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* [Bug middle-end/106811] New: GENERIC and GIMPLE IL undefined behavior needs documenting
@ 2022-09-02 6:49 rguenth at gcc dot gnu.org
2022-11-02 9:59 ` [Bug middle-end/106811] " nunoplopes at sapo dot pt
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2022-09-02 6:49 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106811
Bug ID: 106811
Summary: GENERIC and GIMPLE IL undefined behavior needs
documenting
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: rguenth at gcc dot gnu.org
Target Milestone: ---
Undefined behavior triggered by the GENERIC operations from tree.def (also used
in GIMPLE) need to be documented. In general we follow C here, including GCCs
implementation defined behavior here, but not all C undefined behavior cases
are exploited by the middle-end so the state of those cases is not clear. We
need
- a list of operations with undefined behavior cases that are currently
exploited or seen as OK to exploit
- a list of operations where C documents undefined behavior cases but the
middle-end conciously defines some implementation defined behavior (which
should not be target specific)
- middle-end predicates that can be used to check
- variants of the operations with (implementation) defined behavior
probably best in generic.texi with cross reference from gimple.texi
Note for RTL operations doing the same is necessary.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/106811] GENERIC and GIMPLE IL undefined behavior needs documenting
2022-09-02 6:49 [Bug middle-end/106811] New: GENERIC and GIMPLE IL undefined behavior needs documenting rguenth at gcc dot gnu.org
@ 2022-11-02 9:59 ` nunoplopes at sapo dot pt
2022-11-05 14:22 ` rguenther at suse dot de
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: nunoplopes at sapo dot pt @ 2022-11-02 9:59 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106811
Nuno Lopes <nunoplopes at sapo dot pt> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |nunoplopes at sapo dot pt
--- Comment #1 from Nuno Lopes <nunoplopes at sapo dot pt> ---
I suggest adopting the concept of poison that LLVM has. It allows operations to
have undefined behavior, while still allow them to be moved freely.
I have some slides that may serve as an introduction to the topic:
https://web.ist.utl.pt/nuno.lopes/pres/ub-vmcai19.pdf
Happy to discuss further.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/106811] GENERIC and GIMPLE IL undefined behavior needs documenting
2022-09-02 6:49 [Bug middle-end/106811] New: GENERIC and GIMPLE IL undefined behavior needs documenting rguenth at gcc dot gnu.org
2022-11-02 9:59 ` [Bug middle-end/106811] " nunoplopes at sapo dot pt
@ 2022-11-05 14:22 ` rguenther at suse dot de
2022-11-07 18:07 ` nunoplopes at sapo dot pt
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: rguenther at suse dot de @ 2022-11-05 14:22 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106811
--- Comment #2 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 2 Nov 2022, nunoplopes at sapo dot pt wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106811
>
> Nuno Lopes <nunoplopes at sapo dot pt> changed:
>
> What |Removed |Added
> ----------------------------------------------------------------------------
> CC| |nunoplopes at sapo dot pt
>
> --- Comment #1 from Nuno Lopes <nunoplopes at sapo dot pt> ---
> I suggest adopting the concept of poison that LLVM has. It allows operations to
> have undefined behavior, while still allow them to be moved freely.
> I have some slides that may serve as an introduction to the topic:
> https://web.ist.utl.pt/nuno.lopes/pres/ub-vmcai19.pdf
>
> Happy to discuss further.
I've seen this before but I don't really see how that manages to avoid
issues when making conditional undefined behavior invoking stmts
unconditional.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/106811] GENERIC and GIMPLE IL undefined behavior needs documenting
2022-09-02 6:49 [Bug middle-end/106811] New: GENERIC and GIMPLE IL undefined behavior needs documenting rguenth at gcc dot gnu.org
2022-11-02 9:59 ` [Bug middle-end/106811] " nunoplopes at sapo dot pt
2022-11-05 14:22 ` rguenther at suse dot de
@ 2022-11-07 18:07 ` nunoplopes at sapo dot pt
2022-11-07 18:17 ` rguenther at suse dot de
2023-09-15 10:22 ` rguenth at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: nunoplopes at sapo dot pt @ 2022-11-07 18:07 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106811
--- Comment #3 from Nuno Lopes <nunoplopes at sapo dot pt> ---
> > I suggest adopting the concept of poison that LLVM has. It allows operations to
> > have undefined behavior, while still allow them to be moved freely.
> > I have some slides that may serve as an introduction to the topic:
> > https://web.ist.utl.pt/nuno.lopes/pres/ub-vmcai19.pdf
> >
> > Happy to discuss further.
>
> I've seen this before but I don't really see how that manages to avoid
> issues when making conditional undefined behavior invoking stmts
> unconditional.
It's like a NaN in the sense that you can invoke e.g. arithmetic operations.
You can do 2 + poison. That's poison.
But you can't do 2 / poison. That's undefined behavior (UB), since poison could
be a zero at run time.
So poison enables free movement of instructions that don't trigger UB. For
instructions that may trigger UB, like division, memory accesses, function
calls, etc, you still need to be careful as usual when moving those around.
But the point is that poison explains the correctness of the optimizations that
gcc is already doing today, like hoisting arithmetic operations. You can't
their semantics be full UB on e.g. signed overflow as otherwise you couldn't
hoist them without proving that overflows can't happen.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/106811] GENERIC and GIMPLE IL undefined behavior needs documenting
2022-09-02 6:49 [Bug middle-end/106811] New: GENERIC and GIMPLE IL undefined behavior needs documenting rguenth at gcc dot gnu.org
` (2 preceding siblings ...)
2022-11-07 18:07 ` nunoplopes at sapo dot pt
@ 2022-11-07 18:17 ` rguenther at suse dot de
2023-09-15 10:22 ` rguenth at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenther at suse dot de @ 2022-11-07 18:17 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106811
--- Comment #4 from rguenther at suse dot de <rguenther at suse dot de> ---
> Am 07.11.2022 um 19:07 schrieb nunoplopes at sapo dot pt <gcc-bugzilla@gcc.gnu.org>:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106811
>
> --- Comment #3 from Nuno Lopes <nunoplopes at sapo dot pt> ---
>>> I suggest adopting the concept of poison that LLVM has. It allows operations to
>>> have undefined behavior, while still allow them to be moved freely.
>>> I have some slides that may serve as an introduction to the topic:
>>> https://web.ist.utl.pt/nuno.lopes/pres/ub-vmcai19.pdf
>>>
>>> Happy to discuss further.
>>
>> I've seen this before but I don't really see how that manages to avoid
>> issues when making conditional undefined behavior invoking stmts
>> unconditional.
>
> It's like a NaN in the sense that you can invoke e.g. arithmetic operations.
> You can do 2 + poison. That's poison.
>
> But you can't do 2 / poison. That's undefined behavior (UB), since poison could
> be a zero at run time.
>
> So poison enables free movement of instructions that don't trigger UB. For
> instructions that may trigger UB, like division, memory accesses, function
> calls, etc, you still need to be careful as usual when moving those around.
>
> But the point is that poison explains the correctness of the optimizations that
> gcc is already doing today, like hoisting arithmetic operations. You can't
> their semantics be full UB on e.g. signed overflow as otherwise you couldn't
> hoist them without proving that overflows can't happen.
The point is they are and we are rewriting them to defined overflow when
hoisting. Or in case of division avoid hoisting (because the way to make them
defined is to make them unsigned)
>
> --
> You are receiving this mail because:
> You reported the bug.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/106811] GENERIC and GIMPLE IL undefined behavior needs documenting
2022-09-02 6:49 [Bug middle-end/106811] New: GENERIC and GIMPLE IL undefined behavior needs documenting rguenth at gcc dot gnu.org
` (3 preceding siblings ...)
2022-11-07 18:17 ` rguenther at suse dot de
@ 2023-09-15 10:22 ` rguenth at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-09-15 10:22 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106811
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |documentation
Ever confirmed|0 |1
Status|UNCONFIRMED |ASSIGNED
Last reconfirmed| |2023-09-15
Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot gnu.org
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Mine.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2023-09-15 10:23 UTC | newest]
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2022-09-02 6:49 [Bug middle-end/106811] New: GENERIC and GIMPLE IL undefined behavior needs documenting rguenth at gcc dot gnu.org
2022-11-02 9:59 ` [Bug middle-end/106811] " nunoplopes at sapo dot pt
2022-11-05 14:22 ` rguenther at suse dot de
2022-11-07 18:07 ` nunoplopes at sapo dot pt
2022-11-07 18:17 ` rguenther at suse dot de
2023-09-15 10:22 ` rguenth at gcc dot gnu.org
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