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 <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/usage.c b/usage.c
index 8eab281..40b3de5 100644
--- a/usage.c
+++ b/usage.c
@@ -130,6 +130,7 @@
va_end(params);
}
+#undef error
int error(const char *err, ...)
{
va_list params;
