On Mon, Aug 10, 2026 at 1:20 PM Sammy Al Hashemi <[email protected]> wrote:
>
> The front-end strict aliasing warning was gated on flag_strict_aliasing,
> so -fno-strict-aliasing silenced it even when -Wstrict-aliasing was
> requested explicitly. The front-end check is type-based and does not
> need the optimization to be active, so honor an explicit -Wstrict-aliasing
> regardless of -fstrict-aliasing.
>
> strict_aliasing_warning temporarily forces flag_strict_aliasing around
> the alias set queries so they return meaningful results, but only when
> OPTION_SET_P (warn_strict_aliasing) is true. When the warning is only
> implied (for example by -Wall), the previous behavior of staying quiet
> under -fno-strict-aliasing is preserved.
>
> This affects only the front-end, type-based diagnostic. The additional
> back-end checking, which uses flow-sensitive points-to information for
> multiple-statement cases, still runs only when optimization is enabled.
>
> temp_override and make_temp_override are moved from cp/cp-tree.h to
> c-family/c-common.h so the front-end warning code can use them;
> in_consteval_if_p_temp_override stays in cp-tree.h as it depends on the
> C++-only saved_scope.
>
> gcc/c-family/ChangeLog:
>
> * c-common.h (class temp_override): Move from cp-tree.h.
> (type_identity_t): Likewise.
> (make_temp_override): Likewise.
> * c-warn.cc (strict_aliasing_warning): Do not gate on
> flag_strict_aliasing. Temporarily enable it around the alias
> set queries when -Wstrict-aliasing was requested explicitly.
>
> gcc/cp/ChangeLog:
>
> * cp-tree.h (class temp_override): Move to c-common.h.
> (type_identity_t): Likewise.
> (make_temp_override): Likewise.
>
> gcc/ChangeLog:
>
> * doc/invoke.texi (-Wstrict-aliasing): Document that the
> front-end type-based checking runs without -fstrict-aliasing
> when the warning is requested explicitly.
> (-Wstrict-aliasing=n): Likewise.
>
> gcc/testsuite/ChangeLog:
>
> * c-c++-common/Wstrict-aliasing2-with-fno.c: New test.
> * c-c++-common/Wstrict-aliasing3-with-fno.c: New test.
>
> Signed-off-by: Sammy Al Hashemi <[email protected]>
> ---
> v6:
> - Only bypass the -fstrict-aliasing gate when -Wstrict-aliasing was
> requested explicitly (OPTION_SET_P (warn_strict_aliasing)); an
> implicit -Wall no longer triggers it under -fno-strict-aliasing.
> - Fix the temp_override to span the whole function (v5 scoped it to
> an if-block, so it had no effect on the alias-set queries).
> - Soften the -Wstrict-aliasing documentation: scope the claim to the
> front-end type-based check and restore the note about the back-end
> flow-sensitive pass that runs under optimization.
> - Also move make_temp_override to c-common.h alongside temp_override;
> drop the now-redundant per-file c-family/c-common.h includes.
> - ChangeLog: use present tense and trailing periods.
>
> gcc/c-family/c-common.h | 51 +++++++++++++++++++
> gcc/c-family/c-warn.cc | 11 +++-
> gcc/cp/cp-tree.h | 51 -------------------
> gcc/doc/invoke.texi | 24 ++++++---
> .../c-c++-common/Wstrict-aliasing2-with-fno.c | 12 +++++
> .../c-c++-common/Wstrict-aliasing3-with-fno.c | 12 +++++
> 6 files changed, 100 insertions(+), 61 deletions(-)
> create mode 100644 gcc/testsuite/c-c++-common/Wstrict-aliasing2-with-fno.c
> create mode 100644 gcc/testsuite/c-c++-common/Wstrict-aliasing3-with-fno.c
>
> diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
> index d9ed2b070f5..82d61dbc575 100644
> --- a/gcc/c-family/c-common.h
> +++ b/gcc/c-family/c-common.h
> @@ -1750,4 +1750,55 @@ namespace selftest {
> } // namespace selftest
> #endif /* #if CHECKING_P */
>
> +/* RAII sentinel that saves the value of a variable, optionally
> + overrides it right away, and restores its value when the sentinel
> + id destructed. */
> +
> +template <typename T>
> +class temp_override
> +{
> + T& overridden_variable;
> + T saved_value;
> +public:
> + temp_override(T& var) : overridden_variable (var), saved_value (var) {}
> + temp_override(T& var, T overrider)
> + : overridden_variable (var), saved_value (var)
> + {
> + overridden_variable = overrider;
> + }
> + ~temp_override() { overridden_variable = saved_value; }
> +};
> +
> +/* Wrapping a template parameter in type_identity_t hides it from template
> + argument deduction. */
> +#if __cpp_lib_type_identity
> +using std::type_identity_t;
> +#else
> +template <typename T>
> +struct type_identity { typedef T type; };
> +template <typename T>
> +using type_identity_t = typename type_identity<T>::type;
> +#endif
> +
> +/* Object generator function for temp_override, so you don't need to write
> the
> + type of the object as a template argument.
> +
> + Use as auto x = make_temp_override (flag); */
> +
> +template <typename T>
> +inline temp_override<T>
> +make_temp_override (T& var)
> +{
> + return { var };
> +}
> +
> +/* Likewise, but use as auto x = make_temp_override (flag, value); */
> +
> +template <typename T>
> +inline temp_override<T>
> +make_temp_override (T& var, type_identity_t<T> overrider)
> +{
> + return { var, overrider };
> +}
> +
> #endif /* ! GCC_C_COMMON_H */
> diff --git a/gcc/c-family/c-warn.cc b/gcc/c-family/c-warn.cc
> index 1767d2dc090..4374e7e4752 100644
> --- a/gcc/c-family/c-warn.cc
> +++ b/gcc/c-family/c-warn.cc
> @@ -41,6 +41,7 @@ along with GCC; see the file COPYING3. If not see
> #include "tree-pretty-print.h"
> #include "langhooks.h"
> #include "gcc-urlifier.h"
> +#include "../lto-opts.cc"
I just noticed this include, this seems wrong.
I think you just need opts.h here.
The rest looks good. I will give others some time (a week or so) to
review this but I think once the include is fixed I will push the
patch.
Thanks,
Andrea
>
> /* Print a warning if a constant expression had overflow in folding.
> Invoke this function on every expression that the language
> @@ -701,8 +702,7 @@ strict_aliasing_warning (location_t loc, tree type, tree
> expr)
> STRIP_NOPS (expr);
> tree otype = TREE_TYPE (expr);
>
> - if (!(flag_strict_aliasing
> - && POINTER_TYPE_P (type)
> + if (!(POINTER_TYPE_P (type)
> && POINTER_TYPE_P (otype)
> && !VOID_TYPE_P (TREE_TYPE (type)))
> /* If the type we are casting to is a ref-all pointer
> @@ -710,6 +710,13 @@ strict_aliasing_warning (location_t loc, tree type, tree
> expr)
> || TYPE_REF_CAN_ALIAS_ALL (type))
> return false;
>
> + /* Temporarily enable strict aliasing so that the alias set query
> + functions return meaningful results for the warning.
> + Only do this if the user explicitly asked for `-Wstrict-aliasing` */
> + temp_override<int> save (flag_strict_aliasing,
> + OPTION_SET_P (warn_strict_aliasing) ?
> + 1 : flag_strict_aliasing);
> +
> if ((warn_strict_aliasing > 1) && TREE_CODE (expr) == ADDR_EXPR
> && (DECL_P (TREE_OPERAND (expr, 0))
> || handled_component_p (TREE_OPERAND (expr, 0))))
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index bf477a67a34..9ad4c327e99 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -2255,57 +2255,6 @@ public:
> }
> };
>
> -/* RAII sentinel that saves the value of a variable, optionally
> - overrides it right away, and restores its value when the sentinel
> - id destructed. */
> -
> -template <typename T>
> -class temp_override
> -{
> - T& overridden_variable;
> - T saved_value;
> -public:
> - temp_override(T& var) : overridden_variable (var), saved_value (var) {}
> - temp_override(T& var, T overrider)
> - : overridden_variable (var), saved_value (var)
> - {
> - overridden_variable = overrider;
> - }
> - ~temp_override() { overridden_variable = saved_value; }
> -};
> -
> -/* Wrapping a template parameter in type_identity_t hides it from template
> - argument deduction. */
> -#if __cpp_lib_type_identity
> -using std::type_identity_t;
> -#else
> -template <typename T>
> -struct type_identity { typedef T type; };
> -template <typename T>
> -using type_identity_t = typename type_identity<T>::type;
> -#endif
> -
> -/* Object generator function for temp_override, so you don't need to write
> the
> - type of the object as a template argument.
> -
> - Use as auto x = make_temp_override (flag); */
> -
> -template <typename T>
> -inline temp_override<T>
> -make_temp_override (T& var)
> -{
> - return { var };
> -}
> -
> -/* Likewise, but use as auto x = make_temp_override (flag, value); */
> -
> -template <typename T>
> -inline temp_override<T>
> -make_temp_override (T& var, type_identity_t<T> overrider)
> -{
> - return { var, overrider };
> -}
> -
> /* temp_override for in_consteval_if_p, which can't use make_temp_override
> because it is a bitfield. */
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 339d1d2c97a..fe18ba24fe6 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -8674,17 +8674,25 @@ the implementation.
> @opindex Wstrict-aliasing
> @opindex Wno-strict-aliasing
> @item -Wstrict-aliasing
> -This option is only active when @option{-fstrict-aliasing} is active.
> -It warns about code that might break the strict aliasing rules that the
> -compiler is using for optimization. The warning does not catch all
> -cases, but does attempt to catch the more common pitfalls. It is
> -included in @option{-Wall}.
> +This option warns about code that might break the strict aliasing rules
> +that the compiler uses for optimization. The type-based checking done
> +in the front end is performed even when @option{-fstrict-aliasing} is
> +not active, provided @option{-Wstrict-aliasing} is requested explicitly;
> +it is not performed for @option{-fno-strict-aliasing -Wall} alone. When
> +optimization is enabled, the warning also runs in the back end, where it
> +handles multiple-statement cases using flow-sensitive points-to
> +information. The warning does not catch all cases, but does attempt to
> +catch the more common pitfalls. It is included in @option{-Wall}.
> It is equivalent to @option{-Wstrict-aliasing=3}.
>
> @item -Wstrict-aliasing=@var{n}
> -This option is only active when @option{-fstrict-aliasing} is active.
> -It warns about code that might break the strict aliasing rules that the
> -compiler is using for optimization.
> +This option warns about code that might break the strict aliasing rules
> +that the compiler uses for optimization. The type-based checking done
> +in the front end is performed even when @option{-fstrict-aliasing} is
> +not active, provided @option{-Wstrict-aliasing} is requested explicitly.
> +When optimization is enabled, the warning also runs in the back end,
> +where it handles multiple-statement cases using flow-sensitive points-to
> +information.
> Higher levels correspond to higher accuracy (fewer false positives).
> Higher levels also correspond to more effort, similar to the way @option{-O}
> works.
> diff --git a/gcc/testsuite/c-c++-common/Wstrict-aliasing2-with-fno.c
> b/gcc/testsuite/c-c++-common/Wstrict-aliasing2-with-fno.c
> new file mode 100644
> index 00000000000..c7b223ed91b
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/Wstrict-aliasing2-with-fno.c
> @@ -0,0 +1,12 @@
> +/* Test the usage of option -Wstrict-aliasing. */
> +/* Make sure it's enabled even when -fno-strict-aliasing. */
> +/* Set -Wstrict-aliasing=2 so it warns on casts */
> +/* { dg-do compile } */
> +/* { dg-options "-Wstrict-aliasing=2 -fno-strict-aliasing" } */
> +
> +int main(int argc, char *argv[])
> +{
> + int x;
> + float *q = (float*) &x; /* { dg-warning "strict-aliasing" } */
> + return x;
> +}
> diff --git a/gcc/testsuite/c-c++-common/Wstrict-aliasing3-with-fno.c
> b/gcc/testsuite/c-c++-common/Wstrict-aliasing3-with-fno.c
> new file mode 100644
> index 00000000000..e999b13aec4
> --- /dev/null
> +++ b/gcc/testsuite/c-c++-common/Wstrict-aliasing3-with-fno.c
> @@ -0,0 +1,12 @@
> +/* Test the usage of option -Wstrict-aliasing. */
> +/* Make sure it's enabled even when -fno-strict-aliasing. */
> +/* Set -Wstrict-aliasing=3 so that it only warns on dereference */
> +/* { dg-do compile } */
> +/* { dg-options "-Wstrict-aliasing=3 -fno-strict-aliasing" } */
> +
> +int main(int argc, char *argv[])
> +{
> + int x;
> + *(float*) &x = 42; /* { dg-warning "strict-aliasing" } */
> + return x;
> +}
> --
> 2.55.0
>
