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SubjectRe: [RFT] Port 0x80 I/O speed
Kyle McMartin wrote:
> On Wed, Dec 12, 2007 at 12:31:18AM +0100, Rene Herman wrote:
>> asm volatile ("rdtsc": "=A" (tsc));
>
> rdtsc returns a 64-bit value in two 32-bit regs, you need to do
>
> inline unsigned long long rdtsc(void)
> {
> unsigned int lo, hi;
> asm volatile ("rdtsc": "=a" (lo), "=d" (hi));
> return (unsigned long long)hi << 32 | lo;
> }
>
> as in msr.h, otherwise you'll only be looking at the value in %rax.
>

"=A" works on 32-bit systems (only), obviously, and gcc will generally
produce slightly better code as a result (gcc could really use a
register renaming/copy propagation step *after* multi-register entities
are broken apart, at least on architectures which don't have register
pairs as a hardware constraint.)

-hpa


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