Excess registers pushed - regs_ever_live not right way?

[Andrew Hutchinson <andrewhutchinson at cox dot net>]

Register saves by prolog (pushes) are typically made with reference to 
"df_regs_ever_live_p()" or  "regs_ever_live. "||

If my understanding is correct,  these calls reflect register USEs and 
not register DEFs. So if register is used in a function, but not 
otherwise changed, it will get pushed unnecessarily on stack by prolog.

(as noted in this bug  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32871)

I checked a couple of other ports but they all use 
df_regs_ever_live_p(). Indeed this is noted method in manual.

The question is, what df routine or variable can be used to determine 
which registers are DEFs and hence destructively used by a function?

Maybe:  "df_invalidated_by_call"  in conjunction with:    
"df_get_call_refs perhaps()" perhaps?

Andy