Re: Function Argument Evaluation

[Perry Smith <pedz at easesoftware dot com>]

On Dec 6, 2006, at 10:21 AM, Young, Michael wrote:

> > > Good morning,
> > >
> > > Historically it's always been that arguments passed into  
> > > functions get
> > > evaluated from left to right
> >
> > Not true.
> >
> > > Ie. 'int foo(int a, char *b, float c) { }
> > >
> > > But I'm noticing that with gcc-4.1.1 (on Fedora Core 5), that  
> > > does not
> > > seem to be the case.  We have some code, granted it may be  
> > > fragile or
> > > broken code, that relies on the arguments being evaluated from  
> > > left to
> > > right and seems like that's not the case.  In my code the arguments
> > > seem
> > > to be being evaluated right to left.
> >
> > I /think/ that with -O0 the arguments are evaluated left to right.
> > However, neither the standards nor gcc make any guarantee about
> > argument evaluation order. Not now and not historically either. Any
> > code that relies on a particular order of evaluation is broken.
> >
> > You've just been lucky (or not, depending on the point of view) up to
> > now. It is typical that such bugs are exposed when the optimization
> > level is changed, which is a good reason to test regularly with  
> > both -
> > O0 and -O2.
>
> True - the standard doesn't say anything about the order in which  
> parameter
> values are passed/pushed on the stack.  (BTW - even ABIs aren't  
> "standards",
> but rather "conventions".)  But it is absolutely necessary to know  
> (and be
> able to explicitly define/control) this order if one is doing inter- 
> language
> or cross-tool development; without knowing the stack frame, how can an
> assembler routine call a routine developed in 'C' or vice versa?   
> Most other
> compilers I'm familiar with allow a declspec or define their call  
> linkage
> (standard, pascal, cdecl, etc.) to specify a particular order at  
> compile-time.
> (Obviously, functions defined as such are not candidates for the  
> optimizations
> to which you alluded.  But correctness trumps speed - getting  
> invalid behavior
> or bad results faster get me nowhere.)  Unfortunately, I looked  
> through the gcc
> documentation for something similar about a month ago, and the only  
> thing I
> could find was an option that controls how stack frames are built -  
> but this
> was an option used when compiling gcc/g++ itself.
>
> If anyone knows of switches that control the argument order in g++,  
> please let
> me know - I'll find the details in the docs (if it's there), but I  
> didn't see
> anything when I looked.

Are you mixing two concepts?  The "order of evaluation" is not the  
same as the
calling convention.

For example, on AIX the calling convention is to have the first  
parameter in
register 3, the second in register 4.  But they can get evaluated in  
any order (and
they never really get pushed on the stack).

Perry Smith ( pedz@easesoftware.com )
Ease Software, Inc. ( http://www.easesoftware.com )

Low cost SATA Disk Systems for IBMs p5, pSeries, and RS/6000 AIX systems