12 Feb
2004
12 Feb
'04
4:59 p.m.
Tim Goetze wrote:
can anybody help out with gcc inline assembly syntax
applied to sse
registers/memory locations?
for a simplified example, i'm using
float t[4];
...
asm ("movaps %%xmm1, %0" : : "m" (t[0]));
to move 4 packed floats from xmm1 into 't'.
I couldn't get this to fail in practice - though I didn't
try all that hard - unless t isn't on a 16 byte boundary
in which case it segfaults.
In theory however your code is telling the compiler that
array element t[0] is in memory from which the instruction
reads. It should be more like:
asm ("movaps %%xmm1 %0" : "=m" (t) );
which now tells the compiler that the entire array t
is in memory to which the instruction writes. This
*ought* to discourage the optimiser from doing
anything too drastic. (Maybe/AFAIK/IANAL/etc).
Simon Jenkins
(Bristol, UK)