Implement the software ShadowCallStack for Hexagon.
On Hexagon, r19 is used as the shadow stack pointer (reserved via
-ffixed-r19). On function entry the LR (r31) is saved to the shadow
stack and the pointer is advanced; on exit the LR is restored from the
shadow stack before returning.
Prologue sequence:
r19 = add(r19, #4)
memw(r19+#-4) = r31
Epilogue sequence (between deallocframe/jumpr r31):
r31 = memw(r19+#-4)
r19 = add(r19, #-4)
SCS is only emitted when hasFP() is true: the function uses allocframe.
Leaf functions that never touch the stack do not need protection.
When SCS is active the combined L4_return (deallocframe+jump) is not
used; instead L2_deallocframe and PS_jmpret are kept separate so the
shadow-stack restore can be inserted between them.
Brian Cain
2919ad75af