linux-kernel - Re: [PATCH 1/2] powerpc: rewrite LOAD_REG

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Message-ID: <20190814020803.it7i7mjxyruu4vy3@oak.ozlabs.ibm.com>
Date:   Wed, 14 Aug 2019 12:08:03 +1000
From:   Paul Mackerras <paulus@...abs.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        segher@...nel.crashing.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 1/2] powerpc: rewrite LOAD_REG_IMMEDIATE() as an
 intelligent macro

On Tue, Aug 13, 2019 at 09:59:35AM +0000, Christophe Leroy wrote:

[snip]

> +.macro __LOAD_REG_IMMEDIATE r, x
> +	.if \x & ~0xffffffff != 0
> +		__LOAD_REG_IMMEDIATE_32 \r, (\x) >> 32
> +		rldicr	\r, \r, 32, 31
> +		.if (\x) & 0xffff0000 != 0
> +			oris \r, \r, (\x)@__AS_ATHIGH
> +		.endif
> +		.if (\x) & 0xffff != 0
> +			oris \r, \r, (\x)@l
> +		.endif
> +	.else
> +		__LOAD_REG_IMMEDIATE_32 \r, \x
> +	.endif
> +.endm

Doesn't this force all negative constants, even small ones, to use
the long sequence?  For example, __LOAD_REG_IMMEDIATE r3, -1 will
generate (as far as I can see):

	li	r3, -1
	rldicr	r3, r3, 32, 31
	oris	r3, r3, 0xffff
	ori	r3, r3, 0xffff

which seems suboptimal.

Paul.