On Tue, 11 Feb 2020, Niels Möller wrote:
Marco Bodrato <bodr...@mail.dm.unipi.it> writes:
Ciao,
Il 2020-02-10 18:25 Guillaume Melquiond ha scritto:
When the operand sizes do not match, the mpz_cmp function function just
returns the difference of the signed sizes. Unfortunately, this
difference might not fit inside the "int" return type, when the numbers
are of opposite sign.
In mini-gmp we defined a macro:
#define GMP_CMP(a,b) (((a) > (b)) - ((a) < (b)))
We may use the same idea here too. I mean something like the following:
diff -r f5601c2a8b11 mpz/cmp.c
--- a/mpz/cmp.c Sun Feb 09 16:16:19 2020 +0100
+++ b/mpz/cmp.c Tue Feb 11 14:20:39 2020 +0100
@@ -35,15 +35,15 @@
int
mpz_cmp (mpz_srcptr u, mpz_srcptr v) __GMP_NOTHROW
{
- mp_size_t usize, vsize, dsize, asize;
+ mp_size_t usize, vsize, asize;
mp_srcptr up, vp;
int cmp;
usize = SIZ(u);
vsize = SIZ(v);
- dsize = usize - vsize;
- if (dsize != 0)
- return dsize;
+ cmp = (usize > vsize) - (usize < vsize);
+ if (cmp != 0)
+ return cmp;
I would be tempted to keep it simple,
if (usize != vsize)
return (usize > vsize) ? 1 : -1;
It's not clear to me if this is worth micro optimizing, and ensure we
get only a single branch.
On x86_64, both gcc and clang optimize (usize > vsize) ? 1 : -1 to 2 *
(usize > vsize) - 1 (as a single LEA for gcc, 2 ADD for llvm). So the
generated code may be just as good with the simple code.
--
Marc Glisse
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