middle-end/100394 - avoid DSE/DCE of pure call that throws
There is -fdelete-dead-exceptions now and we're tracking
nothrow and const/pure bits separately and I do remember that
const or pure does _not_ imply nothrow.
Now, in the light of the PR100382 fix which added a
stmt_unremovable_because_of_non_call_eh_p guard to DSEs "DCE"
I wondered how -fdelete-dead-exceptions applies to calls and
whether stmt_unremovable_because_of_non_call_eh_p doing
really should conditionalize itself on
fun->can_throw_non_call_exceptions. In fact DCE happily elides
pure function calls that throw without a LHS (probably a
consistency bug). The following testcase shows this:
int x, y;
int __attribute__((pure,noinline)) foo () { if (x) throw 1; return y; }
int main()
{
int a[2];
x = 1;
try {
int res = foo ();
a[0] = res;
} catch (...) {
return 0;
}
return 1;
}
note that if you wrap foo () into another noinline
wrap_foo () { foo (); return 1; } function then we need to make
sure to not DCE this call either even though it only throws
externally.
2021-05-03 Richard Biener <rguenther@suse.de>
PR middle-end/100394
* calls.c (expand_call): Preserve possibly throwing calls.
* cfgexpand.c (expand_call_stmt): When a call can throw signal
RTL expansion there are side-effects.
* tree-ssa-dce.c (mark_stmt_if_obviously_necessary): Simplify,
mark all possibly throwing stmts necessary unless we can elide
dead EH.
* tree-ssa-dse.c (pass_dse::execute): Preserve exceptions unless
-fdelete-dead-exceptions.
* tree.h (DECL_PURE_P): Add note about exceptions.