[LLVMdev] [RFC, ARM] expanding RET to CopyToReg;BRIND

[Chris Lattner]

On Tue, 30 May 2006, [UTF-8] Rafael Esp?ndola wrote:
> I have changed the way in which the ARM backend generates a function
> return. Instead of expanding a RET to a CopyToReg;RETFLAG, it now
> expands into a CopyToReg;BRIND. I haven't commit it yet, but the patch
> is attached.

Ok, I haven't looked at the code, but you're free to do whatever make 
sense.

> In my opinion the resulting code is easier to understand, but I have
> some questions:
>
> Why all backends use RETFLAG?

The backends seem to be doing the following:

1. For 'ret void', a "ISD::RET" node is left along and not lowered.  As
    such, it gets directly pattern matched.  On PPC, for example, we have:

// Return void support.
def : Pat<(ret), (BLR)>;

    ... which maps it directly to the PPC "blr" instruction.

2. For 'ret value', the targets custom lower the ISD::RET node into some
    number of CopyToReg nodes (to set up the live outs), then need a node
    to represent the return.  The return node has to be flagged do the
    copies, so that the scheduler doesn't make the copies wander from the
    return.

> Why it is named RETFLAG?

Historical reason.  Originally we didn't have nodes that could 
*optionally* have an input flag.  A better design, e.g. on PPC would be to 
have a PPCISD::RET node, which takes an optional input flag, and always 
lower RET to it.

> Why the Copy that places the result must have a Flag operand? If I
> understand correctly, the Flag operand exists in nodes that use a flag
> register (cpsr in ARM).

Flag in the SelectionDAG stuff is so named because it was originally used 
for condition codes.  However, it has since grown to mean "keep these two 
nodes always together".  In the case of return, you want the scheduler to 
produce code like this (on PPC):

   ...
   R3 = outval_virtreg
   blr

not like this:

   ...
   R3 = outval_virtreg
   ...
   blr

So the copy and blr are flagged together.

Another case where flags are useful are for things like the X86 variable 
shift instruction.  There the shift amount is required to be in the CL 
register, so we generate code like this:


    CL = shamt_virtreg
    X = shl Y, CL

We don't want the copy and shift to wander apart from each other (e.g. we 
don't want another shift to get scheduled in between them), so we flag 
them together.  In practice, these copies usually get coallesced away.

-Chris

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