From e208f9cc7574f5980faba498d0aa30b4defeb34f Mon Sep 17 00:00:00 2001 From: Jeff King Date: Sat, 15 Dec 2012 12:37:36 -0500 Subject: [PATCH 1/2] make error()'s constant return value more visible When git is compiled with "gcc -Wuninitialized -O3", some inlined calls provide an additional opportunity for the compiler to do static analysis on variable initialization. For example, with two functions like this: int get_foo(int *foo) { if (something_that_might_fail() < 0) return error("unable to get foo"); *foo = 0; return 0; } void some_fun(void) { int foo; if (get_foo(&foo) < 0) return -1; printf("foo is %d\n", foo); } If get_foo() is not inlined, then when compiling some_fun, gcc sees only that a pointer to the local variable is passed, and must assume that it is an out parameter that is initialized after get_foo returns. However, when get_foo() is inlined, the compiler may look at all of the code together and see that some code paths in get_foo() do not initialize the variable. As a result, it prints a warning. But what the compiler can't see is that error() always returns -1, and therefore we know that either we return early from some_fun, or foo ends up initialized, and the code is safe. The warning is a false positive. If we can make the compiler aware that error() will always return -1, it can do a better job of analysis. The simplest method would be to inline the error() function. However, this doesn't work, because gcc will not inline a variadc function. We can work around this by defining a macro. This relies on two gcc extensions: 1. Variadic macros (these are present in C99, but we do not rely on that). 2. Gcc treats the "##" paste operator specially between a comma and __VA_ARGS__, which lets our variadic macro work even if no format parameters are passed to error(). Since we are using these extra features, we hide the macro behind an #ifdef. This is OK, though, because our goal was just to help gcc. Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- git-compat-util.h | 11 +++++++++++ usage.c | 1 + 2 files changed, 12 insertions(+) diff --git a/git-compat-util.h b/git-compat-util.h index 2e79b8a2f3..9002bca28e 100644 --- a/git-compat-util.h +++ b/git-compat-util.h @@ -288,6 +288,17 @@ extern NORETURN void die_errno(const char *err, ...) __attribute__((format (prin extern int error(const char *err, ...) __attribute__((format (printf, 1, 2))); extern void warning(const char *err, ...) __attribute__((format (printf, 1, 2))); +/* + * Let callers be aware of the constant return value; this can help + * gcc with -Wuninitialized analysis. We have to restrict this trick to + * gcc, though, because of the variadic macro and the magic ## comma pasting + * behavior. But since we're only trying to help gcc, anyway, it's OK; other + * compilers will fall back to using the function as usual. + */ +#ifdef __GNUC__ +#define error(fmt, ...) (error((fmt), ##__VA_ARGS__), -1) +#endif + extern void set_die_routine(NORETURN_PTR void (*routine)(const char *err, va_list params)); extern void set_error_routine(void (*routine)(const char *err, va_list params)); diff --git a/usage.c b/usage.c index 8eab28113a..40b3de51c7 100644 --- a/usage.c +++ b/usage.c @@ -130,6 +130,7 @@ void NORETURN die_errno(const char *fmt, ...) va_end(params); } +#undef error int error(const char *err, ...) { va_list params; From a469a1019352b8efc4bd7003b0bd59eb60fc428c Mon Sep 17 00:00:00 2001 From: Jeff King Date: Sat, 15 Dec 2012 12:42:10 -0500 Subject: [PATCH 2/2] silence some -Wuninitialized false positives There are a few error functions that simply wrap error() and provide a standardized message text. Like error(), they always return -1; knowing that can help the compiler silence some false positive -Wuninitialized warnings. One strategy would be to just declare these as inline in the header file so that the compiler can see that they always return -1. However, gcc does not always inline them (e.g., it will not inline opterror, even with -O3), which renders our change pointless. Instead, let's follow the same route we did with error() in the last patch, and define a macro that makes the constant return value obvious to the compiler. Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- cache.h | 3 +++ config.c | 1 + parse-options.c | 18 +++++++++--------- parse-options.h | 4 ++++ 4 files changed, 17 insertions(+), 9 deletions(-) diff --git a/cache.h b/cache.h index 18fdd18f36..0e8e5d8002 100644 --- a/cache.h +++ b/cache.h @@ -1136,6 +1136,9 @@ extern int check_repository_format_version(const char *var, const char *value, v extern int git_env_bool(const char *, int); extern int git_config_system(void); extern int config_error_nonbool(const char *); +#ifdef __GNUC__ +#define config_error_nonbool(s) (config_error_nonbool(s), -1) +#endif extern const char *get_log_output_encoding(void); extern const char *get_commit_output_encoding(void); diff --git a/config.c b/config.c index fb3f8681ee..526f682374 100644 --- a/config.c +++ b/config.c @@ -1660,6 +1660,7 @@ int git_config_rename_section(const char *old_name, const char *new_name) * Call this to report error for your variable that should not * get a boolean value (i.e. "[my] var" means "true"). */ +#undef config_error_nonbool int config_error_nonbool(const char *var) { return error("Missing value for '%s'", var); diff --git a/parse-options.c b/parse-options.c index c1c66bd408..67e98a6323 100644 --- a/parse-options.c +++ b/parse-options.c @@ -18,15 +18,6 @@ int optbug(const struct option *opt, const char *reason) return error("BUG: switch '%c' %s", opt->short_name, reason); } -int opterror(const struct option *opt, const char *reason, int flags) -{ - if (flags & OPT_SHORT) - return error("switch `%c' %s", opt->short_name, reason); - if (flags & OPT_UNSET) - return error("option `no-%s' %s", opt->long_name, reason); - return error("option `%s' %s", opt->long_name, reason); -} - static int get_arg(struct parse_opt_ctx_t *p, const struct option *opt, int flags, const char **arg) { @@ -594,3 +585,12 @@ static int parse_options_usage(struct parse_opt_ctx_t *ctx, return usage_with_options_internal(ctx, usagestr, opts, 0, err); } +#undef opterror +int opterror(const struct option *opt, const char *reason, int flags) +{ + if (flags & OPT_SHORT) + return error("switch `%c' %s", opt->short_name, reason); + if (flags & OPT_UNSET) + return error("option `no-%s' %s", opt->long_name, reason); + return error("option `%s' %s", opt->long_name, reason); +} diff --git a/parse-options.h b/parse-options.h index 71a39c60d9..e703853749 100644 --- a/parse-options.h +++ b/parse-options.h @@ -177,6 +177,10 @@ 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); +#ifdef __GNUC__ +#define opterror(o,r,f) (opterror((o),(r),(f)), -1) +#endif + /*----- incremental advanced APIs -----*/ enum {