i386: Fix __builtin_floor with FE_DOWNWARD rounding direction [PR96793]

i386: Fix __builtin_floor with FE_DOWNWARD rounding direction [PR96793]

x86_expand_floorceil expander uses x86_sse_copysign_to_positive, which
is unable to change the sign from - to +.  When FE_DOWNWARD rounding
direction is in effect, the expanded sequence that involves subtraction
can trigger x - x = -0.0 special rule.  x86_sse_copysign_to_positive
fails to change the sign of the intermediate value, assumed to always
be positive, back to positive.

The patch adds one extra fabs that strips the sign from the intermediate
value when flag_rounding_math is in effect.

2020-12-22  Uroš Bizjak  <ubizjak@gmail.com>

gcc/
	PR target/96793
	* config/i386/i386-expand.c (ix86_expand_floorceil):
	Remove the sign of the intermediate value for flag_rounding_math.
	(ix86_expand_floorceildf_32): Ditto.

gcc/testsuite/
	PR target/96793
	* gcc.target/i386/pr96793.c: New test.