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Re: [PATCH] Add MIPS32r2/MT ASE pause instruction

On Wed, 7 Dec 2011, Andrew Pinski wrote:

>   This patch adds the MIPS32r2 (or MT  ASE) pause instruction.  Since
> we already have an encoding in the micromips, nothing is needed to be
> done there.

 Ah, that's a late addition to MIPS32r2 ISA it would seem -- added in 
2.60.  As another computationally insignificant encoding of the useful SLL 
instruction it's of course backwards compatible.

> Index: opcodes/mips-opc.c
> ===================================================================
> RCS file: /cvs/src/src/opcodes/mips-opc.c,v
> retrieving revision 1.89
> diff -u -p -r1.89 mips-opc.c
> --- opcodes/mips-opc.c	29 Nov 2011 20:28:55 -0000	1.89
> +++ opcodes/mips-opc.c	7 Dec 2011 22:39:50 -0000
> @@ -1169,6 +1169,7 @@ const struct mips_opcode mips_builtin_op
>  {"ori",     "t,r,i",	0x34000000, 0xfc000000,	WR_t|RD_s,		0,		I1	},
>  {"pabsdiff.ob", "X,Y,Q",0x78000009, 0xfc20003f,	WR_D|RD_S|RD_T|FP_D,	0,		SB1	},
>  {"pabsdiffc.ob", "Y,Q",	0x78000035, 0xfc2007ff,	RD_S|RD_T|FP_D,		WR_MACC,	SB1	},
> +{"pause",   "",		0x00000140, 0xffffffff, TRAP,			0,		I33|MT32},
>  {"pavg.ob", "X,Y,Q",	0x78000008, 0xfc20003f,	WR_D|RD_S|RD_T|FP_D,	0,		SB1	},
>  {"pickf.ob", "X,Y,Q",	0x78000002, 0xfc20003f,	WR_D|RD_S|RD_T|FP_D,	0,		MX|SB1	},
>  {"pickf.ob", "D,S,T",	0x4ac00002, 0xffe0003f,	WR_D|RD_S|RD_T,		0,		N54	},

 I think it's redundant to say |MT32 here as MT32 already implies I33 -- 
the MT ASE requires rev. 2 of the architecture.

 Also please use NODS rather than TRAP -- although both macros expand to 
the same numerical value, they have been semantically split so that the 
former refers to instructions that must not be scheduled into a delay slot 
for architectural reasons (undefined behaviour) and the latter is for 
instructions that are meant to generate a trap and we chose not to 
schedule these instructions into a delay slot merely to make life of other 
software easier (delay-slot execution is just fine).

 Yes, I know most MT instructions use the TRAP flag, but that's because 
there's really no reason I know of they should have this flag at all in 
the first place, so they have been left as classified "for someone's 
convenience" (some can legitimately trap indeed, i.e. FORK when run out of 
thread contexts).  PAUSE's delay-slot operation is on the other hand 
undefined (which actually brings a concern about forward compatibility of 
old software, hmm...).

 I can't approve the change, but it looks good to me otherwise.


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