The -Wstrict-aliasing warning was gated on flag_strict_aliasing, so
passing -fno-strict-aliasing silenced the warning even when explicitly
requested with -Wstrict-aliasing. The warning analysis is purely
type-based and does not depend on the optimization being active, so
temporarily enable flag_strict_aliasing around the alias set query
calls to get meaningful results for the warning.
gcc/c-family/ChangeLog:
* c-warn.cc (strict_aliasing_warning): Remove guard on
flag_strict_aliasing. Temporarily enable it via RAII around
alias set queries so the warning fires independently.
gcc/ChangeLog:
* doc/invoke.texi (-Wstrict-aliasing): Document that the
warning now works independently of -fstrict-aliasing.
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]>
---
Thanks for your feedback, I'm still new to contributing here so I
appreciate it.
Changes from v2:
- Added RAII class (auto_override_flag) to save/restore
flag_strict_aliasing
- Filled in ChangeLog descriptions
gcc/c-family/c-warn.cc | 28 +++++++++++++++++--
gcc/doc/invoke.texi | 16 +++++++----
.../c-c++-common/Wstrict-aliasing2-with-fno.c | 12 ++++++++
.../c-c++-common/Wstrict-aliasing3-with-fno.c | 12 ++++++++
4 files changed, 60 insertions(+), 8 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-warn.cc b/gcc/c-family/c-warn.cc
index 1767d2dc090..072c16f9b64 100644
--- a/gcc/c-family/c-warn.cc
+++ b/gcc/c-family/c-warn.cc
@@ -685,6 +685,27 @@ warn_if_unused_value (const_tree exp, location_t locus,
bool quiet)
}
}
+/* RAII guard to temporarily override an integer flag, restoring its
+ original value when the guard goes out of scope. */
+class auto_override_flag
+{
+public:
+ auto_override_flag (int &flag, int new_value)
+ : m_flag (flag), m_saved (flag)
+ {
+ m_flag = new_value;
+ }
+
+ ~auto_override_flag ()
+ {
+ m_flag = m_saved;
+ }
+
+private:
+ int &m_flag;
+ int m_saved;
+};
+
/* Print a warning about casts that might indicate violation of strict
aliasing rules if -Wstrict-aliasing is used and strict aliasing
mode is in effect. LOC is the location of the expression being
@@ -701,8 +722,7 @@ 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 +730,10 @@ 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. */
+ auto_override_flag save (flag_strict_aliasing, 1);
+
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/doc/invoke.texi b/gcc/doc/invoke.texi
index 339d1d2c97a..b361290cee4 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -8674,17 +8674,21 @@ 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
+This option warns about code that might break the strict aliasing rules
+that the compiler uses for optimization when @option{-fstrict-aliasing}
+is active. This option is independent of @option{-fstrict-aliasing};
+it diagnoses code that would violate strict aliasing rules regardless of
+whether the optimization is enabled. 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 when @option{-fstrict-aliasing}
+is active. This option is independent of @option{-fstrict-aliasing};
+it diagnoses code that would violate strict aliasing rules regardless of
+whether the optimization is enabled.
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;
+}
--
2.54.0
