powerpc targets, long double implementation, and c++ programs

[Edmar Wienskoski]

Joseph S. Myers wrote:

>On Fri, 8 Sep 2006, Edmar Wienskoski wrote:
>
>  
>
>>Ok. I am starting to see the whole picture now.
>>So the whole thing appears to work with --disable-shared, just because the way
>>the linker
>>loads symbols in presence of libgcc_s.so versus libgcc.a.
>>
>>Follow up question:
>>The e500 abi actualy defines long double to be 128bits floats.
>>On rs6000.c, rs6000_init_libfuncs links to __gcc_qadd becasue of
>>TARGET_HARD_FLOAT
>>shouldn't that be TARGET_HARD_FLOAT && TARGET_FPRS
>>and also have:
>>diff -u t-fprules-softfp~ t-fprules-softfp
>>--- t-fprules-softfp~   2006-08-09 14:20:24.000000000 -0500
>>+++ t-fprules-softfp    2006-09-06 12:39:17.000000000 -0500
>>@@ -1,4 +1,4 @@
>>-softfp_float_modes := sf df
>>+softfp_float_modes := sf df tf
>>softfp_int_modes := si di
>>softfp_extensions := sfdf
>>softfp_truncations := dfsf
>>
>>Would that be right ?
>>    
>>
>
>No.
>
>(a) The existing GNU/Linux ABIs use or are intended to use IBM long 
>double, not IEEE long double, and the E500 GNU/Linux ABI should be 
>compatible with the other ABIs in this regard.  The present formal ABI 
>documents are not very relevant to the de facto GNU/Linux ABIs.
>  
>
Well, actually this is part of the problem. We have only one document: 
the  "e500 Sys V ABI",
which was intended to create only one ABI.

>(b) To use IEEE long double with soft-fp you'll need to add sftf dftf to 
>softfp_extensions and tfdf tfsf to softfp_truncations.
>  
>
Humm.

>(c) If using IEEE long double on PowerPC, you should be using the standard 
>_q_* functions defined in the psABI, and not the __*tf* functions at all.  
>glibc does provide the _q_* functions (albeit with a typo meaning _q_utoq 
>is missing), though since they don't get built with -mabi=ieeelongdouble 
>they aren't actually usable.
>  
>
There are 2 issues here:
First, It is libgcc that is generating undefined references to __*tf* functions. If gcc can provide them with "softfp_float_modes := sf df tf", I think is reasonable to do that. For completeness sake you can do as you suggested: change softfp_extensions and softfp_truncations, but they are not absolutely necessary.

Second, is the long double ABI problem. In the past gcc always generated function calls to _q_* functions. (Per ABI Chapter 5)
For this code:
long double foo (long double x, long double y){ return x + y; }
gcc-4.0, target powerpc-eabise and
gcc-4.0, target powerpc-*-linux-gnuspe with -mlong-double-128 option
both generates a call to _q_add.
The same code with gcc-4.2, both targets generates a call to __gcc_qadd.

If there is an intention to change the E500 ABI, then somebody has to step forward and actually change the document (With all the administrative burden that cames with it..).

Edmar