On Wed, Jun 6, 2018 at 3:54 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 6 Jun 2018, Roman Penyaev wrote:
>
>> > Preserving the order of volatile accesses isn't sufficient. The
>> > compiler is still allowed to translate
>> >
>> > r1 = READ_ONCE(x);
>> > if (r1) {
>> > ...
>> > }
>> > WRITE_ONCE(y, r2);
>> >
>> > into something resembling
>> >
>> > r1 = READ_ONCE(x);
>> > WRITE_ONCE(y, r2);
>> > if (r1) {
>> > ...
>> > }
>>
>> Hi Alan,
>>
>> According to the standard C99 Annex C "the controlling expression of
>> a selection statement (if or switch)" are the sequence points, just
>> like a volatile access (READ_ONCE or WRITE_ONCE).
>>
>> "5.1.2.3 Program execution" states:
>> "At certain specified points in the execution sequence called sequence
>> points, all side effects of previous evaluations shall be complete
>> and no side effects of subsequent evaluations shall have taken place."
>>
>> So in the example we have 3 sequence points: "READ_ONCE", "if" and
>> "WRITE_ONCE", which it seems can't be reordered. Am I mistaken
>> interpreting standard? Could you please clarify.
>
> Well, for one thing, we're talking about C11, not C99.
C11 is a n1570, ISO/IEC 9899:2011 ? (according to wiki). Found pdf on
the web contains similar lines, so should not be any differences for
that particular case.
> For another, as far as I understand it, the standard means the program
> should behave _as if_ the side effects are completed in the order
> stated. It doesn't mean that the generated code has to behave that way
> literally.
Then I do not understand what are the differences between "side effects
are completed" and "code generated". Abstract machine state should
provide some guarantees between sequence points, e.g.:
foo(); /* function call */
------------|
*a = 1; |
*b = 12; | Compiler in his right to reorder.
*c = 123; |
------------|
boo(); /* function call */
compiler in his right to reorder memory accesses between foo() and
boo() calls (foo and boo are sequence points, but memory accesses
are not), but:
foo(); /* function call */
*(volatile int *)a = 1;
*(volatile int *)b = 12;
*(volatile int *)c = 123;
boo(); /* function call */
are all sequence points, so compiler can't reorder them.
Where am I mistaken?
> And in particular, the standard is referring to the
> behavior of a single thread, not the interaction between multiple
> concurrent threads.
Yes, that is clear: we are talking about code reordering in one
particular function in a single threaded environment.
--
Roman
