From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jonathan Nieder <jrnieder@gmail.com>
Subject: Re: [PATCH] do not depend on undefined signed integer overflow
Date: Mon, 4 Oct 2010 15:48:48 -0500
Message-ID: <20101004204848.GI6466@burratino>
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Cc: git@vger.kernel.org
To: Erik Faye-Lund <kusmabite@gmail.com>
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Erik Faye-Lund wrote:

> --- 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.