Hi Andrea, I really appreciate the help and guidance along the way. This was my first contribution and I really appreciate your help. I will definitely stay involved in the project :)
Best, Sammy On Wed, Aug 26, 2026, at 9:25 PM, Andrea Pinski wrote: > On Wed, Aug 26, 2026 at 2:50 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: Include opts.h. > > (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]> > > Now pushed as r17-3685-g39ba948f9b86e1. Thanks for the patch and > revisions while we did the review and this last rebase. > Hope you continue to contribute to GCC too. > > Thanks, > Andrea > > > > --- > > v8: > > - Rebase onto current master. Upstream commit 3a578f6fcb8 changed > > temp_override to take type_identity_t<T> as its second constructor > > argument and moved the type_identity_t definition above the class; > > carry that version over to c-common.h, which is what made v7 fail > > to apply. > > - Include "opts.h" directly rather than by relative path. > > > > v7: > > - Replace bad include of "lto-opts.cc" to "opts.h". > > > > 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 b89a8477e7c..197db508c16 100644 > > --- a/gcc/c-family/c-common.h > > +++ b/gcc/c-family/c-common.h > > @@ -1752,4 +1752,55 @@ namespace selftest { > > } // namespace selftest > > #endif /* #if CHECKING_P */ > > > > +/* 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 > > + > > +/* 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, type_identity_t<T> overrider) > > + : overridden_variable (var), saved_value (var) > > + { > > + overridden_variable = overrider; > > + } > > + ~temp_override() { overridden_variable = saved_value; } > > +}; > > + > > +/* 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..a27dda88e12 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 "opts.h" > > > > /* 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 ab7729c0613..0f989eb1648 100644 > > --- a/gcc/cp/cp-tree.h > > +++ b/gcc/cp/cp-tree.h > > @@ -2266,57 +2266,6 @@ public: > > } > > }; > > > > -/* 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 > > - > > -/* 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, type_identity_t<T> overrider) > > - : overridden_variable (var), saved_value (var) > > - { > > - overridden_variable = overrider; > > - } > > - ~temp_override() { overridden_variable = saved_value; } > > -}; > > - > > -/* 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 d30ff18a1f2..e119e149fb1 100644 > > --- a/gcc/doc/invoke.texi > > +++ b/gcc/doc/invoke.texi > > @@ -8751,17 +8751,25 @@ 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 > > >
