Re: [PATCH 2/7] Makefile, x86, LLVM: disable unsupported optimization flags

[Masahiro Yamada]

Hi.


2017-04-06 4:11 GMT+09:00 Michael Davidson <md@xxxxxxxxxx>:
> It "works" for the cases that I currently care about but I have to say
> that I am uneasy about adding -Werror to the cc-option test in this
> way.
>
> Suppose that one of the *other* flags that is implicitly passed to the
> compiler by cc-option - eg something that was explicitly specified in
> $(KBUILD_CFLAGS) - triggers a warning. In that case all calls to
> cc-option will silently fail because of the -Werror and valid options
> will not be detected correctly.

Theoretically, options explicitly specified in KBUILD_CFLAGS
should be always valid.
Options that may not be supported in some cases
should be wrapped with $(call cc-option ).



> If everyone is OK with that because "it shouldn't normally ever
> happen" then that is fine, but if does result in a subtle change from
> existing behavior (and a trap that I almost immediately fell into
> after applying a similar patch).

There is a rare case where a particular combination fails
(such as the conflict between -pg and -ffunction-sections
as reported in https://patchwork.kernel.org/patch/9624573/).

In a such case, we may end up with swapping the order,
but this should not happen quite often.



> On Wed, Apr 5, 2017 at 12:01 PM, Matthias Kaehlcke <mka@xxxxxxxxxxxx> wrote:
>> Hi Masahiro,
>>
>> El Thu, Apr 06, 2017 at 03:08:26AM +0900 Masahiro Yamada ha dit:
>>
>>> 2017-03-17 9:15 GMT+09:00 Michael Davidson <md@xxxxxxxxxx>:
>>> > Unfortunately, while clang generates a warning about these flags
>>> > being unsupported it still exits with a status of 0 so we have
>>> > to explicitly disable them instead of just using a cc-option check.
>>> >
>>> > Signed-off-by: Michael Davidson <md@xxxxxxxxxx>
>>>
>>>
>>> Instead, does the following work for you?
>>> https://patchwork.kernel.org/patch/9657285/
>>
>> Thanks for the pointer, I was about to give this change (or rather its
>> ancestor) a rework myself :)
>>
>>> You need to use
>>> $(call cc-option, ...)
>>> for -falign-jumps=1 and -falign-loops=1
>>
>> I can confirm that this works.
>>
>> Thanks
>>
>> Matthias
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-- 
Best Regards
Masahiro Yamada