http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52593
Bug #: 52593
Summary: Builtin sqrt on x86 is not correctly rounded
Classification: Unclassified
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: bug...@aerifal.cx
The builtin sqrt() on x86 (i387) should be disabled except with -ffast-math
because it is not correctly rounded. For example, sqrt(0x1.fffffffffffffp-1)
yields 1 instead of 0x1.fffffffffffffp-1. Using -fno-builtin-sqrt will give the
correct value assuming your C library/libm is correctly rounded.
Unfortunately bugs like this seem endemic in gcc. I would really like to see
all dubious builtins and other dubious floating point optimizations disabled
except with -ffast-math until somebody takes the time to rigorously test them
and prove their correctness.
