Tom de Vries <tom_devr...@mentor.com> writes:
> On 16-01-14 09:13, Richard Sandiford wrote:
>> Tom de Vries <tom_devr...@mentor.com> writes:
>>> * The set of registers which are clobbered during a call by things
>>> like the plt
>>> - these are not picked up by the use-caller-save optimization. We
>>> need the
>>> hook to inform the compiler about these registers
>>
>> Right, but...
>>
>>> * And finally, registers clobbered in the caller itself during a sequence of
>>> instructions implementing a function call. On mips, that's R6,
>>> which may be
>>> clobbered by the call. Normally that doesn't need mentioning in
>>> the RTL since
>>> it's a call_used_reg, but since use-caller-save might discover a set of
>>> registers for the called function that does not include R6, it becomes
>>> important to record this clobber explicitly. It could be
>>> represented in the
>>> RTL by a clobber on the insn, or a clobber in C_I_F_U. Or it
>>> could just be
>>> part of the registers returned by the hook - but that was
>>> previously deemed
>>> not acceptable (and it doesn't match the description of the hook).
>>
>> ...why do we need two different mechanisms to deal with these two?
>> IMO the set recorded for the callee should contain what the callee
>> instructions clobber and nothing else. CALL_INSN_FUNCTION_USAGE
>> should contain everything clobbered by a call outside the callee,
>> whether that's in the calling function itself, in a PLT, in a MIPS16
>> stub, or whatever.
>>
>
> Richard,
>
> Is this what you mean?
>
> This patch introduces a hook that specifies which registers are implicitly
> clobbered by a call, not including the registers that are clobbered in the
> called function, and then uses that hook to add those registers to
> CALL_INSN_FUNCTION_USAGE.
I don't think a new hook is needed. The call patterns should just add
the registers to CALL_INSN_FUNCTION_USAGE when generating the call insn.
MIPS already does this some cases where normally-call-saved registers
are actually clobbered:
/* If we are handling a floating-point return value, we need to
save $18 in the function prologue. Putting a note on the
call will mean that df_regs_ever_live_p ($18) will be true if the
call is not eliminated, and we can check that in the prologue
code. */
if (fp_ret_p)
CALL_INSN_FUNCTION_USAGE (insn) =
gen_rtx_EXPR_LIST (VOIDmode,
gen_rtx_CLOBBER (VOIDmode,
gen_rtx_REG (word_mode, 18)),
CALL_INSN_FUNCTION_USAGE (insn));
Although we really should have a utility function like use_reg, but for
clobbers, so that the above would become:
clobber_reg (&CALL_INSN_FUNCTION_USAGE (insn), gen_rtx_REG (word_mode, 18));
Thanks,
Richard
