[LLVMdev] Help with Instruction Expansion on Mips

[Reed Kotler]

YOu have to look for which DAG fragments are not being matched.

Then you can create patterns for those using alternate sequences.

When you disablea given instruction, don't just disable it, but also 
look at what pattern it was matching.

Then create a patten for that using remaining instructions or in some 
cases you might have to call a library function, as in the case of 
floating pointing or maybe even integer multiply if you don't implement 
that.

In some cases, the original instruction may be directly emitted by the 
compiler, outside of the framework td files. Then you'll have to modify 
c++ code and emit an alternate sequence.

Here are some simple patterns from the MipsInstrInfo.td

//===----------------------------------------------------------------------===//
//  Arbitrary patterns that map to one or more instructions
//===----------------------------------------------------------------------===//

// Small immediates
def : MipsPat<(i32 immSExt16:$in),
               (ADDiu ZERO, imm:$in)>;
def : MipsPat<(i32 immZExt16:$in),
               (ORi ZERO, imm:$in)>;
def : MipsPat<(i32 immLow16Zero:$in),
               (LUi (HI16 imm:$in))>;

Here are some which have several instructions

multiclass SetgePats<RegisterClass RC, Instruction SLTOp, Instruction 
SLTuOp> {
   def : MipsPat<(setge RC:$lhs, RC:$rhs),
                 (XORi (SLTOp RC:$lhs, RC:$rhs), 1)>;
   def : MipsPat<(setuge RC:$lhs, RC:$rhs),
                 (XORi (SLTuOp RC:$lhs, RC:$rhs), 1)>;
}


On 07/20/2012 07:20 AM, Geraint Yang wrote:
> Thanks for your reply.
>
> We are trying to implement a simple Mips-based CPU with just for 
> teaching purpose, so we delete some instructions which are not 
> commonly used, thus the task won't be too hard for students. I am 
> responsible for modifying the compiler so that the compiler won't emit 
> unsupported instructions.
>
> In order to avoid "can not select" error, I am trying to expand these 
> instructions when meeting them.
>
> So what should I modify to achieve the goal?
>
>
> Thanks,
> Yang
>
>
> On Fri, Jul 20, 2012 at 10:02 PM, Reed Kotler <rkotler at mips.com 
> <mailto:rkotler at mips.com>> wrote:
>
>     why do you want to "ban" certain instructions?
>
>     is this for some architectural variant?
>
>     the compiler is trying to match patterns from the target
>     independent part of the code generator.
>
>     if you remove instructions, the compiler in many cases will no
>     longer be able to match certain patterns
>     and you will get thos "can not select" messages.
>
>
>     On 07/20/2012 03:05 AM, Geraint Yang wrote:
>>     Hi everyone,
>>
>>     I am a newbie to LLVM. I am trying to ban some of instructions in
>>     Mips Instruction, for example, lh, lhu, sh, and etc.
>>     I have tried to directly comment lh, lhu, and sh to make llvm not
>>     to choose these instruction when compiling, however, it usually
>>     cause a 'can not select ...' error when using 'short' data type
>>     in source code.
>>     Then I tried to expand these instructions in
>>     EmitInstrWithCustomInserter in file
>>     lib/Target/Mips/MipsISelLowering.cpp, just as commit in:
>>
>>     https://github.com/geraint0923/LLVM-ThuMips/commit/6a9ca907f3e945c2bb1f9627e0e5fdcbd7964da2
>>
>>     It works fine for lh and lhu, and fails for sh:
>>     when expanding sh, there are four instruction, but in *.s
>>     generated by modified llvm's llc, there are only two instructions
>>     which are both sb, addiu and srl are gone.
>>
>>     Is there anything wrong when expanding?
>>     And I wonder if there is another method to do the same thing.
>>
>>     Any help will be appreciated.
>>
>>
>>     Thanks,
>>     Yang
>>
>>
>>     _______________________________________________
>>     LLVM Developers mailing list
>>     LLVMdev at cs.uiuc.edu  <mailto:LLVMdev at cs.uiuc.edu>          http://llvm.cs.uiuc.edu
>>     http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
>
> -- 
> Geraint Yang
> Tsinghua University Department of Computer Science and Technology
>
>

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