> On Wed, Mar 4, 2026 at 11:05 AM Chris Copeland <[email protected]> wrote:
> >
> > I reported https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124218
> > recently after learning it affects the synchronization interfaces
> > provided by real-time operating systems when using -fwhole-program or
> > -flto. For example, this can leads to moving accesses outside of a
> > section where a FreeRTOS mutex is held. Other ipa passes like
> > ipa-pure-const and ipa-modref handle memory clobbers in their analysis
> > already.
> >
> > I've added a new test that runs on arm-none and will fail without the
> > change to ipa-reference.cc, by having an assembly statement that
> > declares a memory clobber and actually writes a global that the previous
> > version of the analysis would treat as not modified. Please let me know
> > if there's a more appropriate way to test this. Other tests are passing
> > on an aarch64 linux host.
> >
> > PR ipa/124218
> > ---
> > gcc/ChangeLog | 6 ++++++
> > gcc/ipa-reference.cc | 23 +++++++++++++++++++++++
> > gcc/testsuite/gcc.dg/ipa/pr124218.c | 25 +++++++++++++++++++++++++
> > 3 files changed, 54 insertions(+)
> > create mode 100644 gcc/testsuite/gcc.dg/ipa/pr124218.c
> >
> > diff --git a/gcc/ChangeLog b/gcc/ChangeLog
> > index 8f5a62893c1..1be3b678fda 100644
> > --- a/gcc/ChangeLog
> > +++ b/gcc/ChangeLog
> > @@ -1,3 +1,9 @@
> > +2026-03-03 Chris Copeland <[email protected]>
> > +
> > + PR ipa/124218
> > + * ipa-reference.cc (analyze_function): Treat memory clobbers as
> > + reading/writing all_module_statics.
> > +
> > 2026-03-03 H.J. Lu <[email protected]>
> >
> > PR target/124165
> > diff --git a/gcc/ipa-reference.cc b/gcc/ipa-reference.cc
> > index c5699312c8f..0c50b4b2df7 100644
> > --- a/gcc/ipa-reference.cc
> > +++ b/gcc/ipa-reference.cc
> > @@ -42,6 +42,7 @@ along with GCC; see the file COPYING3. If not see
> > #include "backend.h"
> > #include "tree.h"
> > #include "gimple.h"
> > +#include "gimple-iterator.h"
> > #include "tree-pass.h"
> > #include "cgraph.h"
> > #include "data-streamer.h"
> > @@ -550,6 +551,28 @@ analyze_function (struct cgraph_node *fn)
> >
> > if (fn->cannot_return_p ())
> > bitmap_clear (local->statics_written);
> > +
> > + /* If the function body contains any asm statement that clobbers memory,
> > + mark all module statics as read and written. */
>
> An extra IL walk seems excessive for this, I wonder if we (should)
> have recorded such fact
> elsewhere, like in some other IPA summary?
I suppose best fit is fnsummary which holds all kinds of stuff useful
for IPA passes and is always computed.
> > +/* PR ipa/124218: ipa-reference must honor memory clobbers in inline asm.
> > */
> > +
> > +int flag;
> > +
> > +__attribute__ ((noinline))
> > +static void
> > +clobber_and_set (void)
> > +{
> > + __asm__ volatile ("ldr r0, =flag\n\t"
> > + "mov r1, #1\n\t"
> > + "str r1, [r0]"
> > + ::: "r0", "r1", "memory");
At least withtout new Michal's heuristics, for making this safe with LTO
you need used attribute on flag and that disables ipa-reference on it.
Honza
> > +}
> > +
> > +int main (void)
> > +{
> > + flag = 0;
> > + clobber_and_set ();
> > + if (!flag)
> > + __builtin_abort ();
> > + return 0;
> > +}
> > --
> > 2.53.0
> >
