[llvm-dev] Generate Register Indirect mode instruction

[Friedman, Eli via llvm-dev]

On 10/12/2016 2:22 PM, Alex Bradley wrote:
>
>
> > You probably want to look at the x86 backend; it has a lot of 
> instructions which involve both computation and memory.  Take the 
> following IR, a variant of your example:
> >
> > define void @foo(i32 *%a, i32 *%b, i32 *%c) {
> > entry:
> >   %0 = load i32, i32* %a, align 4
> >   %1 = load i32, i32* %b, align 4
> >
> >   %add = add nsw i32 %1, %0
> >   store i32 %add, i32* %c, align 4
> >   ret void
> > }
> >
> > The x86 backend generates the following:
> >
> >         movl    (%rsi), %eax
> >         addl    (%rdi), %eax
> >         movl    %eax, (%rdx)
> >         retq
> >
> > Note in particular the memory operand embedded into the addition.
> >
> > The way the LLVM x86 backend models this is just to pattern match it 
> during instruction selection: it matches a pattern like  (add r, (load 
> addr)) to a single instruction.
>
> Thanks Eli. I will have a look into it. However, the above x86 code 
> loads the content of the memory location into eax register and adds 
> that register with another memory location.
>
> My target loads the address of the memory locations in the registers 
> for both the operands and then uses add operation on the registers in 
> an indirect way. How do I specify that in .td files so that it matches 
> in ISelDAGToDAG select() function? Any small example?
>

Oh, you mean the result goes into memory, not a register?  So, something 
like the following:

define void @foo(i32 *%a) {
entry:
   %0 = load i32, i32* %a, align 4
   %add = add i32 %0, 3
   store i32 %add, i32* %a, align 4
   ret void
}

On x86, this gets turned into:

         addl    $3, (%rdi)
         retq

 From X86InstrArithmetic.td:

// BinOpMI8_RMW - Instructions like "add [mem], imm8".
class BinOpMI8_RMW<string mnemonic, X86TypeInfo typeinfo,
                    SDPatternOperator opnode, Format f>
   : BinOpMI8<mnemonic, typeinfo, f,
              [(store (opnode (load addr:$dst),
                              typeinfo.Imm8Operator:$src), addr:$dst),
               (implicit EFLAGS)]>;

-Eli

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