Re: GIMPLE undefined behavior

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Sep 2, 2022 at 2:03 AM Krister Walfridsson
<krister.walfridsson@gmail.com> wrote:
>
> On Thu, 1 Sep 2022, Richard Biener wrote:
>
> > It's generally poorly documented what is considered 'undefined behavior'.
> > We desparately need a section in the internals manual for this.
> > For the {L,R}SHIFT_EXPR case we assume the shift operand is
> > in range of [0, precision - 1], so in theory value-range propagation could
> > infer that b_8(D) < 32 after it "executed".  But it seems that
> > range-on-exit doesn't do that yet.
> [...]
> > The problem with shifts is that there's not a "do it anway, but without
> > undefined behavior" operation to substitute.
>
> I read this as I should not report these as bugs for now. But I'll
> probably keep this as UB in my tool to get an idea of how often this
> happens...

Can you report a single case as kind-of meta-bug for this particular issue?
That would be nice.

>
> >> Calling f(-3, 0x75181005) makes slsr_9 overflow in the optimized code,
> >> even though the original did not overflow. My understanding is that signed
> >> overflow invokes undefined behavior in GIMPLE, so this is a bug in
> >> ifcombine. Is my understanding correct?
> >
> > Yes, the above would be a bug - again value-range propagation might be
> > leveraged to produce a wrong-code testcase.
>
> OK. I'll open bugs for the signed overflow issues the tool finds.

Thanks.

> >> I would appreciate some comments on which non-memory-related operations I
> >> should treat as invoking undefined behavior (memory operations are more
> >> complicated, and I'll be back with more concrete questions later...).
> >
> > The more "interesting" cases are uninitialized values (registers or memory).
>
> Yes, this is the next thing I was planning to implement. :)
>
>
> > In general what we should worry about most is introducing undefined
> > behavior that, when a later pass can assume it doesn't happen, causes
> > wrong code to be generated.  Likewise when we have late instrumentation
> > that would flag such undefined behavior as a user error.
>
> Agreed. But that comes back to the issue of lacking documentation... :(

Yes.  There's some "unknowns" in what GIMPLE treats as undefined given
the lack of a strict specification but there's also implementation facts for
which cases optimization passes already exploit undefined behavior (but even
that subset is not documented).

Richard.

>     /Krister