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s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=sXKxf2WheKTyVYrbgS1oaunkjJtfisdRmZ807RF1ULg=; b=UKi9m76qt/OU/hFnaw64KgBMFM0zWHLw0zainVYEBCiHk518qWUJAxSXqJv3XQJTfC ANd8iFw0YDjAodUvwxaaV0BgVc9TcM8zdfEUiSgv/dNKgsiNNZEG5D8EeYksFN5717yn IQWuVigxHM3LjqGJLhsNTQbbDLUuZYj7oPepri8hvtct2xAzRJIcFKJ40+aEZVYqqkwy wAxHS+eWdMNrgdTmvuT5kaWkLGvRUWFRiMnX6PndkWjU6zEIJlXl9TZ+1DJ4UVYbAv3h T2v0i8dzrz5EVuMLU0AW9NL0gEcPToInqhITp09lUXs9Xk1q5jj4tI8Ct02gtIAQ2y67 XQvg== X-Gm-Message-State: AOAM533yZf4/+qPSQjdWvPQhHwQjk3GESnB60iGtcCOlFhCxvPiZnUrw Ks7pRk4Uva7rjHeDb+XuPxyFdWFvsL+zQ1PPkUIQKw== X-Google-Smtp-Source: ABdhPJznUWbLFNZFALzU8fQ9YS1h0QQBh8Wqc/a3pb1tgbl1rRtFm+l7R02qCrm4WRhwHDcStYtCj/ZrwiScdaMDdy0= X-Received: by 2002:a2e:b88b:: with SMTP id r11mr2941721ljp.495.1615413756356; Wed, 10 Mar 2021 14:02:36 -0800 (PST) MIME-Version: 1.0 References: <20210225112122.2198845-1-arnd@kernel.org> <20210226211323.arkvjnr4hifxapqu@google.com> <1614559739.p25z5x88wl.astroid@bobo.none> In-Reply-To: From: Nick Desaulniers Date: Wed, 10 Mar 2021 14:02:24 -0800 Message-ID: Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION To: kernel-toolchains@vger.kernel.org Cc: Masahiro Yamada , Nicholas Piggin , Fangrui Song , Ard Biesheuvel , Andrew Scull , Mark Brown , Catalin Marinas , clang-built-linux , David Brazdil , Geert Uytterhoeven , Ionela Voinescu , Kees Cook , Kristina Martsenko , Linux ARM , "linux-kernel@vger.kernel.org" , Mark Rutland , Marc Zyngier , Nathan Chancellor , Vincenzo Frascino , Will Deacon , Nicolas Pitre , Arnd Bergmann X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210310_220238_345493_52D6FD89 X-CRM114-Status: GOOD ( 20.87 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Mar 10, 2021 at 1:08 PM Arnd Bergmann wrote: > > On Wed, Mar 10, 2021 at 9:50 PM Masahiro Yamada wrote: > > On Mon, Mar 1, 2021 at 10:11 AM Nicholas Piggin wrote: > > > Excerpts from Arnd Bergmann's message of February 27, 2021 7:49 pm: > > > > > masahiro@oscar:~/ref/linux$ echo 'void this_func_is_unused(void) {}' > > >> kernel/cpu.c > > masahiro@oscar:~/ref/linux$ export > > CROSS_COMPILE=/home/masahiro/tools/powerpc-10.1.0/bin/powerpc-linux- > > masahiro@oscar:~/ref/linux$ make ARCH=powerpc defconfig > > masahiro@oscar:~/ref/linux$ ./scripts/config -e EXPERT > > masahiro@oscar:~/ref/linux$ ./scripts/config -e LD_DEAD_CODE_DATA_ELIMINATION > > masahiro@oscar:~/ref/linux$ > > ~/tools/powerpc-10.1.0/bin/powerpc-linux-nm -n vmlinux | grep > > this_func > > c000000000170560 T .this_func_is_unused > > c000000001d8d560 D this_func_is_unused > > masahiro@oscar:~/ref/linux$ grep DEAD_CODE_ .config > > CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y > > CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y > > > > > > If I remember correctly, > > LD_DEAD_CODE_DATA_ELIMINATION dropped unused functions > > when I tried it last time. > > > > > > I also tried arm64 with a HAVE_LD_DEAD_CODE_DATA_ELIMINATION hack. > > The result was the same. > > > > > > > > Am I missing something? > > It's possible that it only works in combination with CLANG_LTO now > because something broke. I definitely saw a reduction in kernel > size when both options are enabled, but did not try a simple test > case like you did. > > Maybe some other reference gets created that prevents the function > from being garbage-collected unless that other option is removed > as well? I wish the linker had a debug flag that could let developers discover the decisions it made during --gc-sections as to why certain symbols were retained/kept or not. -- Thanks, ~Nick Desaulniers _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel