https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117302
Bug ID: 117302
Summary: merge + present generates invalid code
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: gigor-ads at yandex dot ru
Target Milestone: ---
Hi!
With the following code, GCC generates wrong code:
real(8) function test1(a) result(r)
implicit none
real(8), intent(in), optional :: a
r = merge(a, 1.0_8, present(a))
end function test1
With gfortran -O3 it is just
test1_:
movsd xmm0, QWORD PTR [rdi]
ret
So, it does not check that present(a) may be false and it put possible
inaccessible value a into r, then it leads to SegFaults if a was not present.
This code should be semantically as the following:
real(8) function test2(a) result(r)
implicit none
real(8), intent(in), optional :: a
if (present(a)) then
r = a
else
r = 1.0_8
end if
end function test2
that has a proper assembler with check of present(a):
test2_:
movsd xmm0, QWORD PTR .LC0[rip]
test rdi, rdi
je .L3
movsd xmm0, QWORD PTR [rdi]
.L3:
ret
.LC0:
.long 0
.long 1072693248
Just note that in some cases, it would be better to set values only after check
without setting default value (for simple types it is OK, but not for
user-defined).
See an example at godbolt: https://godbolt.org/z/969vr763M
