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.
Differential Revision: https://reviews.llvm.org/D120372
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;
--- /dev/null
+// 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();
+
+ do {
+ __attribute__((objc_precise_lifetime)) id y;
+ y = getFoo();
+
+ id y2; // expected-warning {{variable 'y2' set but not used}}
+ y2 = getFoo();
+ } while(0);
+
+ x = ((void *)0);
+}