https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98692
--- Comment #22 from jseward at acm dot org ---
Looking back at the above, it's now clearer what the problem is:
# Park potentially live data in the red zone
_savegpr0_14: std r14,-144(r1)
_savegpr0_15: std r15,-136(r1)
_savegpr0_16: std r16,-128(r1)
_savegpr0_17: std r17,-120(r1)
_savegpr0_18: std r18,-112(r1)
_savegpr0_19: std r19,-104(r1)
_savegpr0_20: std r20,-96(r1)
_savegpr0_21: std r21,-88(r1)
_savegpr0_22: std r22,-80(r1)
_savegpr0_23: std r23,-72(r1)
_savegpr0_24: std r24,-64(r1)
_savegpr0_25: std r25,-56(r1)
_savegpr0_26: std r26,-48(r1)
_savegpr0_27: std r27,-40(r1)
_savegpr0_28: std r28,-32(r1)
_savegpr0_29: std r29,-24(r1)
_savegpr0_30: std r30,-16(r1)
_savegpr0_31: std r31,-8(r1)
std r0, 16(r1)
# And ka-zap! Memcheck paints -288(r1) .. -1(r1) as undefined.
blr
# So now they're all "unusable".
