[LLVMdev] Disabling emission of jump table info

[Richard Osborne]

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

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