Retain the [no_nested_conflict] flag when merging.

The optimization already proves that there are no potential conflicts
between the two merged scopes, so merging them can't introduce a new
nested conflict.

Author: atrick

lib/SILOptimizer/Transforms/AccessEnforcementOpts.cpp

@@ -1089,11 +1089,10 @@ static bool mergeAccesses(
     LLVM_DEBUG(llvm::dbgs()
                << "Merging: " << *childIns << " into " << *parentIns << "\n");
 
-    // Change the no nested conflict of parent:
-    // should be the worst case scenario: we might merge to non-conflicting
-    // scopes to a conflicting one. f the new result does not conflict,
-    // a later on pass will remove the flag
-    parentIns->setNoNestedConflict(false);
+    // Change the no nested conflict of parent if the child has a nested
+    // conflict.
+    if (!childIns->hasNoNestedConflict())
+      parentIns->setNoNestedConflict(false);
 
     // remove end accesses and create new ones that cover bigger scope:
     mergeEndAccesses(parentIns, childIns);

test/SILOptimizer/access_enforcement_opts.sil

@@ -382,7 +382,7 @@ bb0(%0 : $*Int64):
 // CHECK-LABEL: sil @testInoutReadEscapeRead : $@convention(thin) () -> () {
 // CHECK: [[BOX:%.*]] = alloc_box ${ var Int64 }, var, name "x"
 // CHECK: [[BOXADR:%.*]] = project_box [[BOX]] : ${ var Int64 }, 0
-// CHECK: begin_access [read] [static] [[BOXADR]] : $*Int64
+// CHECK: begin_access [read] [static] [no_nested_conflict] [[BOXADR]] : $*Int64
 // CHECK-NOT: begin_access
 // CHECK-LABEL: } // end sil function 'testInoutReadEscapeRead'
 sil @testInoutReadEscapeRead : $@convention(thin) () -> () {
@@ -951,7 +951,7 @@ bb0(%0 : $*Int64, %1 : $@callee_guaranteed () -> ()):
 //
 // CHECK-LABEL: sil @testOldToNewMapRead : $@convention(thin) () -> () {
 // CHECK: [[GLOBAL:%.*]] = global_addr @globalX : $*X
-// CHECK-NEXT: [[BEGIN:%.*]] = begin_access [read] [dynamic] [[GLOBAL]] : $*X
+// CHECK-NEXT: [[BEGIN:%.*]] = begin_access [read] [dynamic] [no_nested_conflict] [[GLOBAL]] : $*X
 // CHECK-NEXT: load [[BEGIN]] : $*X
 // CHECK-NEXT: load [[BEGIN]] : $*X
 // CHECK-NEXT: load [[BEGIN]] : $*X
@@ -981,7 +981,7 @@ bb0:
 //
 // CHECK-LABEL: sil @testOldToNewMapReadMayRelease : $@convention(thin) () -> () {
 // CHECK: [[GLOBAL:%.*]] = global_addr @globalX : $*X
-// CHECK-NEXT: [[BEGIN:%.*]] = begin_access [read] [dynamic] [[GLOBAL]] : $*X
+// CHECK-NEXT: [[BEGIN:%.*]] = begin_access [read] [dynamic] [no_nested_conflict] [[GLOBAL]] : $*X
 // CHECK-NEXT: load [[BEGIN]] : $*X
 // CHECK-NEXT: load [[BEGIN]] : $*X
 // CHECK-NEXT: end_access [[BEGIN]] : $*X
@@ -1293,7 +1293,7 @@ bb4:
 // CHECK-NEXT: load [[BEGIN2]] : $*X
 // CHECK-NEXT: end_access [[BEGIN2]] : $*X
 // CHECK-NEXT: br bb3
-// CHECK: [[BEGIN3:%.*]] = begin_access [read] [dynamic] [[GLOBAL]] : $*X
+// CHECK: [[BEGIN3:%.*]] = begin_access [read] [dynamic] [no_nested_conflict] [[GLOBAL]] : $*X
 // CHECK-NEXT: load [[BEGIN3]] : $*X
 // CHECK: br bb4
 // CHECK: load [[BEGIN3]] : $*X
@@ -1439,7 +1439,7 @@ bb0(%0 : $RefElemNoConflictClass):
 // CHECK-NEXT: load [[BEGIN2]] : $*X
 // CHECK-NEXT: end_access [[BEGIN2]] : $*X
 // CHECK-NEXT: br bb3
-// CHECK: [[BEGIN3:%.*]] = begin_access [read] [dynamic] [[GLOBAL]] : $*X
+// CHECK: [[BEGIN3:%.*]] = begin_access [read] [dynamic] [no_nested_conflict] [[GLOBAL]] : $*X
 // CHECK-NEXT: load [[BEGIN3]] : $*X
 // CHECK: br bb4
 // CHECK: load [[BEGIN3]] : $*X
@@ -1563,7 +1563,7 @@ bb0:
 // CHECK: bb2
 // CHECK: br bb3([[GLOBAL]] : $*X)
 // CHECK: bb3([[GLOBALPHI:%.*]] : $*X):
-// CHECK: [[BEGIN:%.*]] = begin_access [read] [dynamic] [[GLOBALPHI]] : $*X
+// CHECK: [[BEGIN:%.*]] = begin_access [read] [dynamic] [no_nested_conflict] [[GLOBALPHI]] : $*X
 // CHECK-NEXT: load
 // CHECK-NEXT: load
 // CHECK-NEXT: end_access [[BEGIN]] : $*X
@@ -1655,7 +1655,7 @@ bb0(%0 : $TestClass):
 //
 // CHECK-LABEL: sil @testNestedReadMerging : $@convention(thin) () -> () {
 // CHECK: [[GLOBAL:%.*]] = global_addr @globalX : $*X
-// CHECK-NEXT: [[BEGIN:%.*]] = begin_access [read] [dynamic] [[GLOBAL]] : $*X
+// CHECK-NEXT: [[BEGIN:%.*]] = begin_access [read] [dynamic] [no_nested_conflict] [[GLOBAL]] : $*X
 // CHECK-NEXT: load [[BEGIN]] : $*X
 // CHECK-NEXT: load [[BEGIN]] : $*X
 // CHECK-NEXT: [[BEGIN2:%.*]] = begin_access [read] [dynamic] [no_nested_conflict] [[GLOBAL]] : $*X