> On Sep 11, 2020, at 3:05 PM, Segher Boessenkool <seg...@kernel.crashing.org> 
> wrote:
> 
> On Fri, Sep 11, 2020 at 02:40:06PM -0500, Qing Zhao wrote:
>>> On Sep 11, 2020, at 12:13 PM, Segher Boessenkool 
>>> <seg...@kernel.crashing.org> wrote:
>>> On Fri, Sep 11, 2020 at 11:52:29AM -0500, Qing Zhao wrote:
>>>> I don’t understand why it’s not correct if we clearing call-clobbered 
>>>> registers 
>>>> AFTER restoring call-preserved registers?
>>> 
>>> Because the compiler backend (or the linker!  Or the dynamic linker!
>>> Etc.) can use volatile registers for their own purposes.
>> 
>> For the following sequence at the end of a routine:
>> 
>> *...*
>> “restore call-preserved registers”
>> *clear call-clobbered registers"*
>> *ret*
>> 
>> “Clear call-clobbered registers” will only clear the call-clobbered 
>> registers that are not live at the end of the routine.
> 
> And they can be written again right after the routine, by linker-
> generated code for example.  This is a waste.
> 
>> In the new version of the patch,  the implementation of clearing 
>> call-clobbered registers is done in backend, middle end only 
>> computes a hard register set based on user option, source attribute, data 
>> flow information, and function abi information, and
>> Then pass this hard register set to the target hook to generate the clearing 
>> sequence.  The backend will have all the details
>> on the special situations you mentioned. 
>> 
>> Let me know any more concerns here.
> 
> I cannot find that patch?

Haven’t finished yet. -:).

Qing
> 
> 
> Segher

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