From: David Laight
> Sent: 07 August 2023 09:40
....
> with (retyped so it may be wrong):
> #define is_constexpr(x) sizeof(*(0 ? (void *)((long)(x) * 0) : (int *)0)) == 1)
Bah, I know why that works and I still got is backwards :-(
Basically the compiler needs to find a type that is 'compatible'
with both the possible results.
Since '(void *)0' is a valid 'int *' value that gives 'int *'.
But '(void *)(anything_else)' requires the 'int *' be converted
to 'void *'.
Also the following seems to compile to sane code
for all types on 32bit and 64bit x86.
int errno;
#define MAXERRNO 0x4000
#define type int *
type sysret(type arg)
{
if ((unsigned long)arg < 0ul - MAXERRNO)
return arg;
errno = -(long)arg;
return (__typeof(arg))-1;
}
You do get a comparison but no sign extensions.
David
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