On 09/11/2015 02:49 PM, Segher Boessenkool wrote:
On PowerPC, this enables shrink-wrapping of about 2%-3% more functions.
I expected more, but in most cases this would help we cannot yet shrink-
wrap because there are non-volatile registers used, often in the first
block already.
I looked into shrink-wrapping earlier this year and noticed the same. I
tried expanding ira.c:split_live_ranges_for_shrink_wrap() to do a more
general live range splitting than the limited parm_reg->pseudo copies it
tries splitting now, but the splits ended up getting coalesced back
together and a non-volatile chosen. Unfortunately my digging got put on
the back burner and I haven't looked at it for a while.
Another issue I saw for PowerPC that prevented shrink-wrapping was the
case where non-volatile CR's are used somewhere in the function. This
causes a 'mfcr' to be generated in the prolog to save the non-volatile
CR's, which currently lists all CR's as used. This kills shrink-wrapping
since the early exit test will define a CR which then fails the
shrink-wrap requirement of not defining a register used in the prolog. I
tried a simple hack of just removing the volatile CR's from the
'movesi_from_cr' define_insn and it solved the issue, but never
submitted a patch for just that piece.
-Pat
