RE: Coloring problem - Pass 0 for finding allocno costs

["Ian Bolton" <bolton at IceraSemi dot com>]

> -----Original Message-----
> From: gcc-owner@gcc.gnu.org [mailto:gcc-owner@gcc.gnu.org] On Behalf Of
> Michael Matz
> Sent: 18 March 2010 15:13
> To: Frank Isamov
> Cc: gcc@gcc.gnu.org
> Subject: Re: Coloring problem - Pass 0 for finding allocno costs
> 
> Hi,
> 
> On Thu, 18 Mar 2010, Frank Isamov wrote:
> 
> > From the h file:
> >
> > #define REG_CLASS_CONTENTS
> Â Â\
> > Â{
> > Â Â Â Â Â Â \
> > Â Â{0x00000000, 0x00000000, 0x00000000}, /* NO_REGS*/ Â Â Â Â Â\
> > Â Â{0xFFFFFFFF, 0x0000FFFF, 0x00000000}, /* D_REGS*/ Â Â Â Â Â\
> > Â Â{0x00000000, 0xFFFF0000, 0x0000FFFF}, /* R_REGS*/ Â Â Â Â Â \
> >
> > ABI requires use of R registers for arguments and return value. Other
> > than that all of these instructions are more or less symmetrical in
> > sense of using D or R.
> 
> If that is so, you're better off not using two different register
> classes
> at all.  Register classes are there to describe assymetry in the ISA
> register file.  For describing ABI constraints like argument or
> caller-save registers look at FUNCTION_ARG, FUNCTION_ARG_REGNO_P,
> FUNCTION_VALUE_REGNO_P and CALL_USED_REGISTERS (and some friends).
> 
> The compiler will e.g. try to make sure to first use call-clobbered
> registers in leaf functions, so that they don't need to be
> saved/restored
> in the prologue/epilogue.  You don't need register classes to describe
> this.

Good spot!  But ...

In the original post, Frank mentioned that all reg classes in an
insn must match ("all registers in an insn should be from the same
register class"), so I don't think a single class would suffice because
there would be no way to enforce the above requirement.

However, I think a superclass that contains both would work, and the
CALL_USED_REGISTERS define can be used to show just the R_REGS.
Meanwhile, the matching class requirement can still be enforced via the
operand constraints.

I might be wrong, however.  I am still learning about IRA too, so feel
free to teach me something new ;-)