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Date: Fri, 6 Jan 2023 17:57:08 +0100
From: Markus Wichmann <nullplan@....net>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] fix return value of wcs{,n}cmp for near-limits
signed wchar_t values
Hi all,
I thought a bit more about it. It would be possible to compress the
information we need somewhat like this:
int64_t d = (int64_t)*l - *r;
return (d >> 1) | (d & INT_MAX);
The idea is that I need bit 31 of the output to equal bit 32 of the
difference, and bits 0-30 of the output need to be nonzero exactly if
bits 0-31 of the difference were nonzero. So it's one big disjunction.
So I managed to find a branchless function. Not sure if it is actually
worth it to implement it. The branching version is easier to understand
and apparently gives better machine code on i386 and x86_64 (from just
eyeballing it). It is not even implemented with branches on those
architectures. And it is a micro-optimization, anyway.
Ciao,
Markus
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