[llvm-dev] compiler-rt, v4.0: arm\udivsi3.S broken for division by zero

Zhao, Weiming via llvm-dev llvm-dev at lists.llvm.org
Wed Apr 5 12:21:08 PDT 2017

Using "b " is correct as it's a tail call.

However, for Thumb1, it can't use "b" due to  +/- 2 KB range. "BL label" 
has a much wider range of +/- 4MB.

The reason we have two blocks of "divby0": conditional branch in Thumb1 
has even shorter range -252 to +258, so it can't jump to the block 
around line 258.

So we cloned the "divby0" block. I think we can fold the 2nd "divby0" 
into line 39 to make it clear.



On 4/5/2017 11:40 AM, Peter Jakubek wrote:
> I recommend the other incarnation of the call to divby0 at line 256 is 
> also
> modified to use bl instead of b for the same reason.
> Or just remove one of the two to avoid confusion in the future?
> Cheers,
> Peter
> Am 05.04.2017 um 19:14 schrieb Zhao, Weiming:
>> Yes, it's a bug.
>> Please review https://reviews.llvm.org/D31716
>> On 4/5/2017 3:50 AM, Renato Golin wrote:
>>> On 21 March 2017 at 18:32, Peter Jakubek via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>>> I think the current implementation for the call "bl __aeabi_idiv0" in
>>>> builtins\arm\udivsi3.S is broken.
>>>> At least for the case that __aeabi_idiv0 returns. (the provided
>>>> implementation does)
>>>> Since LR is not preserved, the following JMP(lr) results in an
>>>> endless loop.
>>>> Or is this an intentional change of the behavior?
>>> Hi Peter,
>>> That is most certainly a bug. Weiming's patch was supposed to only
>>> introduce Thumb1 code, not transform div0 into a busy loop. :)
>>>> The file contains another implementation enabled by
>>>> This uses "b" instead of "bl".
>>>> (This works as in previous versions)
>>> The comment on the patch makes that clear:
>>> bl  __aeabi_idiv0 // due to relocation limit, can't use b.
>>> I'm not sure why the bottom one is fine and the top one isn't, wrt.
>>> range, but we may have to add a trampoline here.
>>> Weiming, Saleem, comments?
>>> cheers,
>>> --renato

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