[MSSA] Don't check non-AA memory effects when using template access

llvm/lib/Analysis/MemorySSA.cpp

@@ -1732,12 +1732,6 @@ MemoryUseOrDef *MemorySSA::createNewAccess(Instruction *I,
     }
   }
 
-  // Using a nonstandard AA pipelines might leave us with unexpected modref
-  // results for I, so add a check to not model instructions that may not read
-  // from or write to memory. This is necessary for correctness.
-  if (!I->mayReadFromMemory() && !I->mayWriteToMemory())
-    return nullptr;
-
   bool Def, Use;
   if (Template) {
     Def = isa<MemoryDef>(Template);
@@ -1757,6 +1751,12 @@ MemoryUseOrDef *MemorySSA::createNewAccess(Instruction *I,
     }
 #endif
   } else {
+    // Using a nonstandard AA pipelines might leave us with unexpected modref
+    // results for I, so add a check to not model instructions that may not read
+    // from or write to memory. This is necessary for correctness.
+    if (!I->mayReadFromMemory() && !I->mayWriteToMemory())
+      return nullptr;
+
     // Find out what affect this instruction has on memory.
     ModRefInfo ModRef = AAP->getModRefInfo(I, std::nullopt);
     // The isOrdered check is used to ensure that volatiles end up as defs

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
+}