[C] Silence unreachable -Watomic-access diagnostics #140064
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Accessing the member of a structure or union which is _Atomic-qualified
is undefined behavior in C. We currently diagnose that with a warning
that defaults to an error. In turn, this means we reject a valid
program if the access it not reachable because of the error. e.g.,
if (0)
SomeAtomicStruct.Member = 12; // Was diagnosed
This silences the diagnostic if the member access is not reachable.
|
@llvm/pr-subscribers-clang Author: Aaron Ballman (AaronBallman) ChangesAccessing the member of a structure or union which is _Atomic-qualified is undefined behavior in C. We currently diagnose that with a warning that defaults to an error. In turn, this means we reject a valid program if the access it not reachable because of the error. e.g., if (0) This silences the diagnostic if the member access is not reachable. Full diff: https://github.com/llvm/llvm-project/pull/140064.diff 3 Files Affected:
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 31c517338c21f..f927c438c5d80 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -533,6 +533,19 @@ Improvements to Clang's diagnostics
- A new off-by-default warning ``-Wms-bitfield-padding`` has been added to alert to cases where bit-field
packing may differ under the MS struct ABI (#GH117428).
+- ``-Watomic-access`` no longer fires on unreachable code. e.g.,
+
+ ..code-block:: c
+
+ _Atomic struct S { int a; } s;
+ void func(void) {
+ if (0)
+ s.a = 12; // Previously diagnosed with -Watomic-access, now silenced
+ s.a = 12; // Still diagnosed with -Watomic-access
+ return;
+ s.a = 12; // Previously diagnosed, now silenced
+ }
+
Improvements to Clang's time-trace
----------------------------------
diff --git a/clang/lib/Sema/SemaExprMember.cpp b/clang/lib/Sema/SemaExprMember.cpp
index 053414ff7a1a7..39c162c3b835d 100644
--- a/clang/lib/Sema/SemaExprMember.cpp
+++ b/clang/lib/Sema/SemaExprMember.cpp
@@ -1385,7 +1385,7 @@ static ExprResult LookupMemberExpr(Sema &S, LookupResult &R,
// lvalue. Because this is inherently unsafe as an atomic operation, the
// warning defaults to an error.
if (const auto *ATy = BaseType->getAs<AtomicType>()) {
- S.DiagRuntimeBehavior(OpLoc, nullptr,
+ S.DiagRuntimeBehavior(OpLoc, BaseExpr.get(),
S.PDiag(diag::warn_atomic_member_access));
BaseType = ATy->getValueType().getUnqualifiedType();
BaseExpr = ImplicitCastExpr::Create(
diff --git a/clang/test/Sema/atomic-expr.c b/clang/test/Sema/atomic-expr.c
index 7e5219dd3f14a..96571e3e68c87 100644
--- a/clang/test/Sema/atomic-expr.c
+++ b/clang/test/Sema/atomic-expr.c
@@ -114,6 +114,23 @@ void func_16(void) {
(void)sizeof(xp->val);
(void)sizeof(y.ival);
(void)sizeof(yp->ival);
+
+ // Also, do not diagnose in unreachable code paths.
+ {
+ if (0) {
+ x.val = 12;
+ xp->val = 12;
+ (void)y.ival;
+ (void)yp->ival;
+ }
+
+ return;
+
+ x.val = 12;
+ xp->val = 12;
+ (void)y.ival;
+ (void)yp->ival;
+ }
}
// Ensure that we correctly implement assignment constraints from C2x 6.5.16.1.
|
erichkeane
approved these changes
May 15, 2025
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Accessing the member of a structure or union which is _Atomic-qualified is undefined behavior in C. We currently diagnose that with a warning that defaults to an error. In turn, this means we reject a valid program if the access it not reachable because of the error. e.g.,
if (0)
SomeAtomicStruct.Member = 12; // Was diagnosed
This silences the diagnostic if the member access is not reachable.