On 06/18/2013 03:25 AM, Chung-Ju Wu wrote:
2013/6/18 Ian Lance Taylor<iant@xxxxxxxxxx>:
On Mon, Jun 17, 2013 at 10:21 PM, Gene Smith<gds@xxxxxxxxxxxxx> wrote:
Is this difference expected? Should -g cause changes in the actual code
generated and not just add debug symbols to the elf? Possibly it is related
to the optimization level? I have not checked to see if the results differ
with higher or lower levels than -O1.
I have seen several opinions regarding this but no authoritative answer. The
gcc manual also does not really answer this.
If the only change in the command line options is whether you use -g,
you should get exactly the same code. Any difference in code
generation is a bug. Please try to file a bug report according to the
guidelines at http://gcc.gnu.org/bugs/ . Thanks.
Ian
Hi, Gene,
I also suggest to check assembly code sequence by having '-S'
compilation option instead of using objdump to the binary.
So that the influence of assembler/linker can be ruled out.
Best regards,
jasonwucj
Yes, just removing the -g does affect the assembly as seen using -S gcc
flag. However, it appears to be the same difference I see with objdump
on the full elf. Here is a small example of the difference when calling
the same c++ function looking at the -S assembly file output. The
directives differ as expected but there are definitely some added or
different assembly lines between the two. These type of small
differences are throughout all project assembly output files.
w/o -g:
_ZN20MyFunc29guppieSortBoxNumbersEv:
@ args = 0, pretend = 0, frame = 168
@ frame_needed = 0, uses_anonymous_args = 0
push {r4, r5, r6, r7, r8, r9, sl, fp, lr}
sub sp, sp, #196
str r0, [sp, #24]
ldrh r2, [r0, #6]
ldr r3, .L85
umull r1, r3, r3, r2
lsr r3, r3, #2
str r3, [sp, #28]
cmp r3, #9
bhi .L76
mov r3, #0
mov r2, r3
:
:
w/ -g:
_ZN20MyFunc29guppieSortBoxNumbersEv:
.LFB42:
.loc 1 1062 0
.cfi_startproc
@ args = 0, pretend = 0, frame = 176
@ frame_needed = 0, uses_anonymous_args = 0
.LVL63:
push {r4, r5, r6, r7, r8, r9, sl, fp, lr}
.LCFI8:
.cfi_def_cfa_offset 36
sub sp, sp, #204
.LCFI9:
.cfi_def_cfa_offset 240
mov r9, r0
.cfi_offset 14, -4
.cfi_offset 11, -8
.cfi_offset 10, -12
.cfi_offset 9, -16
.cfi_offset 8, -20
.cfi_offset 7, -24
.cfi_offset 6, -28
.cfi_offset 5, -32
.cfi_offset 4, -36
.LBB33:
.loc 1 1063 0
ldrh r2, [r0, #6]
ldr r3, .L82
umull r1, r3, r3, r2
lsr r3, r3, #2
str r3, [sp, #28]
.LVL64:
.loc 1 1065 0
cmp r3, #9
bhi .L76
mov r3, #0
.LBB34:
.loc 1 1075 0
mov r2, r3
:
:
