What is the proper way to inform g++ that a structure is altered by an asm
__volatile__ statement? Adding "memory" to the clobber list works but is an
overkill because then the compiler discards register copies of everything,
not only copies of fields from the relevant structure.
I have been trying on an x86 Ubuntu 9.04 system running in 32-bit mode,
using g++ 4.3.3. Here is the code snippet:
struct int96 {
unsigned long long lo; // 64-bit
unsigned int hi; // 32-bit
unsigned int padding;
//.... bunch of functions
};
int96 sqr96(unsigned long long a) {
// preparations for loads to xmm registers
unsinged long long alo = (unsigned int) a;
unsigned long long ahi = (a >>.32);
// storage for the result
int96 res;
// compute square of 'a' using SSE2
asm __volatile__ (
" // bunch of SSE statements "
: // HERE several things have been tried but nothing compiled
: "rV" (&alo), "rV" (&ahi), "rV" (&res)
: "%xmm5", "%xmm6", "%xmm7", "memory"
);
// return result
return res;
}
The problem is that removing "memory" from the clobber list gives wrong code
because the compiler stores copies of res.lo and res.hi in other registers
and is not aware that these get obsolete. Adding memory output constraints
at "HERE" did not work. I tried for example these two
i) : "=m" (res)
ii) : "=V" (res)
but these generate errors
int96.h:261: Error: junk `(%ebp))' after expression
int96.h:261: Error: missing ')'
[...]
or
int96.h:261: error: inconsistent operand constraints in an ‘asm’
[...]
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