[LLVMdev] BR_CC questions

[deadal nix]

Awesome, I ran into the same thing exactly.

And I have a patch for it. I wasn't sure that there were interest, but
apparently, I'm not the only one.

2014-08-01 14:41 GMT-07:00 Tom Stellard <tom at stellard.net>:

> On Fri, Aug 01, 2014 at 01:23:37PM -0700, Jeff Kuskin wrote:
> > I am implementing a new backend and am pretty sure I don't quite
> understand "the way" one is supposed to implement conditional branches.
> >
> > My target CPU natively supports a conditional branch instruction that
> accepts a condition to test (equal, less than, etc.), two operands (two
> registers, or one register and one immediate), and finally a target PC to
> branch to if the comparison succeeds.
> >
> > Great -- that all seems to mesh directly with the ISD::BR_CC opcode.
> >
> > However... I can't seem to use 'brcc' or 'br_cc' in the .td file.
> Neither is recognized as a valid keyword.  I can use 'brcond', but given
> the capabilities of the CPU I'm targeting, it seems better to implement
> BR_CC directly and do setOperationAction(ISD::BRCOND, MVT::Other, Expand)
> to get BRCOND nodes expanded into BR_CC.  Correct?
> >
> > I tried implementing comparison operations and 'brcond' in the .td file,
> but that yields assembly code that does a comparison instruction followed
> by a branch instruction, when the underlying CPU supports folding the
> comparison into the branch.
> >
> >
> > Is it in fact possible to use BR_CC directly in the TD file?  If so,
> how?  In particular, what pattern would I use?  If not, what is the
> recommended way to proceed (at a high level) when the target CPU supports
> conditional branches as I outlined above?
> >
>
> There is no def in tablegen for br_cc.  I'm not sure why.  If you look in
> include/llvm/Target/TargetSelectionDAG.td, you will see all the SDNodes
> that can be used in TableGen.
>
> You have a few options for handling BR_CC.  You could custom select it
> to a target node during legalization.  It looks like there are a few
> targets that do this.  Or you could manually select it to a MachineSDNode
> in TargetDAGToDAGISel::Select().  You could also define a br_cc node in
> TableGen and write a pattern for it.
>
> -Tom
>
> >
> > Thanks for any help!
> >
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