[LLVMdev] memory-to-memory instructions

[Evan Cheng]

On Aug 4, 2009, at 10:20 AM, Bagel wrote:

> It appears impossible to match memory-to-memory instructions (except  
> for MOV).
>  The MSP430 supports these.  The following test case tries to match  
> the
> pattern that generates "add.w &foo,&bar":
>
> ; RUN: llvm-as < %s | llc -march=msp430 -O3
> target datalayout = "e-p:16:8:8-i8:8:8-i16:8:8-i32:8:8"
> target triple = "msp430-generic-generic"
> @foo = common global i16 0, align 2
> @bar = common global i16 0, align 2
> define void @x() nounwind {
> 	%1 = load i16* @bar
> 	%2 = load i16* @foo
> 	%3 = add i16 %2, %1
> 	store i16 %3, i16* @foo
> 	ret void
> }
>
> In the DAG matching code generated from the .td file there is :
>
> if (N1.getOpcode() == ISD::ADD &&
>     N1.hasOneUse()) {
>    SDValue N10 = N1.getOperand(0);
>    if (N10.getOpcode() == ISD::LOAD &&
>        N10.hasOneUse() &&
>        IsLegalAndProfitableToFold(N10.getNode(), N1.getNode(),  
> N.getNode()) &&
>        (Chain.getNode() == N10.getNode() ||  
> IsChainCompatible(Chain.getNode(),
>            N10.getNode()))) {
> ... NEED TO MATCH ABOVE TO GENERATE MEMORY TO MEMORY PATTERN
>    }
>
> When debugged we get:
> N10.hasOneUse() = 1
> IsLegalAndProfitableToFold(N10.getNode(), N1.getNode(), N.getNode())  
> = 0
> (Chain.getNode() == N10.getNode()) = 0
> IsChainCompatible(Chain.getNode(), N10.getNode()) = 0
>
> IsLegalAndProfitableToFold() rejects the pattern.  (Yes, I had  
> OptLevel > None).
>
> Before I dig further, can anyone answer the question: Does LLVM  
> currently allow
> a matching a pattern with two loads?

The difficulty with matching these kind of patterns is to avoid cyclic  
dag (tm). IsLegalAndProfitableToFold is probably checking for such  
possibilities and rejected the match. X86 isel goes to great pain to  
pre-process the dag to allow (store (op (load))) instructions to  
match. You can take a look at it.

Evan

>
> thanks,
> bagel
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev