[LLVMdev] Questions of instruction target description of MSP430

Arnaud Allard de Grandmaison Arnaud.AllardDeGrandMaison at dibcom.com
Sat Dec 19 04:48:57 CST 2009


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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