Date: Fri, 16 Nov 2007 05:46:15 +1100 From: Peter Jeremy <peterjeremy@optushome.com.au> To: Pete French <petefrench@ticketswitch.com> Cc: stable@freebsd.org, bde@freebsd.org Subject: Re: Float problen running i386 inary on amd64 Message-ID: <20071115184615.GL20992@server.vk2pj.dyndns.org> In-Reply-To: <E1IseEn-000315-Uy@dilbert.ticketswitch.com> References: <20071115062002.GK89746@server.vk2pj.dyndns.org> <E1IseEn-000315-Uy@dilbert.ticketswitch.com>
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--DocE+STaALJfprDB Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable I've Cc'd bde@ because this relates to the FPU initialisation - which he is the expert on. On Thu, Nov 15, 2007 at 12:54:29PM +0000, Pete French wrote: >> On Fri, Nov 02, 2007 at 10:04:48PM +0000, Pete French wrote: >> > int >> > main(int argc, char *argv[]) >> > { >> > if(atof("3.2") =3D=3D atof("3.200")) >> > puts("They are equal"); >> > else >> > puts("They are NOT equal!"); >> > return 0; >> > } >> >> Since the program as defined above does not include any prototype for >> atof(), its return value is assumed to be int. The i386 code for the >> comparison is therefore: > >Sorry, I didn't bother sticking the include lines in when I sent it >to the mailing list as I assumed it would be ovious that you need >to include the prototypes! OK, sorry for the confusion. >Interestingly, if you recode like this: > > double x =3D atof("3.2"); > double y =3D atof("3.200"); > if(x =3D=3D y) > puts("They are equal"); > else > puts("They are NOT equal!"); > >Then the problem goes away! Glancing at the assembly code they both appear= to >be doing the same thing as regards the comparison. The underlying problem is that the amd64 FPU is initialised to 64-bit precision mode, whilst the i386 FPU is initialised to 53-bit precision mode (__INITIAL_FPUCW__ in amd64/include/fpu.h vs __INITIAL_NPXCW__ in i386/include/npx.h). It looks like the FPU is initialised during the machine-dependent CPU initialisation and then inherited by subsequent processes as they are fork()d. The fix is probably to explicitly initialise the FPU for legacy mode processes on the amd64. A work-around would be to call fpsetprec(FP_PD) (see <machine/ieeefp.h>) at the start of main(). --=20 Peter Jeremy Please excuse any delays as the result of my ISP's inability to implement an MTA that is either RFC2821-compliant or matches their claimed behaviour. --DocE+STaALJfprDB Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.4 (FreeBSD) iD8DBQFHPJP3/opHv/APuIcRAorZAJ9mI2AhJwFr1/9d1ZZ1D/s2XhIUzQCgihan X0S1Os/QO9ULgmaUENvE+K4= =aoiE -----END PGP SIGNATURE----- --DocE+STaALJfprDB--
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