[Bug ports/2749] powerpc32 does not build --without-fp

["joseph at codesourcery dot com" <sourceware-bugzilla at sourceware dot org>]

------- Additional Comments From joseph at codesourcery dot com  2006-09-15 16:25 -------
Subject: Re:  powerpc32 does not build --without-fp

On Fri, 15 Sep 2006, sjmunroe at us dot ibm dot com wrote:

> ------- Additional Comments From sjmunroe at us dot ibm dot com  2006-09-15 15:11 -------
> Created an attachment (id=1298)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=1298&action=view)
>  --> (http://sourceware.org/bugzilla/attachment.cgi?id=1298&action=view)
> Updated gcc-4.2 patch with fmsub soft-fp implement
> 
> This patch implements updated darwin-ldouble.c to add a soft-fp implementation
> of fmsub plus fixes to the FP_SUB_Q support.

Note that the soft-fp fix must be accepted for glibc mainline before it 
can go in libgcc's copy, as per FSF policies.  It looks OK to me but I 
have no approval rights for glibc's master copy, only for the GCC copy 
(which effectively means for the build infrastructure integration, i.e. 
the one file t-softfp).

For the fmsub implementation, you do FP_PACK_SEMIRAW_Q between the 
subtraction and the truncation.  This is not correct since it can lead to 
double rounding (first the value is rounded to a quad value exactly half 
way between two double values, then it gets rounded to a double value that 
wasn't the closest to the infinite-precision result).  The truncation 
needs to be done on the output of the subtraction with sticky bits still 
in place, not on the rounded result.

I think the bulk of the fmsub implementation should go in a macro in 
op-common.h, that could be used to implement the standard fma and fmaf 
library functions.  It would need to take fs and wc for both the type in 
question and the double width type, and arguments for the three inputs and 
the result, and for whether it's to calculate a*b+c or a*b-c.  Maybe also 
for the type of the long double temporaries you use, but I hope that you 
can avoid the explicit packing and unpacking into quad representation.



-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=2749

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