[Bug middle-end/104800] reodering of potentially trapping operations and volatile stores

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104800

--- Comment #14 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 9 Mar 2022, muecker at gwdg dot de wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104800
> 
> --- Comment #12 from Martin Uecker <muecker at gwdg dot de> ---
> (In reply to Richard Biener from comment #10)
> > Btw, with -ftrapv it would mean we cannot re-order any signed arithmetic
> > with respect to volatile accesses unless we can prove it does not invoke
> > (undefined,
> > but -ftrapv makes it implementation defined) signed overflow.
> 
> 
> Yes, and I think this would be desirable too. For example, if you safetly turn
> off a machine with a volatile store, you want a later logic error in unrelated
> code not to be able to prevent this.

As said, volatile accesses are not considered to alter control flow and
thus "turning off the machine" is not something GCC "allows", the
program has to resume after the volatile store.

For example volatile accesses are also not considered to abnormally
return.  Consider them raising an interrupt, that triggering a signal
and using siglongjmp - GCC is not prepared for that to happen
(so it's not "allowed").

The following might be eventually a catch-all fix (but too aggressive
as noted in the comment).  With it GCC should consider (on the
GIMPLE / GENERIC level ...) all volatile accesses possibly trapping
and thus altering control flow.

diff --git a/gcc/tree-eh.cc b/gcc/tree-eh.cc
index c37a5845343..21179081be9 100644
--- a/gcc/tree-eh.cc
+++ b/gcc/tree-eh.cc
@@ -2662,8 +2662,15 @@ tree_could_trap_p (tree expr)
     return false;

   code = TREE_CODE (expr);
-  t = TREE_TYPE (expr);
+  /* Volatile accesses need to be considered as altering control flow
+     if they are for example device I/O.
+     ???  We can probably exclude automatic variables and accesses that
+     are known to not map to device memory here.  */
+  if (TREE_CODE_CLASS (code) == tcc_reference
+      && TREE_THIS_VOLATILE (expr))
+    return true;

+  t = TREE_TYPE (expr);
   if (t)
     {
       if (COMPARISON_CLASS_P (expr))