Re: [PATCH 1/2] inline constant return from error() function
From: Junio C Hamano <hidden>
Date: 2016-06-15 23:01:02
Jeff King [off-list ref] writes:
Commit e208f9c introduced a macro to turn error() calls into: (error(), -1) to make the constant return value more visible to the calling code (and thus let the compiler make better decisions about the code). This works well for code like: return error(...); but the "-1" is superfluous in code that just calls error() without caring about the return value. In older versions of gcc, that was fine, but gcc 4.9 complains with -Wunused-value. We can work around this by encapsulating the constant return value in a static inline function, as gcc specifically avoids complaining about unused function returns unless the function has been specifically marked with the warn_unused_result attribute.
That's kind of W*A*T magic, and I generally try to avoid magic, as long as it solves your "can we make both -O2 with new compilers and -O3 happy?" I wouldn't complain ;-)
We also use the same trick for config_error_nonbool and opterror, which learned the same error technique in a469a10. Reported-by: Felipe Contreras <redacted> Signed-off-by: Jeff King <redacted> --- On Mon, May 05, 2014 at 05:29:38PM -0400, Jeff King wrote:quoted
I cannot think of any other way to make the compiler aware of the constant value, but perhaps somebody else is more clever than I am.This came to me in a dream, and seems to work.
quoted hunk
cache.h | 2 +- git-compat-util.h | 6 +++++- parse-options.h | 2 +- 3 files changed, 7 insertions(+), 3 deletions(-)diff --git a/cache.h b/cache.h index 107ac61..e2f12b0 100644 --- a/cache.h +++ b/cache.h@@ -1272,7 +1272,7 @@ extern int git_env_bool(const char *, int); extern int git_config_system(void); extern int config_error_nonbool(const char *); #if defined(__GNUC__) && ! defined(__clang__) -#define config_error_nonbool(s) (config_error_nonbool(s), -1) +#define config_error_nonbool(s) (config_error_nonbool(s), const_error()) #endif extern const char *get_log_output_encoding(void); extern const char *get_commit_output_encoding(void);diff --git a/git-compat-util.h b/git-compat-util.h index f6d3a46..b4c437e 100644 --- a/git-compat-util.h +++ b/git-compat-util.h@@ -331,7 +331,11 @@ extern void warning(const char *err, ...) __attribute__((format (printf, 1, 2))) * using the function as usual. */ #if defined(__GNUC__) && ! defined(__clang__) -#define error(...) (error(__VA_ARGS__), -1) +static inline int const_error(void) +{ + return -1; +} +#define error(...) (error(__VA_ARGS__), const_error()) #endif extern void set_die_routine(NORETURN_PTR void (*routine)(const char *err, va_list params));diff --git a/parse-options.h b/parse-options.h index 3189676..2f9be96 100644 --- a/parse-options.h +++ b/parse-options.h@@ -177,7 +177,7 @@ extern NORETURN void usage_msg_opt(const char *msg, extern int optbug(const struct option *opt, const char *reason); extern int opterror(const struct option *opt, const char *reason, int flags); #if defined(__GNUC__) && ! defined(__clang__) -#define opterror(o,r,f) (opterror((o),(r),(f)), -1) +#define opterror(o,r,f) (opterror((o),(r),(f)), const_error()) #endif /*----- incremental advanced APIs -----*/