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Fri, 14 Mar 2025 17:09:12 -0400 (EDT) From: Junio C Hamano To: git@vger.kernel.org Cc: Jeff King , Patrick Steinhardt Subject: [PATCH v2 1/3] config.mak.dev: enable -Wunreachable-code Date: Fri, 14 Mar 2025 14:09:07 -0700 Message-ID: <20250314210909.3776678-2-gitster@pobox.com> X-Mailer: git-send-email 2.49.0-188-g35fcca2323 In-Reply-To: <20250314210909.3776678-1-gitster@pobox.com> References: <20250307225444.GA42758@coredump.intra.peff.net> <20250314210909.3776678-1-gitster@pobox.com> Precedence: bulk X-Mailing-List: git@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Jeff King Having the compiler point out unreachable code can help avoid bugs, like the one discussed in: https://lore.kernel.org/git/20250307195057.GA3675279@coredump.intra.peff.net/ In that case it was found by Coverity, but finding it earlier saves everybody time and effort. We can use -Wunreachable-code to get some help from the compiler here. Interestingly, this is a noop in gcc. It was a real warning up until gcc 4.x, when it was removed for being too flaky, but they left the command-line option to avoid breaking users. See: https://stackoverflow.com/questions/17249934/why-does-gcc-not-warn-for-unreachable-code However, clang does implement this option, and it finds the case mentioned above (and no other cases within the code base). And since we run clang in several of our CI jobs, that's enough to get an early warning of breakage. We could enable it only for clang, but since gcc is happy to ignore it, it's simpler to just turn it on for all developer builds. Signed-off-by: Jeff King [jc: squashed meson.build change sent by Patrick] Signed-off-by: Junio C Hamano --- config.mak.dev | 1 + meson.build | 1 + 2 files changed, 2 insertions(+) diff --git a/config.mak.dev b/config.mak.dev index 0fd8cc4d35..95b7bc46ae 100644 --- a/config.mak.dev +++ b/config.mak.dev @@ -39,6 +39,7 @@ DEVELOPER_CFLAGS += -Wunused DEVELOPER_CFLAGS += -Wvla DEVELOPER_CFLAGS += -Wwrite-strings DEVELOPER_CFLAGS += -fno-common +DEVELOPER_CFLAGS += -Wunreachable-code ifneq ($(filter clang4,$(COMPILER_FEATURES)),) DEVELOPER_CFLAGS += -Wtautological-constant-out-of-range-compare diff --git a/meson.build b/meson.build index 0064eb64f5..f60f3f49e4 100644 --- a/meson.build +++ b/meson.build @@ -697,6 +697,7 @@ if get_option('warning_level') in ['2','3', 'everything'] and compiler.get_argum '-Woverflow', '-Wpointer-arith', '-Wstrict-prototypes', + '-Wunreachable-code', '-Wunused', '-Wvla', '-Wwrite-strings', From patchwork Fri Mar 14 21:09:08 2025 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Junio C Hamano X-Patchwork-Id: 14017414 Received: from fout-b5-smtp.messagingengine.com (fout-b5-smtp.messagingengine.com [202.12.124.148]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CEFF11C54AA for ; 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Fri, 14 Mar 2025 17:09:14 -0400 (EDT) From: Junio C Hamano To: git@vger.kernel.org Cc: Jeff King Subject: [PATCH v2 2/3] run-command: use errno to check for sigfillset() error Date: Fri, 14 Mar 2025 14:09:08 -0700 Message-ID: <20250314210909.3776678-3-gitster@pobox.com> X-Mailer: git-send-email 2.49.0-188-g35fcca2323 In-Reply-To: <20250314210909.3776678-1-gitster@pobox.com> References: <20250307225444.GA42758@coredump.intra.peff.net> <20250314210909.3776678-1-gitster@pobox.com> Precedence: bulk X-Mailing-List: git@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Jeff King Since enabling -Wunreachable-code, builds with clang on macOS now fail, complaining that the die_errno() call in: if (sigfillset(&all)) die_errno("sigfillset"); is unreachable. On that platform the manpage documents that sigfillset() always returns success, and presumably the implementation is a macro or inline function that does so in a way that is transparent to the compiler. But we should continue to check on other platforms, since POSIX says it may return an error. We could solve this with a compile-time knob to split the two cases (assuming success on macOS and checking for the error elsewhere). But we can also work around it more directly by relying on errno to check the outcome (since POSIX dictates that errno will be set on error). And that works around the compiler's cleverness, since it doesn't know the semantics of errno (though I suppose if sigfillset() is simple enough, it could perhaps realize that no writes to errno are possible; however this does seem to work in practice). Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- run-command.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/run-command.c b/run-command.c index 402138b8b5..d527c46175 100644 --- a/run-command.c +++ b/run-command.c @@ -515,7 +515,15 @@ static void atfork_prepare(struct atfork_state *as) { sigset_t all; - if (sigfillset(&all)) + /* + * Do not use the return value of sigfillset(). It is transparently 0 + * on some platforms, meaning a clever compiler may complain that + * the conditional body is dead code. 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Fri, 14 Mar 2025 17:09:15 -0400 (EDT) From: Junio C Hamano To: git@vger.kernel.org Cc: Jeff King , Patrick Steinhardt Subject: [PATCH v2 3/3] git-compat-util: add NOT_A_CONST macro and use it in atfork_prepare() Date: Fri, 14 Mar 2025 14:09:09 -0700 Message-ID: <20250314210909.3776678-4-gitster@pobox.com> X-Mailer: git-send-email 2.49.0-188-g35fcca2323 In-Reply-To: <20250314210909.3776678-1-gitster@pobox.com> References: <20250307225444.GA42758@coredump.intra.peff.net> <20250314210909.3776678-1-gitster@pobox.com> Precedence: bulk X-Mailing-List: git@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Our hope is that the number of code paths that falsely trigger warnings with the -Wunreachable-code compilation option are small, and they can be worked around case-by-case basis, like we just did in the previous commit. If we need such a workaround a bit more often, however, we may benefit from a more generic and descriptive facility that helps document the cases we need such workarounds. Side note: if we need the workaround all over the place, it simply means -Wunreachable-code is not a good tool for us to save engineering effort to catch mistakes. We are still exploring if it helps us, so let's assume that it is not the case. Introduce NOT_A_CONST() macro, with which, the developer can tell the compiler: Do not optimize this expression out, because, despite whatever you are told by the system headers, this expression should *not* be treated as a constant. and use it as a replacement for the workaround we used that was somewhat specific to the sigfillset case. If the compiler already knows that the call to sigfillset() cannot fail on a particular platform it is compiling for and declares that the if() condition would not hold, it is plausible that the next version of the compiler may learn that sigfillset() that never fails would not touch errno and decide that in this sequence: errno = 0; sigfillset(&all) if (errno) die_errno("sigfillset"); the if() statement will never trigger. Marking that the value returned by sigfillset() cannot be a constant would document our intention better and would not break with such a new version of compiler that is even more "clever". With the marco, the above sequence can be rewritten: if (NOT_A_CONST(sigfillset(&all))) die_errno("sigfillset"); which looks almost like other innocuous annotations we have, e.g. UNUSED. Signed-off-by: Junio C Hamano Signed-off-by: Junio C Hamano --- Makefile | 1 + git-compat-util.h | 9 +++++++++ meson.build | 1 + run-command.c | 12 +++++------- 4 files changed, 16 insertions(+), 7 deletions(-) diff --git a/Makefile b/Makefile index 97e8385b66..2158bf6916 100644 --- a/Makefile +++ b/Makefile @@ -1018,6 +1018,7 @@ LIB_OBJS += ewah/ewah_bitmap.o LIB_OBJS += ewah/ewah_io.o LIB_OBJS += ewah/ewah_rlw.o LIB_OBJS += exec-cmd.o +LIB_OBJS += fbtcdnki.o LIB_OBJS += fetch-negotiator.o LIB_OBJS += fetch-pack.o LIB_OBJS += fmt-merge-msg.o diff --git a/git-compat-util.h b/git-compat-util.h index e283c46c6f..63a3ef6b70 100644 --- a/git-compat-util.h +++ b/git-compat-util.h @@ -1593,4 +1593,13 @@ static inline void *container_of_or_null_offset(void *ptr, size_t offset) ((uintptr_t)&(ptr)->member - (uintptr_t)(ptr)) #endif /* !__GNUC__ */ +/* + * Prevent an overly clever compiler from optimizing an expression + * out, triggering a false positive when building with the + * -Wunreachable-code option. false_but_the_compiler_does_not_know_it_ + * is defined in a compilation unit separate from where the macro is + * used, initialized to 0, and never modified. + */ +#define NOT_A_CONST(expr) ((expr) || false_but_the_compiler_does_not_know_it_) +extern int false_but_the_compiler_does_not_know_it_; #endif diff --git a/meson.build b/meson.build index f60f3f49e4..ce642dcf65 100644 --- a/meson.build +++ b/meson.build @@ -282,6 +282,7 @@ libgit_sources = [ 'ewah/ewah_io.c', 'ewah/ewah_rlw.c', 'exec-cmd.c', + 'fbtcdnki.c', 'fetch-negotiator.c', 'fetch-pack.c', 'fmt-merge-msg.c', diff --git a/run-command.c b/run-command.c index d527c46175..535c73a059 100644 --- a/run-command.c +++ b/run-command.c @@ -516,14 +516,12 @@ static void atfork_prepare(struct atfork_state *as) sigset_t all; /* - * Do not use the return value of sigfillset(). It is transparently 0 - * on some platforms, meaning a clever compiler may complain that - * the conditional body is dead code. Instead, check for error via - * errno, which outsmarts the compiler. + * POSIX says sitfillset() can fail, but an overly clever + * compiler can see through the header files and decide + * it cannot fail on a particular platform it is compiling for, + * triggering -Wunreachable-code false positive. */ - errno = 0; - sigfillset(&all); - if (errno) + if (NOT_A_CONST(sigfillset(&all))) die_errno("sigfillset"); #ifdef NO_PTHREADS if (sigprocmask(SIG_SETMASK, &all, &as->old))