https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126692
Bug ID: 126692
Summary: Fold hypot compared with zero
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Keywords: easyhack, missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ktkachov at gcc dot gnu.org
Target Milestone: ---
hypot never returns less than the larger magnitude of its arguments, and
it returns +Inf whenever either is infinite, so it is zero exactly when
both arguments are zero. The comparison therefore does not need the
call:
hypot (x, y) == 0 -> x == 0 & y == 0
hypot (x, y) != 0 -> x != 0 | y != 0
A nonzero argument cannot underflow the result to zero, since the true
value is at least the smallest subnormal and C requires hypot to compute
it without undue underflow. Dropping the call loses the ERANGE an
overflowing hypot may report, so it needs the right errno flags
int f (double x, double y) { return __builtin_hypot (x, y) == 0.0; }
Generates at -Ofast:
f:
stp x29, x30, [sp, -16]!
mov x29, sp
bl hypot
fcmp d0, #0.0
ldp x29, x30, [sp], 16
cset w0, eq
ret
but could avoid the hypot call altogether
