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    SubjectRe: [PATCH 2/7] Makefile, x86, LLVM: disable unsupported optimization flags
    Hi.


    2017-04-06 4:11 GMT+09:00 Michael Davidson <md@google.com>:
    > 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@chromium.org> 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@google.com>:
    >>> > 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@google.com>
    >>>
    >>>
    >>> 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

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