http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53823
--- Comment #21 from John David Anglin <danglin at gcc dot gnu.org> 2012-08-01
18:44:04 UTC ---
The issue is with the handling of negative constants.
In this bit of code,
max_cost = (set_src_cost (gen_rtx_MULT (mode, fake_reg, op1), speed)
- neg_cost(speed, mode));
if (max_cost > 0
&& choose_mult_variant (mode, -coeff, &algorithm,
&variant, max_cost))
max_cost is computed to be 24 and choose_mult_variant returns 0. This
causes the following hunk to be executed:
max_cost = set_src_cost (gen_rtx_MULT (mode, fake_reg, op1), speed);
if (choose_mult_variant (mode, coeff, &algorithm, &variant, max_cost))
return expand_mult_const (mode, op0, coeff, target,
&algorithm, variant);
max_cost is now 32 and choose_mult_variant succeeds. However,
expand_mult_const
generates incorrect code when coeff is negative.
I hacked the max_cost in the negative case to be 32, and this produces
the correct result and code similar to your cross-compiled output.
If I remember correctly, the xmpyu instruction was introduced in PA 1.1
and this results in different costs for PA 1.0 and 1.1.
