[v2] xtensa: Make register A0 allocable for the CALL0 ABI
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Commit Message
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.
(xtensa_expand_epilogue): Change to emit '(use (reg:SI A0_REG))'
unconditionally after restoring callee-saved registers for
sibling-call functions, in order to prevent misleading that
register A0 is free to use.
---
gcc/config/xtensa/xtensa.cc | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)
Comments
On Fri, Oct 21, 2022 at 3:46 PM Takayuki 'January June' Suwa
<jjsuwa_sys3175@yahoo.co.jp> wrote:
>
> 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.
> (xtensa_expand_epilogue): Change to emit '(use (reg:SI A0_REG))'
> unconditionally after restoring callee-saved registers for
> sibling-call functions, in order to prevent misleading that
> register A0 is free to use.
> ---
> gcc/config/xtensa/xtensa.cc | 14 ++++++++++----
> 1 file changed, 10 insertions(+), 4 deletions(-)
Regtested for target=xtensa-linux-uclibc, no new regressions.
Committed to master.
@@ -3471,15 +3471,14 @@ xtensa_expand_epilogue (bool sibcall_p)
if (xtensa_call_save_reg(regno))
{
rtx x = gen_rtx_PLUS (Pmode, stack_pointer_rtx, GEN_INT (offset));
- rtx reg;
offset -= UNITS_PER_WORD;
- emit_move_insn (reg = gen_rtx_REG (SImode, regno),
+ emit_move_insn (gen_rtx_REG (SImode, regno),
gen_frame_mem (SImode, x));
- if (regno == A0_REG && sibcall_p)
- emit_use (reg);
}
}
+ if (sibcall_p)
+ emit_use (gen_rtx_REG (SImode, A0_REG));
if (cfun->machine->current_frame_size > 0)
{
@@ -4970,6 +4969,13 @@ 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 */