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* [Bug c++/108737] New: Apparent miscompile of infinite loop on gcc trunk in cddce2 pass
@ 2023-02-09 7:44 njs at pobox dot com
2023-02-09 14:04 ` [Bug tree-optimization/108737] [13 Regression] " rguenth at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: njs at pobox dot com @ 2023-02-09 7:44 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108737
Bug ID: 108737
Summary: Apparent miscompile of infinite loop on gcc trunk in
cddce2 pass
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: njs at pobox dot com
Target Milestone: ---
There's a meme going around about strange compilation outputs caused
infinite-loop-related UB, and while discussing it a friend stumbled on this
weird behavior:
https://godbolt.org/z/qfj1jabMW
C++ code is:
extern int foo();
void blah() {
int x = foo();
while (1) {
if(x) foo();
}
}
When compiled with whatever "gcc trunk" means on compiler explorer, and with
-O3 (but not -O2), as C++ (but not C), then gcc reduces this to a single call
to `foo()` followed by an empty infinite loop:
blah():
sub rsp, 8
call foo()
.L2:
jmp .L2
As far as I can tell, there's no UB here -- 'foo' being extern means it might
do anything, and infinite loops with side-effects are well-defined in C++. So
this seems like a straight-up optimizer bug?
Poking through the pass analysis in CE, it looks like the output of "unrolljam
(tree)" is still correct, but then the output of the next pass "cddce2 (tree)"
has deleted everything. Somehow it concludes that the hoisted `if` can be
eliminated? idgi.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/108737] [13 Regression] Apparent miscompile of infinite loop on gcc trunk in cddce2 pass
2023-02-09 7:44 [Bug c++/108737] New: Apparent miscompile of infinite loop on gcc trunk in cddce2 pass njs at pobox dot com
@ 2023-02-09 14:04 ` rguenth at gcc dot gnu.org
2023-02-09 14:32 ` [Bug tree-optimization/108737] [13 Regression] Apparent miscompile of infinite loop on gcc trunk in cddce2 pass since r13-3875 jakub at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-02-09 14:04 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108737
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|Apparent miscompile of |[13 Regression] Apparent
|infinite loop on gcc trunk |miscompile of infinite loop
|in cddce2 pass |on gcc trunk in cddce2 pass
Last reconfirmed| |2023-02-09
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
Target Milestone|--- |13.0
Keywords| |needs-bisection
Component|c++ |tree-optimization
Version|unknown |13.0
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
C++ has the forward progress guarantee, in case 'x' is false there's an endless
loop which is undefined behavior.
Now, the same happens with C and with -fno-finite-loops, so there's something
amiss here.
-funswitch-loops is necessary, but GCC 12 doesn't seem to be affected ...
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/108737] [13 Regression] Apparent miscompile of infinite loop on gcc trunk in cddce2 pass since r13-3875
2023-02-09 7:44 [Bug c++/108737] New: Apparent miscompile of infinite loop on gcc trunk in cddce2 pass njs at pobox dot com
2023-02-09 14:04 ` [Bug tree-optimization/108737] [13 Regression] " rguenth at gcc dot gnu.org
@ 2023-02-09 14:32 ` jakub at gcc dot gnu.org
2023-02-09 14:38 ` rguenth at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: jakub at gcc dot gnu.org @ 2023-02-09 14:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108737
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Priority|P3 |P1
CC| |jakub at gcc dot gnu.org
Summary|[13 Regression] Apparent |[13 Regression] Apparent
|miscompile of infinite loop |miscompile of infinite loop
|on gcc trunk in cddce2 pass |on gcc trunk in cddce2 pass
| |since r13-3875
Keywords|needs-bisection |
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The foo definition could be e.g.
extern "C" void exit (int);
int foo() {
static int cnt;
if (cnt++ == 0)
return 1;
if (cnt > 42)
exit (0);
return 0;
}
Yes, if first invocation of foo returns 0, then it would invoke UB in C++, but
if it returns non-zero and isn't const or pure, it can't be optimized this way.
Started with r13-3875-g9e11ceef165bc074c2fc85b8ddece606b24e710b
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/108737] [13 Regression] Apparent miscompile of infinite loop on gcc trunk in cddce2 pass since r13-3875
2023-02-09 7:44 [Bug c++/108737] New: Apparent miscompile of infinite loop on gcc trunk in cddce2 pass njs at pobox dot com
2023-02-09 14:04 ` [Bug tree-optimization/108737] [13 Regression] " rguenth at gcc dot gnu.org
2023-02-09 14:32 ` [Bug tree-optimization/108737] [13 Regression] Apparent miscompile of infinite loop on gcc trunk in cddce2 pass since r13-3875 jakub at gcc dot gnu.org
@ 2023-02-09 14:38 ` rguenth at gcc dot gnu.org
2023-02-10 9:25 ` rguenth at gcc dot gnu.org
2023-02-13 7:28 ` rguenth at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-02-09 14:38 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108737
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |ASSIGNED
Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot gnu.org
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
I will investigate.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/108737] [13 Regression] Apparent miscompile of infinite loop on gcc trunk in cddce2 pass since r13-3875
2023-02-09 7:44 [Bug c++/108737] New: Apparent miscompile of infinite loop on gcc trunk in cddce2 pass njs at pobox dot com
` (2 preceding siblings ...)
2023-02-09 14:38 ` rguenth at gcc dot gnu.org
@ 2023-02-10 9:25 ` rguenth at gcc dot gnu.org
2023-02-13 7:28 ` rguenth at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-02-10 9:25 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108737
--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think this is another case where control dependences do not work as intended.
Marking useful stmt: foo ();
and we have
<bb 2> [local count: 3508266]:
x_4 = foo ();
if (x_4 != 0)
goto <bb 12>; [33.00%]
else
goto <bb 7>; [67.00%]
<bb 12> [local count: 1157728]:
<bb 3> [local count: 116930483]:
<bb 5> [local count: 116930483]:
foo ();
goto <bb 3>; [100.00%]
and BB5 is control dependent on BB3 (on the edge 3->5), so we mark the
block necessary but since there's nothing in it we do not make its
control dependences necessary.
I think what triggers this latent bug is the "double" forwarder with
the call in the latch block rather than in the header when one tries
this on
extern int foo();
void blah()
{
int x = foo();
if (x)
while (1) foo ();
}
it's probably latent with a GIMPLE testcase.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/108737] [13 Regression] Apparent miscompile of infinite loop on gcc trunk in cddce2 pass since r13-3875
2023-02-09 7:44 [Bug c++/108737] New: Apparent miscompile of infinite loop on gcc trunk in cddce2 pass njs at pobox dot com
` (3 preceding siblings ...)
2023-02-10 9:25 ` rguenth at gcc dot gnu.org
@ 2023-02-13 7:28 ` rguenth at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-02-13 7:28 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108737
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |FIXED
Status|ASSIGNED |RESOLVED
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
I botched the changelog. Fixed with the following, but latent everywhere.
commit 338739645b8e5bf34636d8d4829d7650001ad08c (origin/master, origin/HEAD)
Author: Richard Biener <rguenther@suse.de>
Date: Fri Feb 10 10:28:29 2023 +0100
tree-optimization/106722 - fix CD-DCE edge marking
The following fixes a latent issue when we mark control edges but
end up with marking a block with no stmts necessary. In this case
we fail to mark dependent control edges of that block.
PR tree-optimization/106722
* tree-ssa-dce.cc (mark_last_stmt_necessary): Return
whether we marked a stmt.
(mark_control_dependent_edges_necessary): When
mark_last_stmt_necessary didn't mark any stmt make sure
to mark its control dependent edges.
(propagate_necessity): Likewise.
* gcc.dg/torture/pr108737.c: New testcase.
^ permalink raw reply [flat|nested] 6+ messages in thread
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2023-02-09 7:44 [Bug c++/108737] New: Apparent miscompile of infinite loop on gcc trunk in cddce2 pass njs at pobox dot com
2023-02-09 14:04 ` [Bug tree-optimization/108737] [13 Regression] " rguenth at gcc dot gnu.org
2023-02-09 14:32 ` [Bug tree-optimization/108737] [13 Regression] Apparent miscompile of infinite loop on gcc trunk in cddce2 pass since r13-3875 jakub at gcc dot gnu.org
2023-02-09 14:38 ` rguenth at gcc dot gnu.org
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