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

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Message-ID: <3d4e757f-84cd-7a48-0b5a-d39c44ea33ea@c-s.fr>
Date:   Wed, 14 Aug 2019 11:46:41 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Paul Mackerras <paulus@...abs.org>
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



Le 14/08/2019 à 04:08, Paul Mackerras a écrit :
> 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.

Ah yes, thanks. And it is also buggy when \x is over 0x80000000 because 
lis is a signed ops

I'll send v2

Christophe