llvm dev - Apr 2014 - [LLVMdev] Why "I = ++I" instead of "++I" in COFFDump.cpp ?

Oops, meant to send this to the mailing list instead of to Reid
privately. (Why cc the mailing list instead of just sending to the
mailing list?)

In article <CACs=tyJ6zaHeiS0eNhBkdcZE--JY4k7yH9_P1yFbGqod6uymMw at
mail.gmail.com>,
    Reid Kleckner <rnk at google.com> writes:
> Looks like a bug.  This can probably be simplified with C++11.
> 
> On Fri, Apr 4, 2014 at 10:39 AM, Robison, Arch <arch.robison at
intel.com>wrote:
> 
> >   for (; I != E; I = ++I) {
> >       ...
> >   }
> >
> > Is there a reason for writing "I = ++I" instead of
"++I" ?
I don't see how it's a bug because functionality isn't impacted by
this.  It's simply a redundant assignment.

I also don't see what C++11 has to do with this.

    i = i;

has always been legal since the very first implementation of C and has
always been a pointless expression.

Had the code been:

    i = i++;

then we would have had a real problem since i++ returns the old value
of i before it was incremented and the loop would be infinite.
-- 
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<http://tinyurl.com/d3d-pipeline>
     The Computer Graphics Museum <http://ComputerGraphicsMuseum.org>
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On Mon, Apr 07, 2014 at 08:38:58AM -0600, Richard wrote:
> Oops, meant to send this to the mailing list instead of to Reid
> privately. (Why cc the mailing list instead of just sending to the
> mailing list?)
> 
> In article <CACs=tyJ6zaHeiS0eNhBkdcZE--JY4k7yH9_P1yFbGqod6uymMw at
mail.gmail.com>,
>     Reid Kleckner <rnk at google.com> writes:
> 
> > Looks like a bug.  This can probably be simplified with C++11.
> > 
> > On Fri, Apr 4, 2014 at 10:39 AM, Robison, Arch <arch.robison at
intel.com>wrote:
> > 
> > >   for (; I != E; I = ++I) {
> > >       ...
> > >   }
> > >
> > > Is there a reason for writing "I = ++I" instead of
"++I" ?
> 
> I don't see how it's a bug because functionality isn't impacted
by
> this.  It's simply a redundant assignment.
It is a bug as it violates the sequence point rule. One memory location
shall not be modified twice.

Joerg

In article <20140407190957.GA11923 at britannica.bec.de>,
    Joerg Sonnenberger <joerg at britannica.bec.de> writes:
> On Mon, Apr 07, 2014 at 08:38:58AM -0600, Richard wrote:
> > In article <CACs=tyJ6zaHeiS0eNhBkdcZE--JY4k7yH9_P1yFbGqod6uymMw at
mail.gmail.com>,
> >     Reid Kleckner <rnk at google.com> writes:
> > 
> > > Looks like a bug.  This can probably be simplified with C++11.
> > > 
> > > On Fri, Apr 4, 2014 at 10:39 AM, Robison, Arch <arch.robison
at intel.com>wrote:
> > > > Is there a reason for writing "I = ++I" instead of
"++I" ?
> > 
> > I don't see how it's a bug because functionality isn't
impacted by
> > this.  It's simply a redundant assignment.
> 
> It is a bug as it violates the sequence point rule. One memory location
> shall not be modified twice.
Hrm.  Then it's really two bugs?

1) the code violates the sequence point rule.
2) gcc/clang/MSVC didn't complain when compiling this code

If you could point me to the appropriate section of the Nov.  2012 Draft
Standard where I could read about the sequence point rule, I would be very
appreciative as I would like to understand this better.

Thanks!
-- 
"The Direct3D Graphics Pipeline" free book
<http://tinyurl.com/d3d-pipeline>
     The Computer Graphics Museum <http://ComputerGraphicsMuseum.org>
         The Terminals Wiki <http://terminals.classiccmp.org>
  Legalize Adulthood! (my blog) <http://LegalizeAdulthood.wordpress.com>

On Mon, Apr 7, 2014 at 12:09 PM, Joerg Sonnenberger <joerg at
britannica.bec.de
> wrote:
> On Mon, Apr 07, 2014 at 08:38:58AM -0600, Richard wrote:
> > Oops, meant to send this to the mailing list instead of to Reid
> > privately. (Why cc the mailing list instead of just sending to the
> > mailing list?)
> >
> > In article <CACs> tyJ6zaHeiS0eNhBkdcZE--JY4k7yH9_P1yFbGqod6uymMw
at mail.gmail.com>,
> >     Reid Kleckner <rnk at google.com> writes:
> >
> > > Looks like a bug.  This can probably be simplified with C++11.
> > >
> > > On Fri, Apr 4, 2014 at 10:39 AM, Robison, Arch <arch.robison
at intel.com
> >wrote:
> > >
> > > >   for (; I != E; I = ++I) {
> > > >       ...
> > > >   }
> > > >
> > > > Is there a reason for writing "I = ++I" instead of
"++I" ?
> >
> > I don't see how it's a bug because functionality isn't
impacted by
> > this.  It's simply a redundant assignment.
>
> 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.)
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