[ira] Minor fix to REG_CLASS_UNION computation

[Jeff Law]

Let's say a port has defined some register classes.  DATA_REGS,
EXTENDED_REGS and DATA_OR_EXTENDED_REGS.    Pretty normal stuff, two
fundamental classes and a union of those classes.

Now on the mn103, the registers in EXTENED_REGS are not available in the
base mn103 chip -- they are only available on follow-on chips.  So, when
compiling for the bare mn103, we consider the registers in EXTENED_REGS
to be unavailable.  In that situation I'd expect the compiler to never
choose EXTENDED_REGS or DATA_OR_EXTENED_REGS for any preferred or
alternate register class.

However, if we look at the dumps when IRA is enabled we see:

     a80 (r128,l4) best DATA_OR_EXTENDED_REGS, cover DATA_OR_ADDRESS_REGS
     a66 (r126,l3) best DATA_OR_EXTENDED_REGS, cover DATA_OR_ADDRESS_REGS
     a49 (r123,l2) best DATA_OR_EXTENDED_REGS, cover DATA_OR_ADDRESS_REGS
     a28 (r122,l1) best DATA_OR_EXTENDED_REGS, cover DATA_OR_ADDRESS_REGS
     a27 (r121,l1) best DATA_OR_EXTENDED_REGS, cover DATA_OR_ADDRESS_REGS
     a24 (r118,l1) best DATA_OR_EXTENDED_REGS, cover DATA_OR_ADDRESS_REGS


This is a trivial problem in how IRA computes union sets.  It should
find the smallest (according to the enumeration values in REG_CLASS) set
which is the union of the input sets.  Instead it'll find the largest
(according to the enumeration values in REG_CLASS) --  note carefully
that the active contents are the same since the extra registers in
DATA_OR_EXTENED_REGS are unavailable when compiling for the base mn103 port.


With this trivial fix we get the much more sane looking:


     a80 (r128,l4) best DATA_REGS, cover DATA_OR_ADDRESS_REGS
     a66 (r126,l3) best DATA_REGS, cover DATA_OR_ADDRESS_REGS
     a49 (r123,l2) best DATA_REGS, cover DATA_OR_ADDRESS_REGS
     a28 (r122,l1) best DATA_REGS, cover DATA_OR_ADDRESS_REGS
     a27 (r121,l1) best DATA_REGS, cover DATA_OR_ADDRESS_REGS
     a24 (r118,l1) best DATA_REGS, cover DATA_OR_ADDRESS_REGS


This change just tells the union code to ignore larger enumerated sets
which are identical to any union set that has already been found.  It 
shouldn't ever change the generated code, just the debugging dumps.

	* ira.c (setup_reg_class_intersect_union): Choose smallest
	union class containing the desired registers, not the largest
	union class.





-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: ira.c.patch
URL: <http://gcc.gnu.org/pipermail/gcc-patches/attachments/20080823/23af928e/attachment.ksh>