[LLVMdev] Disabling emission of jump table info

[Jim Grosbach]

Thanks, Richard. I'll have a look at that PR.

-jim

On Mar 11, 2010, at 7:05 AM, Richard Osborne wrote:

> Thanks for reviewing this. Committed in r98255 and r98256. The bug against the ARM backend is 6581:
> 
> http://llvm.org/bugs/show_bug.cgi?id=6581
> 
> On 10/03/10 21:45, Chris Lattner wrote:
>> Typo "responisbility", otherwise looks great to me, please apply.  For ARM, please just file a bugzilla suggesting that the ARM backend adopt this.  Thanks Richard!
>> 
>> -Chris
>> 
>> On Mar 9, 2010, at 6:06 AM, Richard Osborne wrote:
>> 
>>   
>>> On 02/03/10 00:11, Jim Grosbach wrote:
>>>     
>>>> On Mar 1, 2010, at 4:09 PM, Richard Osborne wrote:
>>>> 
>>>>       
>>>>> On 01/03/10 21:14, Chris Lattner wrote:
>>>>> 
>>>>>         
>>>>>> On Mar 1, 2010, at 10:52 AM, Richard Osborne wrote:
>>>>>> 
>>>>>>           
>>>>>>> On 23/02/10 14:58, Richard Osborne wrote:
>>>>>>> 
>>>>>>> 
>>>>>>>             
>>>>>>>> I've recently changed the XCore target to implement BR_JT as a jump to a
>>>>>>>> series jumps. The jump table entries are expand inline in the function
>>>>>>>> so there is no longer a need to emit jump tables at the end of the
>>>>>>>> function. However the emission of jump tables at the end of a function
>>>>>>>> is done inside the AsmPrinter base class and there seems to be no way of
>>>>>>>> disabling this.
>>>>>>>> 
>>>>>>>> This also seems to effect code produced for ARM Thumb2. Again each jump
>>>>>>>> table is emitted twice - once inline in the code and once at the end of
>>>>>>>> the function. The jump tables at the end of the function are unused.
>>>>>>>> 
>>>>>>>> What would be the preferred way of dealing with this? Should I add a
>>>>>>>> variable to AsmPrinter that can be set by targets to disable this
>>>>>>>> emission at the end?
>>>>>>>> 
>>>>>>>> 
>>>>>>>>               
>>>>>>> The attached patch adds a bool to MCAsmInfo that can be used to disable the emission of jump tables at the end of a function and updates the XCore target to set this. Is this OK to commit?
>>>>>>> 
>>>>>>> 
>>>>>>>             
>>>>>> Hi Richard,
>>>>>> 
>>>>>> This shouldn't go into MCAsmInfo, it's a "lowering" issue, not a "assembly syntax" issue.  ARM also has inline jump tables, how does it work?
>>>>>> 
>>>>>> -Chris
>>>>>> 
>>>>>>           
>>>>> I've attached the result of running llc -march=arm on test/CodeGen/XCore/switch.ll. The jump table entries are emitted twice, once inside the function after the label .LJTI1_0_0 and then again outside the function after the label .LJTI1_0. The .LJTI1_0 label at the start of the second set of jumptable entries is completely unreferenced.
>>>>> 
>>>>>         
>>>> Interesting. Any idea when that started happening? I'm pretty sure that didn't used to be the case.
>>>> 
>>>> -Jim
>>>> 
>>>>       
>>> On ARM the problem started after this commit:
>>> 
>>> http://llvm.org/viewvc/llvm-project?view=rev&revision=94719
>>> 
>>> I've attached a new patch which adds a new jump table encoding type to indicate jump
>>> tables are emitted inline. This fixes the problem on the XCore target.
>>> 
>>> I can put together a patch to switch over ARM as well but I can't easily test it. After this the TargetJITInfo::hasCustomJumpTables() hook would no longer be needed and could be removed.
>>> 
>>> -- 
>>> Richard Osborne | XMOS
>>> http://www.xmos.com
>>> 
>>> <jt.patch>
>>>     
>>   
> 
> 
> -- 
> Richard Osborne | XMOS
> http://www.xmos.com
>