[LLVMdev] proposed change to class BasicTTI

reed kotler rkotler at mips.com
Fri Mar 22 15:25:51 PDT 2013

On 03/22/2013 03:22 PM, Nadav Rotem wrote:
> Hi Reed,
> We will need to reconstruct the target machine and the TTI chain when 
> the function attributes change. We currently don't have code for doing 
> that but I suggest that you talk with Bill Wendling about the best way 
> to implement this.

Cool. Bill's new attribute work has already made this task much easier 
for me.

I'm willing to contribute to the compiler in this area and any advice is 
greatly appreciated.

If Bill is already scheduled to do this then i can also wait like I did 
for his attribute work.


> Thanks,
> Nadav
> On Mar 22, 2013, at 11:30 AM, Reed Kotler <rkotler at mips.com 
> <mailto:rkotler at mips.com>> wrote:
>> Just realized that BasicTransformInfoClass is an immutable pass.
>> Not sure how to reconcile this with fact that there will be different 
>> answers needed depending on the subtarget.
>> Seems like BasicTansformInfoClass should become a function pass that 
>> does not modify  anything.
>> On 03/22/2013 09:43 AM, Reed Kotler wrote:
>>> Another way to do this would to be to have a reset virtual function
>>> which is passed the Function, and the address of TLI so that it could be
>>> modified.
>>> This seems somewhat cleaner.
>>> The reset virtual function would be added to base class 
>>> TargetLoweringBase.
>>> On 03/22/2013 09:22 AM, reed kotler wrote:
>>>> For being able to change subtargets within a compilation unit, among
>>>> other things, I need to be able to change the target lowering class 
>>>> that
>>>> is used by BasicTTI
>>>> For example we have a mips16 and non mips16 version.
>>>> On the original call that creates this class, I'd like to pass the
>>>> address of the address of the TargetLoweringBase class.
>>>> That way I can insert a function pass before this pass which examines
>>>> the function attributes and changes the pointer.
>>>> So we would get:
>>>> ImmutablePass *
>>>> llvm::createBasicTargetTransformInfoPass(const TargetLoweringBase
>>>> **TLI_) {
>>>>   return new BasicTTI(TLI);
>>>> }
>>>> we would add the variable
>>>> TargetLoweringBase **TLI_
>>>> and at the beginning of each pass
>>>> TLI = *TLI_;
>>>> Maybe there is a more elegant way to do this but this is essentially
>>>> what I need to do.
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