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Re: Reorganize -ffast-math code.
- To: guerby at acm dot org
- Subject: Re: Reorganize -ffast-math code.
- From: Jim Wilson <wilson at cygnus dot com>
- Date: Thu, 8 Mar 2001 12:05:10 -0800
- Cc: gcc at gcc dot gnu dot org
- References: <Pine.LNX.4.31.0103081045390.26305-100000@penguin.transmeta.com>
In article <200103081946.UAA01461@ulmo> you write:
>Don't we have the same problem with IA64? I read the beast spec a
>while ago and seem to remember that some extended precision is still
>there (82 bits or something, can't find where I put the manual...).
There is no such problem with IA-64. IA-64 has all of the necessary
single/double/double-extended FP instructions that x86 is lacking, and hence
does not suffer from this FP rounding/truncation/excess-precision problem.
IA-64 does have 82-bit FP registers, but that is a hardware implementation
detail invisible to people using high level languages like C and Fortran.
One extra bit is for encoding NatVals (Not a Thing Values). These are
non-numeric values obtained by failed speculative loads, and indicate that
the value must be re-read from memory. One extra bit is for correct rounding
of divide/sqrt sequences that use reciprocal approximation plus Newton-Raphson
iteration.
Jim