https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99816
Bug ID: 99816
Summary: fortran do loop line table issue
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: fortran
Assignee: unassigned at gcc dot gnu.org
Reporter: andrew.burgess at embecosm dot com
Target Milestone: ---
Consider this Fortran program:
subroutine sub(a,n)
dimension a(n)
do 100 i = 1, n
a(i) = i
100 continue ! Breakpoint here.
return
end subroutine sub
program test
dimension a(10)
write(*,*) 'This is a test.'
call sub(a,10)
write(*,*) a
end program test
The line marked 'Breakpoint here' is line #5. I compile the test as:
gfortran -g3 -O0 -o bug.x bug.f90
The run this GDB session:
$ gdb bug.x
(gdb) b 5
Breakpoint 1 at 0x4011de: file bug.f90, line 5.
(gdb) r
Starting program: /home/andrew/tmp/bug.x
This is a test.
Breakpoint 1, sub (a=..., n=10) at bug.f90:5
5 100 continue ! Breakpoint here.
(gdb) c
Continuing.
1.00000000 2.00000000 3.00000000 4.00000000
5.00000000 6.00000000 7.00000000 8.00000000 9.00000000
10.0000000
[Inferior 1 (process 1170395) exited normally]
Notice that the breakpoint was only hit once, despite line #5 being the last
statement _within_ the loop. My expectation was that this breakpoint would be
hit 10 times.
I then look at the line table (I've truncated the output, but included the
lines we care about):
$ objdump --dwarf=decodedline bug.x
bug.x: file format elf64-x86-64
Contents of the .debug_line section:
CU: ./bug.f90:
File name Line number Starting address View
Stmt
bug.f90 1 0x401176
x
bug.f90 1 0x401183
x
bug.f90 3 0x4011a7
x
bug.f90 3 0x4011b4
bug.f90 4 0x4011c1
x
bug.f90 3 0x4011d8
x
bug.f90 5 0x4011de
x
bug.f90 6 0x4011df
x
bug.f90 7 0x4011e0
x
bug.f90 9 0x4011e3
x
Finally, here's a snippet of the generated assembler code (from objdump -d)
with an annotation of my own:
....
4011b4: 39 55 ec cmp %edx,-0x14(%rbp)
4011b7: 0f 9f c0 setg %al
4011ba: 0f b6 c0 movzbl %al,%eax
4011bd: 85 c0 test %eax,%eax
4011bf: 75 1d jne 4011de <sub_+0x68>
4011c1: 8b 45 ec mov -0x14(%rbp),%eax
4011c4: 48 98 cltq
4011c6: 48 8d 48 ff lea -0x1(%rax),%rcx
4011ca: f3 0f 2a 45 ec cvtsi2ssl -0x14(%rbp),%xmm0
4011cf: 48 8b 45 d8 mov -0x28(%rbp),%rax
4011d3: f3 0f 11 04 88 movss %xmm0,(%rax,%rcx,4)
4011d8: 83 45 ec 01 addl $0x1,-0x14(%rbp)
4011dc: eb d6 jmp 4011b4 <sub_+0x3e>
4011de: 90 nop <----------- Line 5 is
recorded as here!
4011df: 90 nop
4011e0: 5b pop %rbx
4011e1: 5d pop %rbp
4011e2: c3 retq
You can see that line 5 is clearly recorded at a point after the loop, hence
why we only hit the breakpoint once.
I've tested this with gfortran versions 'GNU Fortran (GCC) 9.3.1 20200408 (Red
Hat 9.3.1-2)' (my distro's default version) and also with current HEAD of
master branch (e19afa0645f).
