linux-kernel - Re: [PATCH v3 3/3] powerpc/64: optimise LOAD_REG_IMMEDIATE

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Message-Id: <1566226318.3km27rs0o4.astroid@bobo.none>
Date:   Tue, 20 Aug 2019 01:05:16 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Segher Boessenkool <segher@...nel.crashing.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH v3 3/3] powerpc/64: optimise LOAD_REG_IMMEDIATE_SYM()

Segher Boessenkool's on August 20, 2019 12:24 am:
> On Mon, Aug 19, 2019 at 01:58:12PM +0000, Christophe Leroy wrote:
>> -#define LOAD_REG_IMMEDIATE_SYM(reg,expr)	\
>> -	lis     reg,(expr)@highest;		\
>> -	ori     reg,reg,(expr)@higher;	\
>> -	rldicr  reg,reg,32,31;		\
>> -	oris    reg,reg,(expr)@__AS_ATHIGH;	\
>> -	ori     reg,reg,(expr)@l;
>> +#define LOAD_REG_IMMEDIATE_SYM(reg, tmp, expr)	\
>> +	lis	reg, (expr)@highest;		\
>> +	lis	tmp, (expr)@__AS_ATHIGH;	\
>> +	ori	reg, reg, (expr)@higher;	\
>> +	ori	tmp, reg, (expr)@l;		\
>> +	rldimi	reg, tmp, 32, 0
> 
> That should be
> 
> #define LOAD_REG_IMMEDIATE_SYM(reg, tmp, expr)	\
> 	lis	tmp, (expr)@highest;		\
> 	ori	tmp, tmp, (expr)@higher;	\
> 	lis	reg, (expr)@__AS_ATHIGH;	\
> 	ori	reg, reg, (expr)@l;		\
> 	rldimi	reg, tmp, 32, 0
> 
> (tmp is the high half, reg is the low half, as inputs to that rldimi).

I guess the intention was also to try to fit the independent ops into
the earliest fetch/issue cycle possible.

#define LOAD_REG_IMMEDIATE_SYM(reg, tmp, expr)	\
	lis	tmp, (expr)@highest;		\
	lis	reg, (expr)@__AS_ATHIGH;	\
	ori	tmp, tmp, (expr)@higher;	\
	ori	reg, reg, (expr)@l;		\
	rldimi	reg, tmp, 32, 0

Very cool series though.

Thanks,
Nick