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Michael S <already5chosen@yahoo.com> schrieb:On Fri, 13 Sep 2024 21:39:39 -0000 (UTC)
Thomas Koenig <tkoenig@netcologne.de> wrote:
Michael S <already5chosen@yahoo.com> schrieb:>On Fri, 13 Sep 2024 04:12:21 -0700
Tim Rentsch <tr.17687@z991.linuxsc.com> wrote:
Michael S <already5chosen@yahoo.com> writes:>struct {
char x[8]
int y;
} bar;
bar.y = 0; bar.x[8] = 42;
>
IMHO, here behavior should be fully defined by implementation.
And in practice it is. Just not in theory.
Do you mean union rather than struct? And do you mean bar.x[7]
rather than bar.x[8]? Surely no one would expect that storing
into bar.x[8] should be well-defined behavior.
If the code were this
union {
char x[8];
int y;
} bar;
bar.y = 0; bar.x[7] = 42;
and assuming sizeof(int) == 4, what is it that you think should
be defined by the C standard but is not? And the same question
for a struct if that is what you meant.
>
No, I mean struct and I mean 8.
And I mean that a typical implementation-defined behavior would
be bar.y==42 on LE machines and bar.y==42*2**24 on BE machines.
As it actually happens in reality with all production compilers.
Ah, you want to re-introduce Fortran's storage association and
common blocks, but without the type safety. Good idea, that.
That created *really* interesting bugs, and Real Programmers (TM)
have to have something that pays their salaries, right?
SCNR
What I wrote is how all production C compilers work today. So it
will add no new bugs.
Maybe I should be a little bit more precise in why I think this
is an extemely bad idea.
struct {
char x[8]
int y;
} bar;
Assume
bar.y = 1234;
bar.x[i] = 42; // The compiler does not know i
// Do something with bar.y
The compiler should then treat the access to bar.x[i] as if bar.y
was clobbered by the assignment statement, and reload bar.y if
it was kept in a register? That is the semantics you propose.
So, either bar.y is treated as if it was volatile, or hard-to-detect
bugs would appear because, with optimization, the assignment would
sometimes change the value of bar.y and sometimes not.
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