[Transforms/ObjCARC] Fix non-deterministic output of `ObjCARCOptPass`

[akyrtzi]

Summary

ProvenanceAnalysis::related() was assuming that the order of parameters for relatedCheck() was not affecting
the result but this was not the case when both parameters were PHINodes.
Due to this assumption ProvenanceAnalysis::related() was ordering the parameters based on pointer value which resulted in
non-deterministic behavior.

To address this change relatedPHI() so that it gives the same result independent of the parameter order.

rdar://100325456

[ahatanak]

llvm/lib/Transforms/ObjCARC/ProvenanceAnalysis.cpp

80–81

This function returns true if either comparePHISources(PNB, A) or comparePHISources(A, B) returns true. Should it return false if either comparePHISources(PNB, A) or comparePHISources(A, B) returns false instead? I think related returns true when the two pointers are related but also when it doesn't know the answer (i.e., it's being conservative). On the other hand, it returns false only when it's certain that the two pointers aren't related.

[akyrtzi]

llvm/lib/Transforms/ObjCARC/ProvenanceAnalysis.cpp

80–81

To make sure I understand, the semantics we want here is that two PHINodes are only considered related if they both have a source value that is related to the other, and if only one PHINode has a source value related to the other, but not the other PHINode, then they should not be considered related. Is this correct?

[ahatanak]

llvm/lib/Transforms/ObjCARC/ProvenanceAnalysis.cpp

80–81

Yes, that was what I was thinking.

[akyrtzi]

ProvenanceAnalysis::relatedPHI: Return true only if both comparePHISources() calls return true

[akyrtzi]

llvm/lib/Transforms/ObjCARC/ProvenanceAnalysis.cpp

80–81

Thank you for the feedback! 🙇🏻‍♂️ See updated patch.

[akyrtzi]

Fix the patch.

[ahatanak]

LGTM

llvm/test/Transforms/ObjCARC/related-check.ll

110

Instead of running opt twice and comparing the output using diff, you can just check (using FileCheck) that the phi in this block is eliminated and the release below (the second one) is moved.

[akyrtzi]

llvm/test/Transforms/ObjCARC/related-check.ll

110

This test is less about the specific effects of the transformation and more about checking that the behavior of the transformation is deterministic.

[ahatanak]

llvm/test/Transforms/ObjCARC/related-check.ll

110

OK, I see.

llvm/lib/Transforms/ObjCARC/ProvenanceAnalysis.cpp

if (A->getCondition() == SB->getCondition())
              related(A->getFalseValue(), SB->getFalseValue());
 
   // Check both arms of the Select node individually.
   return related(A->getTrueValue(), B) || related(A->getFalseValue(), B);
 }
 
 bool ProvenanceAnalysis::relatedPHI(const PHINode *A,
                                     const Value *B) {
-  // If the values are PHIs in the same block, we can do a more precise as well
-  // as efficient check: just check for relations between the values on
+
+  auto comparePHISources = [this](const PHINode *PNA, const Value *B) -> bool {
+    // Check each unique source of the PHI node against B.
+    SmallPtrSet<const Value *, 4> UniqueSrc;
+    for (Value *PV1 : PNA->incoming_values()) {
+      if (UniqueSrc.insert(PV1).second && related(PV1, B))
+        return true;
+    }
+
+    // All of the arms checked out.
+    return false;
+  };
+
+  if (const PHINode *PNB = dyn_cast<PHINode>(B)) {
+    // If the values are PHIs in the same block, we can do a more precise as
+    // well as efficient check: just check for relations between the values on
     // corresponding edges.
-  if (const PHINode *PNB = dyn_cast<PHINode>(B))
     if (PNB->getParent() == A->getParent()) {
       for (unsigned i = 0, e = A->getNumIncomingValues(); i != e; ++i)
         if (related(A->getIncomingValue(i),
                     PNB->getIncomingValueForBlock(A->getIncomingBlock(i))))
           return true;
       return false;
     }
 
-  // Check each unique source of the PHI node against B.
-  SmallPtrSet<const Value *, 4> UniqueSrc;
-  for (Value *PV1 : A->incoming_values()) {
-    if (UniqueSrc.insert(PV1).second && related(PV1, B))
-      return true;
+    if (!comparePHISources(PNB, A))
+      return false;
   }
 
-  // All of the arms checked out.
-  return false;
+  return comparePHISources(A, B);
 }
 
 /// Test if the value of P, or any value covered by its provenance, is ever
 /// stored within the function (not counting callees).
 static bool IsStoredObjCPointer(const Value *P) {
   SmallPtrSet<const Value *, 8> Visited;
   SmallVector<const Value *, 8> Worklist;
   Worklist.push_back(P);
bool ProvenanceAnalysis::related(const Value *A, const Value *B) {
   // compute the real answer to guard against recursive queries.
   if (A > B) std::swap(A, B);
   std::pair<CachedResultsTy::iterator, bool> Pair =
     CachedResults.insert(std::make_pair(ValuePairTy(A, B), true));
   if (!Pair.second)
     return Pair.first->second;
 
   bool Result = relatedCheck(A, B);
+  assert(relatedCheck(B, A) == Result &&
+         "relatedCheck result depending on order of parameters!");
   CachedResults[ValuePairTy(A, B)] = Result;
   return Result;
 }

llvm/test/Transforms/ObjCARC/related-check.ll

+; Make sure this succeeds without hitting an assertion and the output is deterministic
+; RUN: mkdir -p %t
+; RUN: opt -objc-arc %s -S -o %t/out1.ll
+; RUN: opt -objc-arc %s -S -o %t/out2.ll
+; RUN: diff -u %t/out1.ll %t/out2.ll
+
+%0 = type opaque
+%struct._class_t = type { %struct._class_t*, %struct._class_t*, %struct._objc_cache*, i8* (i8*, i8*)**, %struct._class_ro_t* }
+%struct._objc_cache = type opaque
+%struct._class_ro_t = type { i32, i32, i32, i8*, i8*, %struct.__method_list_t*, %struct._objc_protocol_list*, %struct._ivar_list_t*, i8*, %struct._prop_list_t* }
+%struct.__method_list_t = type { i32, i32, [0 x %struct._objc_method] }
+%struct._objc_method = type { i8*, i8*, i8* }
+%struct._objc_protocol_list = type { i64, [0 x %struct._protocol_t*] }
+%struct._protocol_t = type { i8*, i8*, %struct._objc_protocol_list*, %struct.__method_list_t*, %struct.__method_list_t*, %struct.__method_list_t*, %struct.__method_list_t*, %struct._prop_list_t*, i32, i32, i8**, i8*, %struct._prop_list_t* }
+%struct._ivar_list_t = type { i32, i32, [0 x %struct._ivar_t] }
+%struct._ivar_t = type { i32*, i8*, i8*, i32, i32 }
+%struct._prop_list_t = type { i32, i32, [0 x %struct._prop_t] }
+%struct._prop_t = type { i8*, i8* }
+%struct.__NSConstantString_tag = type { i32*, i32, i8*, i64 }
+
+@.str = private unnamed_addr constant [8 x i8] c"%s: %s\0A\00", align 1
+@OBJC_METH_VAR_NAME_ = private unnamed_addr constant [25 x i8] c"fileSystemRepresentation\00", section "__TEXT,__objc_methname,cstring_literals", align 1
+@OBJC_SELECTOR_REFERENCES_ = internal externally_initialized global i8* getelementptr inbounds ([25 x i8], [25 x i8]* @OBJC_METH_VAR_NAME_, i32 0, i32 0), section "__DATA,__objc_selrefs,literal_pointers,no_dead_strip", align 8
+@"OBJC_CLASS_$_NSString" = external global %struct._class_t
+@"OBJC_CLASSLIST_REFERENCES_$_" = internal global %struct._class_t* @"OBJC_CLASS_$_NSString", section "__DATA,__objc_classrefs,regular,no_dead_strip", align 8
+@__CFConstantStringClassReference = external global [0 x i32]
+@.str.1 = private unnamed_addr constant [3 x i8] c"%@\00", section "__TEXT,__cstring,cstring_literals", align 1
+@_unnamed_cfstring_ = private global %struct.__NSConstantString_tag { i32* getelementptr inbounds ([0 x i32], [0 x i32]* @__CFConstantStringClassReference, i32 0, i32 0), i32 1992, i8* getelementptr inbounds ([3 x i8], [3 x i8]* @.str.1, i32 0, i32 0), i64 2 }, section "__DATA,__cfstring", align 8 #0
+@OBJC_METH_VAR_NAME_.2 = private unnamed_addr constant [18 x i8] c"stringWithFormat:\00", section "__TEXT,__objc_methname,cstring_literals", align 1
+@OBJC_SELECTOR_REFERENCES_.3 = internal externally_initialized global i8* getelementptr inbounds ([18 x i8], [18 x i8]* @OBJC_METH_VAR_NAME_.2, i32 0, i32 0), section "__DATA,__objc_selrefs,literal_pointers,no_dead_strip", align 8
[e-mail] = appending global [5 x i8*] [i8* getelementptr inbounds ([25 x i8], [25 x i8]* @OBJC_METH_VAR_NAME_, i32 0, i32 0), i8* bitcast (i8** @OBJC_SELECTOR_REFERENCES_ to i8*), i8* bitcast (%struct._class_t** @"OBJC_CLASSLIST_REFERENCES_$_" to i8*), i8* getelementptr inbounds ([18 x i8], [18 x i8]* @OBJC_METH_VAR_NAME_.2, i32 0, i32 0), i8* bitcast (i8** @OBJC_SELECTOR_REFERENCES_.3 to i8*)], section "llvm.metadata"
+
+; Function Attrs: optsize ssp uwtable(sync)
+define i32 @main(i32 noundef %argc, i8** nocapture noundef readnone %argv) local_unnamed_addr #1 {
+entry:
+  %persistent = alloca i32, align 4
+  %personalized = alloca i32, align 4
+  %cmp31 = icmp sgt i32 %argc, 1
+  br i1 %cmp31, label %for.body.lr.ph, label %for.cond.cleanup
+
+for.body.lr.ph:                                   ; preds = %entry
+  %0 = bitcast i32* %persistent to i8*
+  %1 = bitcast i32* %personalized to i8*
+  %2 = load i8*, i8** @OBJC_SELECTOR_REFERENCES_, align 8
+  %3 = load i8*, i8** @OBJC_SELECTOR_REFERENCES_.3, align 8
+  br label %for.body
+
+for.cond.cleanup.loopexit:                        ; preds = %if.end19
+  br label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %for.cond.cleanup.loopexit, %entry
+  ret i32 0
+
+for.body:                                         ; preds = %for.body.lr.ph, %if.end19
+  %i.032 = phi i32 [ 1, %for.body.lr.ph ], [ %inc, %if.end19 ]
+  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %0) #4
+  store i32 0, i32* %persistent, align 4
+  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %1) #4
+  store i32 0, i32* %personalized, align 4
+  %call = call zeroext i1 @lookupType(i32* noundef nonnull %persistent, i32* noundef nonnull %personalized) #8, !clang.arc.no_objc_arc_exceptions !15
+  br i1 %call, label %if.then, label %if.end19
+
+if.then:                                          ; preds = %for.body
+  %4 = load i32, i32* %persistent, align 4
+  %cmp1.not = icmp eq i32 %4, 0
+  br i1 %cmp1.not, label %if.end, label %if.then2
+
+if.then2:                                         ; preds = %if.then
+  %call34 = call %0* bitcast (%0* (...)* @getnsstr to %0* ()*)() #8 [ "clang.arc.attachedcall"(i8* (i8*)* @llvm.objc.retainAutoreleasedReturnValue) ], !clang.arc.no_objc_arc_exceptions !15
+  call void (...) @llvm.objc.clang.arc.noop.use(%0* %call34) #4
+  call void @llvm.objc.release(i8* null) #4, !clang.imprecise_release !15
+  %call56 = call %0* bitcast (%0* (...)* @getnsstr to %0* ()*)() #8 [ "clang.arc.attachedcall"(i8* (i8*)* @llvm.objc.retainAutoreleasedReturnValue) ], !clang.arc.no_objc_arc_exceptions !15
+  call void (...) @llvm.objc.clang.arc.noop.use(%0* %call56) #4
+  call void @llvm.objc.release(i8* null) #4, !clang.imprecise_release !15
+  br label %if.end
+
+if.end:                                           ; preds = %if.then2, %if.then
+  %path.0 = phi %0* [ %call34, %if.then2 ], [ null, %if.then ]
+  %name.0 = phi %0* [ %call56, %if.then2 ], [ null, %if.then ]
+  %5 = load i32, i32* %personalized, align 4
+  %cmp7.not = icmp eq i32 %5, 0
+  br i1 %cmp7.not, label %if.end11, label %if.then8
+
+if.then8:                                         ; preds = %if.end
+  %call910 = call %0* bitcast (%0* (...)* @getnsstr to %0* ()*)() #8 [ "clang.arc.attachedcall"(i8* (i8*)* @llvm.objc.retainAutoreleasedReturnValue) ], !clang.arc.no_objc_arc_exceptions !15
+  call void (...) @llvm.objc.clang.arc.noop.use(%0* %call910) #4
+  %6 = bitcast %0* %path.0 to i8*
+  call void @llvm.objc.release(i8* %6) #4, !clang.imprecise_release !15
+  br label %if.end11
+
+if.end11:                                         ; preds = %if.then8, %if.end
+  %path.1 = phi %0* [ %call910, %if.then8 ], [ %path.0, %if.end ]
+  %cmp12.not = icmp eq %0* %path.1, null
+  br i1 %cmp12.not, label %if.else, label %if.then13
+
+if.then13:                                        ; preds = %if.end11
+  %7 = bitcast %0* %path.1 to i8*
+  %call14 = call i8* bitcast (i8* (i8*, i8*, ...)* @objc_msgSend to i8* (i8*, i8*)*)(i8* noundef nonnull %7, i8* noundef %2) #8, !clang.arc.no_objc_arc_exceptions !15
+  %call15 = call i32 (i8*, ...) @printf(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([8 x i8], [8 x i8]* @.str, i64 0, i64 0), i8* noundef %call14) #8, !clang.arc.no_objc_arc_exceptions !15
+  br label %if.end18
+
+if.else:                                          ; preds = %if.end11
+  %8 = load i8*, i8** bitcast (%struct._class_t** @"OBJC_CLASSLIST_REFERENCES_$_" to i8**), align 8
+  %call1617 = call i8* (i8*, i8*, %0*, ...) bitcast (i8* (i8*, i8*, ...)* @objc_msgSend to i8* (i8*, i8*, %0*, ...)*)(i8* noundef %8, i8* noundef %3, %0* noundef nonnull bitcast (%struct.__NSConstantString_tag* @_unnamed_cfstring_ to %0*), %0* noundef null) #8 [ "clang.arc.attachedcall"(i8* (i8*)* @llvm.objc.retainAutoreleasedReturnValue) ], !clang.arc.no_objc_arc_exceptions !15
+  call void (...) @llvm.objc.clang.arc.noop.use(i8* %call1617) #4
+  call void @llvm.objc.release(i8* %call1617) #4, !clang.imprecise_release !15
+  br label %if.end18
+
+if.end18:                                         ; preds = %if.else, %if.then13
+  %.pre-phi = phi i8* [ null, %if.else ], [ %7, %if.then13 ]
+  %9 = bitcast %0* %name.0 to i8*
+  call void @llvm.objc.release(i8* %9) #4, !clang.imprecise_release !15
+  call void @llvm.objc.release(i8* %.pre-phi) #4, !clang.imprecise_release !15
+  br label %if.end19
+
+if.end19:                                         ; preds = %if.end18, %for.body
+  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %1) #4
+  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %0) #4
+  %inc = add nuw nsw i32 %i.032, 1
+  %exitcond.not = icmp eq i32 %inc, %argc
+  br i1 %exitcond.not, label %for.cond.cleanup.loopexit, label %for.body
+}
+
+; Function Attrs: argmemonly mustprogress nocallback nofree nosync nounwind willreturn
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #2
+
+; Function Attrs: argmemonly mustprogress nocallback nofree nosync nounwind willreturn
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture) #2
+
+; Function Attrs: inaccessiblememonly mustprogress nocallback nofree nosync nounwind willreturn
+declare void @llvm.objc.clang.arc.noop.use(...) #5
+
+declare zeroext i1 @lookupType(i32* noundef, i32* noundef) #2
+
+declare %0* @getnsstr(...) #2
+
+; Function Attrs: nounwind
+declare i8* @llvm.objc.retainAutoreleasedReturnValue(i8*) #3
+
+; Function Attrs: nounwind
+declare void @llvm.objc.release(i8*) #3
+
+declare i32 @printf(i8* noundef, ...) #2
+
+; Function Attrs: nonlazybind
+declare i8* @objc_msgSend(i8*, i8*, ...) #4
+
+attributes #0 = { "objc_arc_inert" }
+
+!15 = !{}