linux-kernel - [PATCH] Fix constant folding and poor optimization in byte swapping code

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Message-ID: <Pine.LNX.4.58.0702161617330.16341@shell2.speakeasy.net>
Date:	Fri, 16 Feb 2007 16:19:20 -0800 (PST)
From:	Trent Piepho <xyzzy@...akeasy.org>
To:	Linux Kernel Mailing list <linux-kernel@...r.kernel.org>
cc:	Andrew Morton <akpm@...l.org>,
	Francois-Rene Rideau <fare@...es.org>
Subject: [PATCH] Fix constant folding and poor optimization in byte swapping
 code

Constant folding does not work for the swabXX() byte swapping functions,
and the C versions optimize poorly.

Attempting to initialize a global variable to swab16(0x1234) or put
something like "case swab32(42):" in a switch statement will not compile.
It can work, swab.h just isn't doing it correctly.  This patch fixes
that.

Contrary to the comment in asm-i386/byteorder.h, gcc does not recognize
the "C" version of swab16 and turn it into efficient code.  gcc can do
this, just not with the current code.  The simple function:

u16 foo(u16 x) { return swab16(x); }

Would compile to:
        movzwl  %ax, %eax
        movl    %eax, %edx
        shrl    $8, %eax
        sall    $8, %edx
        orl     %eax, %edx

With this patch, it will compile to:
        rolw    $8, %ax

I also attempted to document the maze different macros/inline functions
that are used to create the final product.
View attachment "swab-fix.patch" of type "TEXT/PLAIN" (7074 bytes)