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Date:   Thu, 9 May 2019 09:47:32 -0700
From:   Nick Desaulniers <>
To:     Masahiro Yamada <>
Cc:     Sedat Dilek <>,
        Linux Kbuild mailing list <>,
        Nathan Chancellor <>,
        Arnd Bergmann <>,
        Clang-Built-Linux ML <>,
        Michal Marek <>,
        Linux Kernel Mailing List <>,
        chromeos-toolchain <>,
        Android-LLVM <>
Subject: Re: [PATCH] kbuild: add some extra warning flags unconditionally

From: Masahiro Yamada <>
> On Thu, May 9, 2019 at 4:11 PM Sedat Dilek <> wrote:
> > BTW, is this a speedup when doing CC/LD FLAGS etc checks unconditionally?
> Yes.
> cc-option is somewhat costly because it invoked the compiler to
> check if the given flag is supported.
> So, I want to get rid of as many cc-option calls as possible.
> > Asking in general - do you have any numbers :-)?
> Removing a couple of cc-options does not make
> a measurable difference in general use-cases.
> But, this might be more beneficial for chrome OS
> because $(CC) is a wrapper and invoking it is much more expensive.

Android does too, which we plan on removing as we recently measured
the performance cost of 5% for having a Python wrapper to aid in

Anyways, I checked these options with clang 4 and gcc 4.6.4 in godbolt.
Tested-by: Nick Desaulniers <>
~Nick Desaulniers

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