[LLVMdev] Why "I = ++I" instead of "++I" in COFFDump.cpp ?

[Richard]

In article <CAOfiQq=gtrW-qHLLn-5bfmwZ6Qr4wv3mi=NP78AetEOpbTJVqA at mail.gmail.com>,
    Richard Smith <richard at metafoo.co.uk> writes:

> On Mon, Apr 7, 2014 at 12:09 PM, Joerg Sonnenberger <joerg at britannica.bec.de
> > wrote:
> 
> > It is a bug as it violates the sequence point rule. One memory location
> > shall not be modified twice.
> 
> 
> Not in C++11, and (probably) not in C11. The side-effect of pre-increment
> is sequenced before its value computation, and its value computation is
> sequenced before the assignment. (C++ doesn't have sequence points any
> more, and C only has them as a vestigial remnant of the pre-memory-model
> world.)

OK, still looking on how to find this in the standard.

Discussion of postfix ++ and --:

5.2 Postfix expressions
5.2.6 Increment and decrement

	"The value computation of the ++ expression is sequenced before
	the modification of the operand object."

Discussion of prefix ++ and --:

5.3 Unary expressions
5.3.2 Increment and decrement

	...doesn't say anything about sequencing

It seems that with postfix operators the reason the code in 1.9.15
("i = i++ + 1", for example) was yielding undefined behavior was because
although the value computation of postfix++ is sequenced before the
modification of the operand object (5.2.6), the standard doesn't guarantee
anything about the relative sequencing of the postfix++ modification
and the assignment of the computed rvalue to the lvalue.

So a conforming implementation could do either:

    // initially i = 9
    i = i++ + 1;
    //   1. evalute i++ to get 9
    //   2. evalute 1 to get 1
    //   3. add 9 + 1 to get 10
    //   4. assign 10 to i
    //   5. increment i to 11

-or-

    // initially i = 9
    i = i++ + 1;
    //   1. evalute i++ to get 9
    //   2. increment i to 10
    //   3. evalute 1 to get 1
    //   4. add 9 + 1 to get 10
    //   5. assign 10 to i

Am I getting closer?
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