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SubjectRe: [RFT] Port 0x80 I/O speed
On 12-12-07 12:20, linux@horizon.com wrote:

> Here are a variety of machines:

Thanks much for all! Collecting all data now...

> With -O2, the cycle counts come out (before division) as
> out: 0xFFFFFFFFFFEA6F4F
> in: 0xFFFFFFFFFCE68BB6
> I think the "A" constraint doesn't work quite the same in
> 64-bit code. The compiler seems to be using %rdx rather than
> %edx:%eax.

Yes indeed, that tripped me up. Have been using the "=A" locally for a while
for similar timing tests. Will use a manual "=a" (lo), "=d" (hi) I guess for
amd64 compatibility from now on.

If I'd care deeply I'd probably categorize this as a backwards compatibility
bug in GCC though. Things were probably never guaranteed but they certainly
worked that way...

Rene.


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