on 2022/8/10 05:10, Segher Boessenkool wrote:
> Hi!
>
> On Tue, Aug 09, 2022 at 08:51:59PM +0800, Kewen.Lin wrote:
>> on 2022/8/9 18:35, Segher Boessenkool wrote:
>>>> + /* As ELFv2 ABI shows, the allowable bytes past the global entry
>>>> + point are 0, 4, 8, 16, 32 and 64. Considering there are two
>>>> + non-prefixed instructions for global entry (8 bytes), the count
>>>> + for patchable NOPs before local entry would be 2, 6 and 14. */
>>>
>>> The other option is to allow other numbers of nops, but in that case not
>>> have a local entry point (so, always use the global entry point).
>>
>> Good point, it's doable, but it means for the other counts of NOPs, the
>> patched function has to pay the cost of TOC initialization all the time,
>> IMHO it may not be what we want.
>
> It isn't very expensive: the main benefit of the LEP is not not having
> to do those two insns, but having the r2 setter earlier, allowing loads
> via the TOC reg to execute earlier.
>
OK.
>>> I don't know if that is useful for any users of this support (if there
>>> even are such users :-P )
>>
>> Yeah, as the discussions in PR98125, powerpc linux kernel doesn't adopt
>> this feature. :-P
>
> Right, -mprofile-kernel is more efficient.
>
> So maybe just say in the comment that it is possible to support those
> other nop pad sizes, by not doing a LEP at all? Instead of sasying it
> cannot be done :-)
OK, I'll update the comments like:
/* As ELFv2 ABI shows, the allowable bytes past the global entry
point are 0, 4, 8, 16, 32 and 64 when there is a local entry.
Considering there are two non-prefixed instructions for global
entry (8 bytes), the count for patchable NOPs before local entry
would be 2, 6 and 14. It's possible to support those other
counts of NOPs by not doing a local entry at all, but we don't
have clear user cases for them, so leave them unsupported for
now. */
>
>>
>>>
>>>> + if (patch_area_entry > 0)
>>>> + {
>>>> + if (patch_area_entry != 2
>>>> + && patch_area_entry != 6
>>>> + && patch_area_entry != 14)
>>>> + error ("for %<-fpatchable-function-entry=%u,%u%>, patching "
>>>> + "%u NOP(s) before function entry is invalid, it can "
>>>> + "cause assembler error",
>>>
>>> I would not say "it can [etc.]" at all. Oh, and "NOP" (capitals) isn't
>>> a thing, it is not an acronym or such ;-)
>>>
>>
>> Poor at wording. :( Could you help to suggest some words here?
>
> I'll try...
>
> "unsupported number of nops before function entry (%u)"
>
Nice, will update with this.
>>>> +/* { dg-require-effective-target powerpc_elfv2 } */
>>>> +/* Specify -mcpu=power9 to ensure global entry is needed. */
>>>> +/* { dg-options "-mdejagnu-cpu=power9" } */
>>>
>>> Why would it be needed for p9, and not older, or newer?
>>>
>>
>> It can be p8 or p9, but not p10 and later.
>>
>> It's meant to exclude pc-relative feature which can make the case not
>> generate a global entry point prologue and the test point will become
>> unavailable. I thought about adding -mno-pcrel, but guessed it's safer
>> to use one cpu type which doesn't support pcrel at all, since it can
>> exclude all possibilities that pcrel gets re-enabled.
>>
>> Do you think -mno-pcrel is more elegant and relatively safe?
>> Or just update the comments to make it more meaningful?
>
> Just use { ! powerpc_pcrel } ? I don't think you can put that in a
> dg-require-effective-target, but you can do for example
> dg-do compile { target { ! powerpc_pcrel } }
> or similar.
>
Good idea, I'll send out one new version of patch after some testings.
BR,
Kewen
