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Date: Fri, 6 Jan 2023 06:17:24 -0500
From: Rich Felker <dalias@...c.org>
To: Markus Wichmann <nullplan@....net>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] fix return value of wcs{,n}cmp for near-limits
signed wchar_t values
On Fri, Jan 06, 2023 at 10:20:10AM +0100, Markus Wichmann wrote:
> On Wed, Jan 04, 2023 at 04:07:19PM +0100, Gabriel Ravier wrote:
> > int wcscmp(const wchar_t *l, const wchar_t *r)
> > {
> > for (; *l==*r && *l && *r; l++, r++);
>
> I just noticed this line. Isn't the "&& *r" part superfluous? If r is a
> prefix of l, then *r and *l will be unequal, and the loop will terminate
> because of the first condition alone. (If l is a prefix of r, we need
> the second condition to terminate the loop.)
Yes, but I would assume the compiler would make the same optimization
anyway. The original motivation here may have just been writing it
symmetrically in l and r.
> > - return *l - *r;
> > + return *l < *r ? -1 : *l > *r;
> > }
>
> Ah, that old bug. The problem is that the difference between two 32-bit
> values takes up 33 bits to save. I wonder if it would be worth it to
> just cast the values to 64 bits, then dividing the result by two. You
> know, like
>
> return ((int64_t)*l - *r) >> 1;
>
> Although that does presuppose that wchar_t is smaller than 64 bits,
> which the proposed change does not require.
As you noted later this doesn't work but I think the core flaw here is
not the same as the classic bug. Rather, I probably had a wrong
initial assumption that the function was intended only to work for
valid wide character values of wchar_t not arbitrary integers that fit
in wchar_t arrays. In that case they would be 21-bit and the
difference would never overflow.
Rich
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