https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108878
Bug ID: 108878
Summary: Mis-optimization with splitting floating point into a
significand and exponent.
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: kargl at gcc dot gnu.org
Target Milestone: ---
Fortran does not have an equivalent to C's frexp(), which splits a floating
point value into its significand and exponent. Fortran has exponent() and
fraction() to grab the exponent and significand, respectively. Consider,
% cat a.f90
subroutine foo(x, f, m)
real, intent(in) :: x
real, intent(out) :: f
integer, intent(out) :: m
m = exponent(x)
f = fraction(x)
end subroutine
% gfortran -Ofast -fdump-tree-optimized -S a.f90
% more a.f90.254t.optimized
...
<bb 2> [local count: 1073741824]:
_5 = *x_4(D);
__builtin_frexpf (_5, &D.4261);
_1 = D.4261;
*m_7(D) = _1;
D.4261 ={v} {CLOBBER(eol)};
_2 = __builtin_frexpf (_5, &D.4263);
*f_11(D) = _2;
D.4263 ={v} {CLOBBER(eol)};
return;
As can be seen, __builtin_frexpf() is called twice. The generated assembly
also shows two calls to frexpf().
% more a.s
...
.cfi_offset 3, -32
movq %rsi, %rbx
subq $32, %rsp
movss (%rdi), %xmm1
leaq -20(%rbp), %rdi
movaps %xmm1, %xmm0
movss %xmm1, -36(%rbp)
call frexpf
movl -20(%rbp), %eax
movss -36(%rbp), %xmm1
leaq -20(%rbp), %rdi
movl %eax, (%r12)
movaps %xmm1, %xmm0
call frexpf
movss %xmm0, (%rbx)
addq $32, %rsp
popq %rbx
popq %r12
popq %rbp
.cfi_def_cfa 7, 8
...
