[LLVMdev] Questions of instruction target description of MSP430

[Heyu Zhu]

Hi Arnaud,

Thank you for your patient explanation.

Regards

2009/12/19 Arnaud Allard de Grandmaison <
Arnaud.AllardDeGrandMaison at dibcom.com>

> Hi,
>
> 1. This instruction is not selected automatically by the instruction
> selector. The instruction combine / select stages insert registercopies,
>  and they are expanded later on by the copyRegToReg() function provided by
> the MSP430InstrInfo to this MOV16rr.
>
> 2. ReMaterializable means there is no need to find a way to preserve the
> value in a register : the instruction can be just be reissued whenever the
> value is needed. This remove some pressure on register alloc, and avoid
> unnecessary spilling.
>
> 3. It says that beside doing an add, it also affects (implicitly) the
> condition codes. This enables to link this node to a following conditional
>  instruction, using the condition codes. It is important that those 2 nodes
> stay stitched together to ensure the condition codes are not clobbered by
> another instruction, for example during instruction scheduling.
>
> Best regards,
> --
> Arnaud de Grandmaison
> ________________________________________
> From: llvmdev-bounces at cs.uiuc.edu [llvmdev-bounces at cs.uiuc.edu] On Behalf
> Of Heyu Zhu [zhu.heyu at gmail.com]
> Sent: Friday, December 18, 2009 3:52 PM
> To: llvmdev at cs.uiuc.edu
> Subject: [LLVMdev] Questions of instruction target description of MSP430
>
> Hi everyone,
>
> I am puzzled by several instruction defines in MSP430.
>
> 1
> def MOV16rr : Pseudo<(outs GR16:$dst), (ins GR16:$src),
>                     "mov.w\t{$src, $dst}",
>                     [ ]>;
> Because it's an empty  dag pattern[ ], by what does instuction selector
> select intruction 'MOV16rr'?
>
>  2
> let canFoldAsLoad = 1, isReMaterializable = 1, mayHaveSideEffects = 1 in {
> def MOV16rm : Pseudo<(outs GR16:$dst), (ins memsrc:$src),
>                "mov.w\t{$src, $dst}",
>                [(set GR16:$dst, (load addr:$src))]>;
> }
> Please gvie me an explaination of the property IsReMaterialiable and the
> benefit if it is true with a simple sample
>
>  3
> def ADD16rr : Pseudo<(outs GR16:$dst), (ins GR16:$src1, GR16:$src2),
>                     "add.w\t{$src2, $dst}",
>                     [(set GR16:$dst, (add GR16:$src1, GR16:$src2)),
>                      (implicit SRW)]>;
> What informaton instuction selector will get by '(implicit SRW)'?
> I can't understand 'implicit SRW' completely.
>
>
> Regards
>
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