[llvm-dev] Tail call optimization is getting affected due to local function related optimization with IPRA

[Matthias Braun via llvm-dev]

> On Jun 28, 2016, at 11:34 AM, Mehdi Amini via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
> 
> Sent from my iPhone
> 
> On Jun 28, 2016, at 2:27 PM, Matthias Braun <matze at braunis.de <mailto:matze at braunis.de>> wrote:
> 
>> 
>>> On Jun 28, 2016, at 10:09 AM, Mehdi Amini via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> 
>>> 
>>> 
>>> Sent from my iPhone
>>> 
>>> On Jun 28, 2016, at 12:53 PM, vivek pandya <vivekvpandya at gmail.com <mailto:vivekvpandya at gmail.com>> wrote:
>>> 
>>>> 
>>>> 
>>>> On Tue, Jun 28, 2016 at 8:11 PM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>>>> 
>>>>> On Jun 27, 2016, at 12:25 PM, vivek pandya <vivekvpandya at gmail.com <mailto:vivekvpandya at gmail.com>> wrote:
>>>>> 
>>>>> Hello ,
>>>>> 
>>>>> To solve this bug locally I have given preference to tail call optimization over local function related optimization in IPRA. I have added following method to achieve this:
>>>>> 
>>>>> bool isEligibleForTailCallOptimization(Function *F) {
>>>>>   CallingConv::ID CC = F->getCallingConv();
>>>>>   if (CC == CallingConv::Fast || CC == CallingConv::GHC || CC == CallingConv::HiPE)
>>>>>     return true;
>>>>>   return false;
>>>>> }
>>>>> 
>>>>> Any other suggestions are always welcomed.
>>>> 
>>>> Why aren’t checking for the presence of a tail call?
>>>> Are you asking about if tail call optimization is enable or not?  If not then above method is inspired from X86ISelLowering::canGuaranteeTCO(). 
>>>> 
>>> 
>>> Are we turning calls into tail calls during codegen?
>>> My assumption is that tail call is inferred on the IR, so you can inspect every *call site*.
>> The final decision on whether to tail call or not is done during instruction selection (it is part of X86TargetLowering::LowerCall()/IsEligibleForTailCallOptimization() for example).
> 
> Sorry i don't have access to the source code right now, can you clarify if the backend can tail call when the IR didn't mark the call as such, or if what you're referring to is "not honoring the tail call From the IR and demoting to a normal call?

The backend does only tail call if the middleend marked the call with the "tail" or "musttail" marker. But that happens for most calls. We can only really transform a franction of those into real tail calls later.

- Matthias
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