[LLVMdev] Disabling emission of jump table info

[Jim Grosbach]

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

> Given that MCAsmInfo is the wrong place would it make sense to describe it instead as a separate type of jump table encoding and add it to the MachineJumpTableInfo::JTEntryKind enum?