Re: PR 34998: Tracking of subreg liveness in global.c and ra-conflict.c

[Ian Lance Taylor <iant at google dot com>]

David Edelsohn <dje@watson.ibm.com> writes:

> >>>>> Richard Sandiford writes:
> 
> > So I think the patch really does model the intended behaviour of the
> > pseudo-register access in this case.  E.g. if we allocate an SImode
> > pseudo FOO to two HImode registers on a 32-bit target, a set of
> > (subreg:HI (reg:SI FOO) ...) really should clobber both of the HImode
> > registers.  ("..." is either 0 or 2, depending on endianness.  The other
> > value isn't valid.)
> 
> 	The analysis and patch seem to contradict changes made to the
> PowerPC port in 2000:
> 
> http://gcc.gnu.org/ml/gcc-patches/2000-01/msg00899.html
> http://gcc.gnu.org/ml/gcc-patches/2000-01/msg00921.html
> 
> A pattern implemented
> 
> (set (subreg:SI (reg:DI foo)) (reg:SI bar))
> 
> and the lifetime analysis of the period decided the pseudo was live since
> the beginning.  Geoff Keating and RTH agreed that the SUBREG should be
> changed to an UNSPEC specifically to express that the pattern killed the
> entire pseudo.  Maybe the semantics have changed / been clarified in the
> intervening time.

Richard is talking about HImode => SImode.  You are talking about
SImode => DImode, for a 32-bit target.  These are different cases,
because multi-word subregs operate one word at a time.  Richard's
patch effectively expands the set of the HImode subreg to clobber the
whole SImode.  His patch would not do anything for a set of an SImode
subreg on a 32-bit target.

Ian