linux-kernel - Re: [PATCH] powerpc/32: Remove one insn in _

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Message-ID: <20160811221119.GA26763@gate.crashing.org>
Date:	Thu, 11 Aug 2016 17:11:19 -0500
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Gabriel Paubert <paubert@...m.es>
Cc:	Christophe Leroy <christophe.leroy@....fr>,
	linux-kernel@...r.kernel.org, Scott Wood <oss@...error.net>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/32: Remove one insn in __bswapdi2

On Thu, Aug 11, 2016 at 11:34:37PM +0200, Gabriel Paubert wrote:
> On the other hand gcc did at the time a very poor job (quite an
> understatement) at bswapdi when compiling for 64 bit processors 
> (see the example).
> 
> But what do modern compilers generate for bswapdi these days? Do they
> still call the library or not?

Nope.

> After all, bswapdi on 32 bit processors only takes 6 instructions if the
> input and output registers don't overlap.

For this testcase:
===
typedef unsigned long long u64;
u64 bs(u64 x) { return __builtin_bswap64(x); }
===

we get with -m32:
===
bs:
	mr 9,3
	rotlwi 3,4,24
	rlwimi 3,4,8,8,15
	rlwimi 3,4,8,24,31
	rotlwi 4,9,24
	rlwimi 4,9,8,8,15
	rlwimi 4,9,8,24,31
	blr
===

and with -m64:
===
.L.bs:
	srdi 10,3,32
	mr 9,3
	rotlwi 3,3,24
	rotlwi 8,10,24
	rlwimi 3,9,8,8,15
	rlwimi 8,10,8,8,15
	rlwimi 3,9,8,24,31
	rlwimi 8,10,8,24,31
	sldi 3,3,32
	or 3,3,8
	blr
===

Neither as tight as possible, but neither horrible either.


Segher