* Re: Division by ZERO on GIMPLE?
2013-12-08 12:17 ` Bin.Cheng
@ 2013-12-08 12:47 ` Bin.Cheng
2013-12-08 19:04 ` Ian Lance Taylor
2013-12-09 3:26 ` Jeff Law
2 siblings, 0 replies; 7+ messages in thread
From: Bin.Cheng @ 2013-12-08 12:47 UTC (permalink / raw)
To: Ian Lance Taylor; +Cc: gcc-help, Richard Biener
On Sun, Dec 8, 2013 at 8:16 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> On Sun, Dec 8, 2013 at 9:29 AM, Ian Lance Taylor <iant@google.com> wrote:
>> On Sat, Dec 7, 2013 at 1:48 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>>>
>>> I just found some GIMPLE like below is generated during boostrap gcc
>>> for x86, tree-vect-loop.c
>>>
>>>
>>> <bb 637>:
>>> # group_size_1432 = PHI <1(497), group_size_1017(660)>
>>> # scalar_dest_1287 = PHI <scalar_dest_1228(497), scalar_dest_1012(660)>
>>> ratio_1497 = group_size_1432 / 0;
>>> goto <bb 501>;
>>>
>>> The compilation ends fine, but what is the division by 0?
>>
>> Pretty hard to say without more information. I just tried, and I
>> don't see it myself. I would guess that some optimization split out
>> the ratio == 0 case for some reason.
>
> It disappeared with Richard's checkin r205730. Also I am not sure is
> why the rest of compiler can live with that code.
>
> The configuration option for me is :
> --disable-nls --disable-libmudflap --disable-libada --disable-libssp
> --disable-libquadmath --disable-libgomp --disable-libffi
> --disable-libstdcxx-pch --disable-tls --with-gnu-as --with-gnu-ld ...
> --enable-languages=c,c++
>
And the assembly finally generated is like:
.L4387:
movl 40(%esp), %eax # 6902 *movsi_internal/1 [length = 4]
xorl %ecx, %ecx # 7442 *movsi_xor [length = 2]
<----------------------
xorl %edx, %edx # 7443 *movsi_xor [length = 2]
divl %ecx # 7177 *udivmodsi4_noext [length = 2]
<----------------------
movl %eax, 68(%esp) # 6903 *movsi_internal/2 [length = 4]
jmp .L4474 # 8252 jump [length = 5]
Thanks,
bin
--
Best Regards.
^ permalink raw reply [flat|nested] 7+ messages in thread* Re: Division by ZERO on GIMPLE?
2013-12-08 12:17 ` Bin.Cheng
2013-12-08 12:47 ` Bin.Cheng
@ 2013-12-08 19:04 ` Ian Lance Taylor
2013-12-09 1:57 ` Bin.Cheng
2013-12-09 3:26 ` Jeff Law
2 siblings, 1 reply; 7+ messages in thread
From: Ian Lance Taylor @ 2013-12-08 19:04 UTC (permalink / raw)
To: Bin.Cheng; +Cc: gcc-help, Richard Biener
On Sun, Dec 8, 2013 at 4:16 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> On Sun, Dec 8, 2013 at 9:29 AM, Ian Lance Taylor <iant@google.com> wrote:
>> On Sat, Dec 7, 2013 at 1:48 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>>>
>>> I just found some GIMPLE like below is generated during boostrap gcc
>>> for x86, tree-vect-loop.c
>>>
>>>
>>> <bb 637>:
>>> # group_size_1432 = PHI <1(497), group_size_1017(660)>
>>> # scalar_dest_1287 = PHI <scalar_dest_1228(497), scalar_dest_1012(660)>
>>> ratio_1497 = group_size_1432 / 0;
>>> goto <bb 501>;
>>>
>>> The compilation ends fine, but what is the division by 0?
>>
>> Pretty hard to say without more information. I just tried, and I
>> don't see it myself. I would guess that some optimization split out
>> the ratio == 0 case for some reason.
>
> It disappeared with Richard's checkin r205730. Also I am not sure is
> why the rest of compiler can live with that code.
The compiler doesn't much care if you divide by zero. Even if the
programmer doesn't do such a thing, there are many ways that CSE can
produce an integer division by zero. Code that divides an integer by
zero is valid C/C++ as long as it is never executed. In fact, code
that divides an integer by zero is valid Go even if it does get
executed--Go defines what happens when an integer division by zero
occurs (you get a runtime panic that can be caught). So while I
understand that the code looks odd, there is no reason the compiler
shouldn't be able to handle it.
Ian
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Division by ZERO on GIMPLE?
2013-12-08 19:04 ` Ian Lance Taylor
@ 2013-12-09 1:57 ` Bin.Cheng
0 siblings, 0 replies; 7+ messages in thread
From: Bin.Cheng @ 2013-12-09 1:57 UTC (permalink / raw)
To: Ian Lance Taylor; +Cc: gcc-help, Richard Biener
On Mon, Dec 9, 2013 at 3:03 AM, Ian Lance Taylor <iant@google.com> wrote:
> On Sun, Dec 8, 2013 at 4:16 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>> On Sun, Dec 8, 2013 at 9:29 AM, Ian Lance Taylor <iant@google.com> wrote:
>>> On Sat, Dec 7, 2013 at 1:48 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>>>>
>>>> I just found some GIMPLE like below is generated during boostrap gcc
>>>> for x86, tree-vect-loop.c
>>>>
>>>>
>>>> <bb 637>:
>>>> # group_size_1432 = PHI <1(497), group_size_1017(660)>
>>>> # scalar_dest_1287 = PHI <scalar_dest_1228(497), scalar_dest_1012(660)>
>>>> ratio_1497 = group_size_1432 / 0;
>>>> goto <bb 501>;
>>>>
>>>> The compilation ends fine, but what is the division by 0?
>>>
>>> Pretty hard to say without more information. I just tried, and I
>>> don't see it myself. I would guess that some optimization split out
>>> the ratio == 0 case for some reason.
>>
>> It disappeared with Richard's checkin r205730. Also I am not sure is
>> why the rest of compiler can live with that code.
>
> The compiler doesn't much care if you divide by zero. Even if the
> programmer doesn't do such a thing, there are many ways that CSE can
> produce an integer division by zero. Code that divides an integer by
> zero is valid C/C++ as long as it is never executed. In fact, code
> that divides an integer by zero is valid Go even if it does get
> executed--Go defines what happens when an integer division by zero
> occurs (you get a runtime panic that can be caught). So while I
> understand that the code looks odd, there is no reason the compiler
> shouldn't be able to handle it.
Hi Ian, thanks for your explanation.
Thanks,
bin
>
> Ian
--
Best Regards.
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Division by ZERO on GIMPLE?
2013-12-08 12:17 ` Bin.Cheng
2013-12-08 12:47 ` Bin.Cheng
2013-12-08 19:04 ` Ian Lance Taylor
@ 2013-12-09 3:26 ` Jeff Law
2 siblings, 0 replies; 7+ messages in thread
From: Jeff Law @ 2013-12-09 3:26 UTC (permalink / raw)
To: Bin.Cheng, Ian Lance Taylor; +Cc: gcc-help, Richard Biener
On 12/08/13 05:16, Bin.Cheng wrote:
> On Sun, Dec 8, 2013 at 9:29 AM, Ian Lance Taylor <iant@google.com> wrote:
>> On Sat, Dec 7, 2013 at 1:48 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>>>
>>> I just found some GIMPLE like below is generated during boostrap gcc
>>> for x86, tree-vect-loop.c
>>>
>>>
>>> <bb 637>:
>>> # group_size_1432 = PHI <1(497), group_size_1017(660)>
>>> # scalar_dest_1287 = PHI <scalar_dest_1228(497), scalar_dest_1012(660)>
>>> ratio_1497 = group_size_1432 / 0;
>>> goto <bb 501>;
>>>
>>> The compilation ends fine, but what is the division by 0?
>>
>> Pretty hard to say without more information. I just tried, and I
>> don't see it myself. I would guess that some optimization split out
>> the ratio == 0 case for some reason.
>
> It disappeared with Richard's checkin r205730. Also I am not sure is
> why the rest of compiler can live with that code.
Well, in an ideal world we'd detect this, isolate the path, emit a trap
after the /0, remove all the outgoing edges and issue a warning. I'll
put it on the TODO list.
As long as the compiler doesn't try to evaluate that code, then it
should not cause an problems. It's not significantly different than if
there was a divide by an SSA_NAME where the SSA_NAME might have the
value zero.
jeff
^ permalink raw reply [flat|nested] 7+ messages in thread