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Balog Pal
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 Posted: Fri Feb 20, 2004 8:05 pm Post subject: volatile vs. assignment |
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on clcm I read a claim that conforming to the standard having a volatile
object, assignment to it will have a lvalue->rvalue conversion at the end
that shall not be discarded. Thus
extern volatile int v;
v=1;
will emit code with a write to the location, *and then a read access* to
that same location. What may lead to unexpected events if v is some memory
mapped i/o register.
A recent post counters it:
- ---------
"Ben Hutchings" <do-not-spam-benh (AT) bwsint (DOT) com> wrote
(on clc++m)
| Quote: | Jack Klein wrote:
snip
Note that under the current C++ standard, assigning to a volatile
object yields an lvalue result. An lvalue in an expression calls for
an lvalue-to-rvalue conversion in the abstract machine, and can
normally be optimized away if the rvalue is not used.
snip
The lvalue-to-rvalue conversion is done in "a context where an rvalue
is expected" (3.10/7), e.g. where the lvalue appears as a sub-
expression, initialiser or return statement which requires an rvalue.
An expression statement is not such a context (6.2/1), so
b = a;
requires lvalue-to-rvalue conversion on a but not on b. However,
c = b = a;
requires lvalue-to-rvalue conversion on both a and b but not c.
- ------------------- |
What is the truth in this question -- is that read mandatory, optional or
forbidden?
Paul
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