mwyman created this revision.
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The `objc_precise_lifetime` attribute is applied to Objective-C pointers to
ensure the optimizer does not prematurely release an object under Automatic
Reference Counting (ARC). It is a common enough pattern to assign values to
these variables but not reference them otherwise, and annotating them with
`__unused` is not really correct as they //are// being used to ensure an
object's lifetime.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D120372
Files:
clang/lib/Sema/SemaDecl.cpp
clang/test/SemaObjC/objc-precise-lifetime-unused-variable.m
Index: clang/test/SemaObjC/objc-precise-lifetime-unused-variable.m
===================================================================
--- /dev/null
+++ clang/test/SemaObjC/objc-precise-lifetime-unused-variable.m
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -triple x86_64-apple-macos11 -fsyntax-only -fobjc-arc
-fblocks -verify -Wunused-but-set-variable -Wno-objc-root-class %s
+
+id getFoo(void);
+
+void test() {
+ // no diagnostics for objects with precise lifetime semantics.
+ __attribute__((objc_precise_lifetime)) id x;
+ x = getFoo();
+
+ id x2; // expected-warning {{variable 'x2' set but not used}}
+ x2 = getFoo();
+}
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -2008,6 +2008,12 @@
if (VD->hasAttr<BlocksAttr>() && Ty->isObjCObjectPointerType())
return;
+ // Don't warn about Objective-C pointer variables with precise lifetime
+ // semantics; they can be used to ensure ARC releases the object at a known
+ // time, which may mean assignment but no other references.
+ if (VD->hasAttr<ObjCPreciseLifetimeAttr>() && Ty->isObjCObjectPointerType())
+ return;
+
auto iter = RefsMinusAssignments.find(VD);
if (iter == RefsMinusAssignments.end())
return;
Index: clang/test/SemaObjC/objc-precise-lifetime-unused-variable.m
===================================================================
--- /dev/null
+++ clang/test/SemaObjC/objc-precise-lifetime-unused-variable.m
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -triple x86_64-apple-macos11 -fsyntax-only -fobjc-arc -fblocks -verify -Wunused-but-set-variable -Wno-objc-root-class %s
+
+id getFoo(void);
+
+void test() {
+ // no diagnostics for objects with precise lifetime semantics.
+ __attribute__((objc_precise_lifetime)) id x;
+ x = getFoo();
+
+ id x2; // expected-warning {{variable 'x2' set but not used}}
+ x2 = getFoo();
+}
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -2008,6 +2008,12 @@
if (VD->hasAttr<BlocksAttr>() && Ty->isObjCObjectPointerType())
return;
+ // Don't warn about Objective-C pointer variables with precise lifetime
+ // semantics; they can be used to ensure ARC releases the object at a known
+ // time, which may mean assignment but no other references.
+ if (VD->hasAttr<ObjCPreciseLifetimeAttr>() && Ty->isObjCObjectPointerType())
+ return;
+
auto iter = RefsMinusAssignments.find(VD);
if (iter == RefsMinusAssignments.end())
return;
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