[PATCH] xtensa: Make register A0 allocable for the CALL0 ABI

[Takayuki 'January June' Suwa <jjsuwa_sys3175@yahoo.co.jp>]

This patch offers an additional allocable register by RA for the CALL0
ABI.

> Register a0 holds the return address upon entry to a function, but
> unlike the windowed register ABI, it is not reserved for this purpose
> and may hold other values after the return address has been saved.
  - Xtensa ISA Reference Manual,
                   8.1.2 "CALL0 Register Usage and Stack Layout" [p.589]

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (xtensa_conditional_register_usage):
	Remove register A0 from FIXED_REGS if the CALL0 ABI.
---
 gcc/config/xtensa/xtensa.cc | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/gcc/config/xtensa/xtensa.cc b/gcc/config/xtensa/xtensa.cc
index 950eb5a59be..f30725113dc 100644
--- a/gcc/config/xtensa/xtensa.cc
+++ b/gcc/config/xtensa/xtensa.cc
@@ -4970,6 +4970,13 @@ xtensa_conditional_register_usage (void)
   /* Remove hard FP register from the preferred reload registers set.  */
   CLEAR_HARD_REG_BIT (reg_class_contents[(int)RL_REGS],
 		      HARD_FRAME_POINTER_REGNUM);
+
+  /* Register A0 holds the return address upon entry to a function
+     for the CALL0 ABI, but unlike the windowed register ABI, it is
+     not reserved for this purpose and may hold other values after
+     the return address has been saved.  */
+  if (!TARGET_WINDOWED_ABI)
+    fixed_regs[A0_REG] = 0;
 }
 
 /* Map hard register number to register class */
-- 
2.30.2