Leonitis wrote:
> [working with ARM XScale processor and GCC cross-compiler]
>
> I am trying to write an inline function that does a 32-bit by 32-bit
> multiply (32x32->64).
> I want to save the result into a long long type. I also want to use the
> SMULL instruction followed by a STRD instruction.
>
> SMULL: Returns 64-bit product of 32x32 multiplication in two registers
> STRD: Stores contents of two consecutive registers into two consecutive
> memory slots
>
> So far I have:
> static inline void MULT32_32_64(volatile long long* result, int a, int b)
> {
> asm volatile("SMULL r2, r3, %[a], %[b]\n\t"
> "STRD r2, %[result]\n\t"
> : [res] "=m" (*result)
> : [a] "r" (a), [b] "r" (b)
> : "r2", "r3", "memory" );
> return;
> }
>
> Here's what's strange: it works correctly for all optimization levels except
> -O2 (so it works for -O3). With -O2, the 64-bit output gets some very
> strange value (146406845186048), regardless of input.
Please send a test case that displays the problem.
The test you just sent doesn't even compile for me.
This does:
#include <stdio.h>
static inline void MULT32_32_64(volatile long long* result, int a, int b)
{
asm volatile("SMULL r2, r3, %[a], %[b]\n\t"
"STRD r2, %[res]\n\t"
: [res] "=m" (*result)
: [a] "r" (a), [b] "r" (b)
: "r2", "r3", "memory" );
return;
}
long long test (int a, int b)
{
long long result;
MULT32_32_64(&result, a, b);
return result;
}
void main()
{
printf ("%llx\n", test (0x7fffffff, 0x7fffffff));
printf ("%llx\n", (0x7fffffffLL * 0x7fffffffLL));
}
Andrew.
