[Bug target/64154] New: enable fuse-caller-save for Thumb1

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64154

            Bug ID: 64154
           Summary: enable fuse-caller-save for Thumb1
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org

For PR 63718, we disabled -fuse-caller-save for Thumb1 in arm.c:
...
  /* In Thumb1 mode, we emit the epilogue in RTL, but the last insn
     - epilogue_insns - does not accurately model the corresponding insns
     emitted in the asm file.  In particular, see the comment in thumb_exit
     'Find out how many of the (return) argument registers we can corrupt'.
     As a consequence, the epilogue may clobber registers without
     fuse-caller-save finding out about it.  Therefore, disable
fuse-caller-save
     in Thumb1 mode.
     TODO: Accurately model clobbers for epilogue_insns and reenable
     fuse-caller-save.  */
  if (TARGET_THUMB1)
    flag_use_caller_save = 0;
...

We want to fix the target to properly model the clobbers in the rtl insn
epilogue_insns, such that we can re-enable -fuse-caller-save.