Re: [PATCH] strub: disable on rl78

[Jeff Law <jeffreyalaw@gmail.com>]

On 12/8/23 19:18, Alexandre Oliva wrote:
> Hello, Jeff, DJ,
> 
> Thanks for the info.
> 
> On Dec  7, 2023, Jeff Law <jeffreyalaw@gmail.com> wrote:
> 
>> On 12/6/23 15:03, DJ Delorie wrote:
>>> Alexandre Oliva <oliva@gnu.org> writes:
>>>> This looks like a latent bug in the port.
>>> I'm not surprised, that port was weird.
>>>
>>>> This was just a plain asm insn in strub.c:
>>>>
>>>> /* Make sure the stack overwrites are not optimized away.  */
>>>> asm ("" : : "m" (end[0]));
>>>>
>>>> whose constraint passes during reload, rl78_alloc_physical_registers
>>>> leaves it alone and clears virt_insns_ok, so when cprog_hardreg attempts
>>>> to extract constraints again, rl78_as_legitimate_address rejects r8 as
>>>> the address because virt insns are no longer ok.
>>> Some background: the "virtual" registers are memory-mapped registers
>>> that use a common addressing scheme to access non-mapped registers.
>>> When we convert from virtual to physical, we can map that reg to a
>>> physical reg, or we replace the reg with a mem, but then usually have to
>>> split up the insn.
>>> For this insn, "converting" should have mapped or reloaded r8 to a
>>> valid
>>> address register.  I doubt we have a way to have two patterns for user
>>> asms like we do for, say, addhi3.
> 
> *nod*, the virt-to-phys "reloader" should care of it it, but it doesn't
> because it explicitly punts on most ASMs, and it sort-of implicitly
> punts on inputs-only asms like the one in strub.c.
> 
>> Given we don't know the semantics of what goes on inside the MEM I
>> think rewriting would be extraordinarily difficult.
> 
> ISTM that it would need to know what's inside, any more than it (or
> reload) does.  It just needs to find (or make) some available phys
> register and use that as the address instead of the virt register, like
> it presumably does for other kinds of insns.
> 
>> I wouldn't lose any sleep if we had a way to simply disable strub for rl78.
> 
> Now that we have easy means to do that, it became irresistible ;-)
> 
> Tested building libgcc (all multilibs) with a cross rl78-elf.  Ok to install?
> 
> 
> rl78 allocation of virtual registers to physical registers doesn't
> operate on asm statements, and strub uses asm statements in the
> runtime and in the generated code, to the point that the runtime
> won't build.  Force strub disabled on that target.
> 
> 
> for  gcc/ChangeLog
> 
> 	* config/rl78/rl78.c (TARGET_HAVE_STRUB_SUPPORT_FOR): Disable.
> ---
>   gcc/config/rl78/rl78.cc |    5 +++++
>   1 file changed, 5 insertions(+)
OK for the trunk.  THanks.

jeff

>