On 2/5/20 1:19 AM, Richard Biener wrote:
On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <mse...@gmail.com> wrote:
On 2/4/20 2:31 PM, Jeff Law wrote:
On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
On 2/4/20 12:15 PM, Richard Biener wrote:
On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <l...@redhat.com> wrote:
On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <mse...@gmail.com> wrote:
PR 93519 reports a false positive -Wrestrict issued for an inlined
call
to strcpy that carefully guards against self-copying. This is
caused
by the caller's arguments substituted into the call during inlining
and
before dead code elimination.
The attached patch avoids this by removing -Wrestrict from the
folder
and deferring folding perfectly overlapping (and so undefined)
calls
to strcpy (and mempcpy, but not memcpy) until much later. Calls to
perfectly overlapping calls to memcpy are still folded early.
Why do we bother to warn at all for this case? Just DWIM here.
Warnings like
this can be emitted from the analyzer?
They potentially can, but the analyzer is and will almost always
certainly be considerably slower. I would not expect it to be used
nearly as much as the core compiler.
WHether or not a particular warning makes sense in the core compiler or
analyzer would seem to me to depend on whether or not we can reasonably
issue warnings without interprocedural analysis. double-free
realistically requires interprocedural analysis to be effective. I'm
not sure Wrestrict really does.
That is, I suggest to simply remove the bogus warning code from
folding
(and _not_ fail the folding).
I haven't looked at the patch, but if we can get the warning out of the
folder that's certainly preferable. And we could investigate deferring
self-copy removal.
I think the issue is as usual, warning for code we'll later remove as dead.
Warning at folding is almost always premature.
In this instance the code is reachable (or isn't obviously unreachable).
GCC doesn't remove it, but provides benign (and reasonable) semantics
for it(*). To me, that's one aspect of quality. Letting the user know
that the code is buggy is another. I view that as at least as important
as folding the ill-effects away because it makes it possible to fix
the problem so the code works correctly even with compilers that don't
provide these benign semantics.
If you look at the guts of what happens at the point where we issue the
warning from within gimple_fold_builtin_strcpy we have:
DCH_to_char (char * in, char * out, int collid)
{
int type;
char * D.2148;
char * dest;
char * num;
long unsigned int _4;
char * _5;
;; basic block 2, loop depth 0
;; pred: ENTRY
;; succ: 4
;; basic block 4, loop depth 0
;; pred: 2
;; succ: 5
;; basic block 5, loop depth 0
;; pred: 4
;; succ: 6
;; basic block 6, loop depth 0
;; pred: 5
if (0 != 0)
goto <bb 7>; [53.47%]
else
goto <bb 8>; [46.53%]
;; succ: 7
;; 8
;; basic block 7, loop depth 0
;; pred: 6
strcpy (out_1(D), out_1(D));
;; succ: 8
;; basic block 8, loop depth 0
;; pred: 6
;; 7
_4 = __builtin_strlen (out_1(D));
_5 = out_1(D) + _4;
__builtin_memcpy (_5, "foo", 4);
;; succ: 3
;; basic block 3, loop depth 0
;; pred: 8
return;
;; succ: EXIT
}
Which shows the code is obviously unreachable in the case we're warning
about. You can't see this in the dumps because it's exposed by
inlining, then cleaned up before writing the dump file.
In the specific case of the bug the code is of course eliminated
because it's guarded by the if (s != d). I was referring to
the general (unguarded) case of:
char *s = "", *p;
int main (void)
{
p = strcpy (s, s);
puts (p);
}
where GCC folds the assignment 'p = strcpy(s, s);' to effectively
p = s; That's perfectly reasonable but it could equally as well
leave the call alone, as it does when s is null, for instance.
I think folding it away is not only reasonable but preferable to
making the invalid call, but it's done only rarely. Most of
the time GCC does emit the undefined access (it does that with
calls to library functions as well as with direct stores and
reads). (I am hoping we can change that in the future so that
these kinds of problems are handled with some consistency.)
ISTM this would be a case we could handle with the __builtin_warning
stuff.
I think the question is do we want to do anything about it this cycle?
If so, I think Martin's approach is quite reasonable. It disables
folding away the self-copies from gimple-fold and moves the warning
into the expander. So if there's such a call in the IL at expansion
time we get a warning (-O0).
I'd hazard a guess that the diagnostic was added to the strlen pass to
capture the missed warning when we're optimizing and the self-copy has
survived until that point. There's a couple issues that raises though.
First, it's insufficient. DSE (for example) can do self-copy removal,
so it needs the same handling. There may be others places too.
Second, if the code becomes unreachable after strlen, then we've got
new false positive issues.
It's the classic problems we have with all middle end based warnings.
But I could live with those if we can show that using __builtin_warning
to handle this stuff in gcc-11 works... ISTM we emit the
__builtin_warning call before any self-copy like that, whenever we
happen to spot them. They'll naturally get removed if the path becomes
unreachable. We'd warn during expansion for calls to
__builtin_warning. We could even optionally warn when removing a call
to __bulitin_warning.
Thoughts?
The patch has pretty much the same effect as emitting __builtin_warning
from the folder would. It defers the folding until much later, and if
the code isn't eliminated, it issues a warning and folds the call away.
But it affects subsequent optimizations - the call is more expensive
in any size heuristics, it posses an (alias-set zero) memory write
barrier (unless you start to optimize no-op copies in the alias oracle),
it is a _call_ - passes like the vectorizer are not happy about a call.
It prevents SRA of the accessed object, ...
This is a strcpy call copying over itself. It's undefined code,
and so hardly anything that's common or so performance sensitive
to make a noticeable difference.
So no, leaving in the call is _not_ equivalent to sticking in a
__builtin_warning() call (or however we actually implement it,
I'd prefer a stmt in the "debug" category so it's simply ignored
or elided by most passes by means of existing code).
That said, I'd prefer to not do anything about this bug. Iff then
in the inliner try doing a CFG cleanup before folding stmts
(it's doing delayed folding anyway). But not for GCC 10.
One could also mark stmts with no-warning before the inliner
folds them (and then mark back) to avoid those classes of
folding warnings.
I think this would very unfortunate for GCC 10. The user's code
is clearly correct -- they take pains to avoid the overlapping
copy by guarding against it just before it -- and GCC simply emits
an invalid warning because of how it does inlining. All that will
be accomplished by not fixing it is we will release a worse quality
compiler than we otherwise can, unnecessarily eroding our users'
confidence in the value of GCC's diagnostic.
I'll look into your suggestions for the inliner in stage 1 but
please reconsider for GCC 10.
Martin
