Re: strict aliasing question

[Howard Chu <hyc at highlandsun dot com>]

Andrew Pinski wrote:
> On Fri, 2006-11-10 at 23:05 -0800, Howard Chu wrote:
>   
> > Daniel Berlin wrote:
> >     
> > > We ask the TBAA analyzer "can a store to a short * touch i.
> > > In this case, it says "no", because it's not legal.
> > >
> > >       
> > If you know the code is not legal, why don't you abort the compilation 
> > with an error code?
> >     
>
> The code is legal but undefined at runtime.

Ah... Now we see why people are so easily confused by the overall issue 
- ask the question and get conflicting answers on what's legal, 
implementation-defined, or undefined.

Back in the gcc 1.x days "#pragma" was implementation-defined, so the 
preprocessor would try to execute hack, rogue, and a few other toys 
whenever it was encountered in source, whichever got located first. 
Eventually someone made the pragmatic decision that gcc should do its 
best to actually do something useful when encountering undefined situations.

>   There was a defect report
> to the C standard about undefined code at runtime and rejecting that
> code and the C standard committee decided it was not a defect.
> http://std.dkuug.dk/JTC1/SC22/WG14/www/docs/dr_109.html
>
> Here is the rational from that Defect report about not rejecting the
> undefined behavior:
> A conforming implementation must not fail to translate a strictly
> conforming program simply because some possible execution of that
> program would result in undefined behavior. Because foo might never be
> called, the example given must be successfully translated by a
> conforming implementation.
>   

What does "successfully translated" mean? Shouldn't "translation" mean 
the source code is translated into object code? Shouldn't that mean it 
should actually generate code that actually executes, and not get 
ignored? Otherwise, "successfully translated" may just as well mean 
"invokes nethack, rogue, larn..." at that instant.

--
 -- Howard Chu
 Chief Architect, Symas Corp.  http://www.symas.com
 Director, Highland Sun        http://highlandsun.com/hyc
 OpenLDAP Core Team            http://www.openldap.org/project/