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@xxxxxxxxxxxxxxxx )
Ease Software, Inc. ( http://www.easesoftware.com )
Low cost SATA Disk Systems for IBMs p5, pSeries, and RS/6000 AIX systems
