Re: [PATCH] do not depend on undefined signed integer overflow

From: Jonathan Nieder <jrnieder@gmail.com>
Date: Oct 4, 2010, 20:48 UTC
Message-ID: <20101004204848.GI6466@burratino>
In-Reply-To: <1286221388-4908-1-git-send-email-kusmabite@gmail.com>

Erik Faye-Lund wrote:

Show 9 quoted lines

> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -28,6 +28,15 @@
>  #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
>  #define bitsizeof(x)  (CHAR_BIT * sizeof(x))
>  
> +/*
> + * Signed integer overflow is undefined in C and some compilers take
> + * advantage of this when optimizing

[...]

> + */
> +#define signed_add_overflows(a, b) \
> +    ((b) > ((INTMAX_MAX >> (bitsizeof(uintmax_t) - bitsizeof(a))) - (a)))

Clever.  I like it.