---------- Forwarded message ---------
From: Jason McKesson <jmckesson_at_[hidden]>
Date: Thu, Jun 9, 2022 at 8:32 PM
Subject: Re: [std-proposals] Use of volatile as function argument
should not be deprecated.
To: Edward Catmur <ecatmur_at_[hidden]>


On Thu, Jun 9, 2022 at 7:40 PM Edward Catmur <ecatmur_at_[hidden]> wrote:
>
> On Thu, 9 Jun 2022 at 18:29, Jason McKesson via Std-Proposals <std-proposals_at_[hidden]> wrote:
>>
>> On Thu, Jun 9, 2022 at 11:45 AM Hyman Rosen via Std-Proposals
>> <std-proposals_at_[hidden]> wrote:
>> >
>> > The problem started with code like
>> >
>> > volatile char v[4] = 0;
>> > ++v[2];
>> >
>> > On architectures that do not provide byte-level memory access, the abstract machine cannot be followed exactly. Reading or writing one bye from that array could require reading or writing all four. Rather than trying to come up with wording to cover such situations, the C standard made volatile access implementation-defined, with the understanding that compilers should implement volatile semantics as best they can in weird situations. But once the optimizationists captured the standardization processes, they took that implementation-defined behavior as permission to disregard volatile semantics altogether even on normal platforms.
>> >
>> > Here is an interesting case.
>> >
>> > void foo(int *p) {
>> > if (!p) { printf("null pointer\n"); }
>> > volatile bool b = false;
>> > if (b) { abort(); }
>> > *p = 0;
>> > }
>> >
>> > It's implementing a poor-man's contract check that the argument is not null and trying to print a message if it is (but continue going). With correctly implemented volatile semantics, the message will print because printing is a side effect that must happen before the volatile access which is also a side effect. But the Microsoft compiler elides the volatile variable and test altogether, then sees that if the initial test is true then undefined behavior must result, and eliminates that test and the print.
>>
>> OK, I'm a normal C++ programmer and I read over that code. My first
>> thought will be "why is there a pointless variable here?" The idea
>> that the presence of `if(b)` should change *anything* about the
>> execution of this code is absurd. This only makes sense to those who
>> already know way too much about the language.
>>
>> It would make far more sense if you could just stick some kind of
>> syntax that obviously says, "Follow the abstract machine exactly"
>> there. Like:
>>
>> > void foo(int *p) {
>> > [[exact_code]] {
>> > if (!p) { printf("null pointer\n"); }
>> > *p = 0;
>> > }
>> > }
>>
>> That is readable. It tells you what is happening. It makes it much
>> more clear not just what is happening but why it is there. And the
>> scope applied to the attribute tells you how much of the function it
>> covers.
>
>
> I am not sure what it means to "follow the abstract machine exactly". Could you explain in a manner that could be followed by a compiler?

Not really. That's why it is an attribute. The idea is to say "don't
mess around with this code." What that means is up to compilers and
QOI.

The closest I could say to an actual, standard-defined set of behavior
would be to not have UB propagate backwards in time. That is, if there
is some runtime construct that generates UB, every observable
side-effect that happens-before the UB-invoking construct should
execute as directed, whenever possible.

> As for scope, we really aren't interested in covering more than a single read or write (in the benchmarking case) of an object (which may involve more than one read or write of scalars). Quite possibly this could be accomplished by std::<bikeshed>_load and std::<bikeshed>_store functions acting as an optimizer barrier.

The example that was presented is about the propagation of UB. That
is, because `if(!p)` can only ever be true if the final statement
yields UB. And since compilers are allowed to do *anything* in the
event of UB, if the user passes a NULL `p`, then UB will result.
Therefore, the compiler may freely assume that `p` will never be NULL,
since UB doesn't promise anything about the state of the abstract
machine *before* the UB was triggered. And therefore, it can optimize
out the `if`.

There is no way to make this a property of a variable or of a memory
read. It's a property of the relationship between the `if(!p)` and the
`*p` code. You need to tell the compiler that you want it to actually
do the `if` test and you actually want to see the results if possible,
even if you will get UB in a moment.

> Or semantics could be given to volatile automatic variables if there is indeed a consensus among the people who use them as to what those semantics are; I think it would be sufficient to specify that each volatile qualified automatic object (and function parameter) has a single, consistent (throughout its lifetime) storage location that is distinct from the storage location of every other volatile qualified object (concurrently within its lifetime) and every object of static storage duration; it would follow that reads and writes to that volatile qualified object must result in reads and writes to that storage location.
>
>> I'm not arguing against the desire for this. But spelling it `virtual`
>> is clearly an artifact, a thing people do because it work(ed), not
>> because it makes any kind of obvious sense that it does what it did.
>>
>> Important aspects of the language should not be buried under layers of
>> obfuscation.
>
>
> Yes, `virtual` automatic variables are a bit obscure. But we should consider backwards compatibility, and compatibility with C.