Re: [linux-audio-dev] Traps in floating point code

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Subject: Re: [linux-audio-dev] Traps in floating point code
From: Tim Goetze (tim_AT_quitte.de)
Date: Thu Jul 01 2004 - 15:41:13 EEST

[Ruben van Royen]
>please note that SSE2 has support for 64bit floats (doubles) and contains an
>instruction that truncates to int, irregardless of controlwords. A new enough
>gcc with (-march=pentium4 or -msse2) and -mfpmath=sse will use sse instead of
>the old fp unit. This has more advantages, since sse math uses normal
>registers instead of the stack in the old fp unit.
>
>The disadvantage is of course that it does not run on older processors. I'm
>also not sure what level of sse athlon currently supports. The last time I
>looked, it only supported sse. This is also good, but it lacks support for
>double precision floatingpoint.

afaik, the athlon XP here only has SSE (not ~2), but the instruction
set includes this (quote taken from the NASM documentation, section
B.4):

CVTTSD2SI reg32,xmm/mem32 ; F3 0F 2C /r [KATMAI,SSE]

CVTTSS2SI converts a single-precision FP value in the source operand
to a signed doubleword in the destination operand. If the result is
inexact, it is truncated (rounded toward zero).

The destination operand is a general purpose register. The source can
be either an XMM register or a 32-bit memory location. If the source
is a register, the input value is in the low doubleword.

-

the operand requirements are quite different from "fistpl" so
replacing one with the other requires some additional instructions
to move the data around.

tim

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