llvm dev - Aug 2009 - [LLVMdev] Methods for filing delay slots.

Hi,

I was looking over the experimental MIPS backend and noticed that it has a delay
slot pass which just inserts nops into the delay slots. I assume it should be
possible to do a bit better than this. Is there an existing pass which
"fills" delay slots or would I have to write one if I wanted slightly
more optimal code? (anyone have any references?)

Thanks in advance.

On Sun, Aug 2, 2009 at 2:06 PM, Carter Cheng<carter_cheng at yahoo.com>
wrote:
> I was looking over the experimental MIPS backend and noticed that it has a
delay slot pass which just inserts nops into the delay slots. I assume it should
be possible to do a bit better than this. Is there an existing pass which
"fills" delay slots or would I have to write one if I wanted slightly
more optimal code? (anyone have any references?)
I don't think there's an existing optimizing delay slot filler.  The
LLVM SPARC implementation also has a delay slot filler, but it's
almost identical to the MIPS one.

It's not that difficult to move an instruction; you'll get the idea
pretty quickly looking at MachineBasicBlock.h.  Of course, finding an
instruction which can be moved is the tricky part; as a first step,
you could try moving the instruction immediately before a direct
unconditional branch into the delay slot.

-Eli

Thanks. 

--- On Sun, 8/2/09, Eli Friedman <eli.friedman at gmail.com> wrote:
> From: Eli Friedman <eli.friedman at gmail.com>
> Subject: Re: [LLVMdev] Methods for filing delay slots.
> To: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Date: Sunday, August 2, 2009, 2:31 PM
> On Sun, Aug 2, 2009 at 2:06 PM,
> Carter Cheng<carter_cheng at yahoo.com>
> wrote:
> > I was looking over the experimental MIPS backend and
> noticed that it has a delay slot pass which just inserts
> nops into the delay slots. I assume it should be possible to
> do a bit better than this. Is there an existing pass which
> "fills" delay slots or would I have to write one if I wanted
> slightly more optimal code? (anyone have any references?)
> 
> I don't think there's an existing optimizing delay slot
> filler.  The
> LLVM SPARC implementation also has a delay slot filler, but
> it's
> almost identical to the MIPS one.
> 
> It's not that difficult to move an instruction; you'll get
> the idea
> pretty quickly looking at MachineBasicBlock.h.  Of
> course, finding an
> instruction which can be moved is the tricky part; as a
> first step,
> you could try moving the instruction immediately before a
> direct
> unconditional branch into the delay slot.
> 
> -Eli
> 
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