[LLVMdev] How do I model MUL with multiply-accumulate instruction?

Evan Cheng evan.cheng at apple.com
Thu Apr 16 17:15:45 PDT 2009

On Apr 16, 2009, at 2:19 PM, Greg McGary wrote:

> The only multiplication instruction on my target CPU is
> multiply-and-accumulate.  The result goes into a special register that
> can destructively read at the end of a sequence of multiply-adds.  The
> following sequence is required to so a simple multiply:
>    acc r0   # clear accumulator, discarding its value (r0 reads as 0,
> and sinks writes)
>    mac rSRC1, rSRC2   # multiply sources, store result in accumulator
>    acc rDEST   # fetch accumulator value to rDEST
> What's the best way to model simple MUL as this 3-insn sequence in the
> LLVM backend?
> Should the internal accumulation register be explicitly modeled as its
> own register class with a pattern to copy its value to a general  
> register?
> Is it possible to code the three insn sequence in TableGen alone, or
> must I resort to custom C++ code?

Probably c++ code. You want to lower it into the target node followed  
by a copyfromreg from the accumulator to a virtual register.


> G
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