoopUnswitch/memssa-readnone-access.ll

@@ -0,0 +1,117 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -S -passes="loop-mssa(loop-instsimplify,simple-loop-unswitch<nontrivial>)" < %s | FileCheck %s
+
+@vtable = constant ptr @foo
+
+declare void @foo() memory(none)
+declare void @bar()
+
+; The call becomes known readnone after simplification, but still have a
+; MemoryAccess. Make sure this does not lead to an assertion failure.
+define void @test(i1 %c) {
+; CHECK-LABEL: define void @test(
+; CHECK-SAME: i1 [[C:%.*]]) {
+; CHECK-NEXT:    [[C_FR:%.*]] = freeze i1 [[C]]
+; CHECK-NEXT:    br i1 [[C_FR]], label [[DOTSPLIT_US:%.*]], label [[DOTSPLIT:%.*]]
+; CHECK:       .split.us:
+; CHECK-NEXT:    br label [[LOOP_US:%.*]]
+; CHECK:       loop.us:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    br label [[EXIT_SPLIT_US:%.*]]
+; CHECK:       exit.split.us:
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       .split:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    br label [[LOOP]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+  br label %loop
+
+loop:
+  %fn = load ptr, ptr @vtable, align 8
+  call void %fn()
+  br i1 %c, label %exit, label %loop
+
+exit:
+  ret void
+}
+
+; Variant with another access after the call.
+define void @test2(i1 %c, ptr %p) {
+; CHECK-LABEL: define void @test2(
+; CHECK-SAME: i1 [[C:%.*]], ptr [[P:%.*]]) {
+; CHECK-NEXT:    [[C_FR:%.*]] = freeze i1 [[C]]
+; CHECK-NEXT:    br i1 [[C_FR]], label [[DOTSPLIT_US:%.*]], label [[DOTSPLIT:%.*]]
+; CHECK:       .split.us:
+; CHECK-NEXT:    br label [[LOOP_US:%.*]]
+; CHECK:       loop.us:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    call void @bar()
+; CHECK-NEXT:    br label [[EXIT_SPLIT_US:%.*]]
+; CHECK:       exit.split.us:
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       .split:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    call void @bar()
+; CHECK-NEXT:    br label [[LOOP]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+  br label %loop
+
+loop:
+  %fn = load ptr, ptr @vtable, align 8
+  call void %fn()
+  call void @bar()
+  br i1 %c, label %exit, label %loop
+
+exit:
+  ret void
+}
+
+; Variant with another access after the call and no access before the call.
+define void @test3(i1 %c, ptr %p) {
+; CHECK-LABEL: define void @test3(
+; CHECK-SAME: i1 [[C:%.*]], ptr [[P:%.*]]) {
+; CHECK-NEXT:    [[C_FR:%.*]] = freeze i1 [[C]]
+; CHECK-NEXT:    br i1 [[C_FR]], label [[DOTSPLIT_US:%.*]], label [[DOTSPLIT:%.*]]
+; CHECK:       .split.us:
+; CHECK-NEXT:    br label [[LOOP_US:%.*]]
+; CHECK:       loop.us:
+; CHECK-NEXT:    br label [[SPLIT_US:%.*]]
+; CHECK:       split.us:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    call void @bar()
+; CHECK-NEXT:    br label [[EXIT_SPLIT_US:%.*]]
+; CHECK:       exit.split.us:
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       .split:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    br label [[SPLIT:%.*]]
+; CHECK:       split:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    call void @bar()
+; CHECK-NEXT:    br label [[LOOP]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+  br label %loop
+
+loop:
+  %fn = load ptr, ptr @vtable, align 8
+  br label %split
+
+split:
+  call void %fn()
+  call void @bar()
+  br i1 %c, label %exit, label %loop
+
+exit:
+  ret void
+}