GIMPLE undefined behavior

[Krister Walfridsson]

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...


>> 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.


>> 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... :(

    /Krister