mpn/ia64/divrem_2.asm do not restore f17 register

[Bill Allombert]

Dear GMP developers,

I am forwarding a bug I already report to the Debian BTS 
(http://bugs.debian.org/708264) with libgmp 5.1.1 on IA64.

It seems the file mpn/ia64/divrem_2.asm do not restore f17 register
which sort of contradict mpn/ia64/README which says that only
f6-f15 f32-f127 are caller-save.

Thus programs calling e.g. mpn_sqrtrem might end up have having
corrupted registers (f16 to f18)

libgmp works fine when rebuild with the file removed. I did not test
libgmp 5.1.2 yet but the file mpn/ia64/divrem_2.asm is identical so it
is likely the bug is still there.

I attach a test case (which uses a lot of double in the hope the compiler
use f17). I tested gcc 4.6, gcc 4.7 and 4.8 (debian build).

The correct output (without call to mpn_sqrtrem)
%gcc gmptest.c -g -O1 -lgmp -o test && ./test
8.53996 1.56883 8.15584 44.3965
10.1088 6.97113 52.5523 -36.2406
3.142 2.718 0.5772 14.13
8.53996 1.56883 8.15584 44.3965
33.3916

The corrupted output (with a call to mpn_sqrtrem)
%gcc gmptest.c -g -O1 -lgmp -o test -DSQRTN && ./test
8.53996 1.56883 8.15584 44.3965
9.84742e-309 5.14474e-309 1.51044e+19 -36.2406
3.142 2.718 0.5772 14.13
8.53996 1.56883 8.15584 44.3965
33.3916

Cheers,
Bill.
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