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

[vivek pandya via llvm-dev]

I have tried out the following code which examines each call site in a
module for tail call and do not perform optimization in such case:


On Wed, Jun 29, 2016 at 12:34 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:

>
> On Jun 28, 2016, at 3:01 PM, Matthias Braun <matze at braunis.de> wrote:
>
>
> 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> wrote:
>
>
> On Jun 28, 2016, at 10:09 AM, Mehdi Amini via llvm-dev <
> 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> wrote:
>
>
>
> On Tue, Jun 28, 2016 at 8:11 PM, Mehdi Amini <mehdi.amini at apple.com>
> wrote:
>
>>
>> On Jun 27, 2016, at 12:25 PM, vivek pandya <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.
>
>
> Thanks, so back to my original point: if we have to disable the CSR
> optimization on function that “may be tail called”, it would still be
> better IMO to do something like `llvm::any_of(callsites, isTailCall)`
> instead of IsEligibleForTailCallOptimization().
>
> —
> Mehdi
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160629/dc8cede9/attachment.html>