C++ Logo


Advanced search

Re: [std-proposals] Use of volatile as function argument should not be deprecated.

From: Edward Catmur <ecatmur_at_[hidden]>
Date: Fri, 10 Jun 2022 00:39:48 +0100
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?

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.

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.

Received on 2022-06-09 23:40:01