[LLVMdev] Disabling emission of jump table info

[Chris Lattner]

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>