[MSSAUpdater] Handle simplified accesses when updating phis (#78272)

This is a followup to #76819. After those changes, we can still run into
an assertion failure for a slight variation of the test case: When
fixing up MemoryPhis, we map the incoming access to the access of the
cloned instruction -- which may now no longer exist.

Fix this by reusing the getNewDefiningAccessForClone() helper, which
will look upwards for a new defining access in that case.

Author: nikic

llvm/lib/Analysis/MemorySSAUpdater.cpp

@@ -692,25 +692,9 @@ void MemorySSAUpdater::updateForClonedLoop(const LoopBlocksRPO &LoopBlocks,
         continue;
 
       // Determine incoming value and add it as incoming from IncBB.
-      if (MemoryUseOrDef *IncMUD = dyn_cast<MemoryUseOrDef>(IncomingAccess)) {
-        if (!MSSA->isLiveOnEntryDef(IncMUD)) {
-          Instruction *IncI = IncMUD->getMemoryInst();
-          assert(IncI && "Found MemoryUseOrDef with no Instruction.");
-          if (Instruction *NewIncI =
-                  cast_or_null<Instruction>(VMap.lookup(IncI))) {
-            IncMUD = MSSA->getMemoryAccess(NewIncI);
-            assert(IncMUD &&
-                   "MemoryUseOrDef cannot be null, all preds processed.");
-          }
-        }
-        NewPhi->addIncoming(IncMUD, IncBB);
-      } else {
-        MemoryPhi *IncPhi = cast<MemoryPhi>(IncomingAccess);
-        if (MemoryAccess *NewDefPhi = MPhiMap.lookup(IncPhi))
-          NewPhi->addIncoming(NewDefPhi, IncBB);
-        else
-          NewPhi->addIncoming(IncPhi, IncBB);
-      }
+      NewPhi->addIncoming(
+          getNewDefiningAccessForClone(IncomingAccess, VMap, MPhiMap, MSSA),
+          IncBB);
     }
     if (auto *SingleAccess = onlySingleValue(NewPhi)) {
       MPhiMap[Phi] = SingleAccess;

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

@@ -115,3 +115,107 @@ split:
 exit:
   ret void
 }
+
+; Variants of the above test with swapped branch destinations.
+
+define void @test1_swapped(i1 %c) {
+; CHECK-LABEL: define void @test1_swapped(
+; CHECK-SAME: i1 [[C:%.*]]) {
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    [[C_FR:%.*]] = freeze i1 [[C]]
+; CHECK-NEXT:    br i1 [[C_FR]], label [[START_SPLIT_US:%.*]], label [[START_SPLIT:%.*]]
+; CHECK:       start.split.us:
+; CHECK-NEXT:    br label [[LOOP_US:%.*]]
+; CHECK:       loop.us:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    br label [[LOOP_US]]
+; CHECK:       start.split:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+start:
+  br label %loop
+
+loop:
+  %fn = load ptr, ptr @vtable, align 8
+  call void %fn()
+  br i1 %c, label %loop, label %exit
+
+exit:
+  ret void
+}
+
+define void @test2_swapped(i1 %c, ptr %p) {
+; CHECK-LABEL: define void @test2_swapped(
+; 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 [[LOOP_US]]
+; CHECK:       .split:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    call void @bar()
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; 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 %loop, label %exit
+
+exit:
+  ret void
+}
+
+define void @test3_swapped(i1 %c, ptr %p) {
+; CHECK-LABEL: define void @test3_swapped(
+; 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 [[LOOP_US]]
+; 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 [[EXIT:%.*]]
+; 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 %loop, label %exit
+
+exit:
+  ret void
+}