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Re: libgcc: strange optimization
On Tue, Aug 2, 2011 at 2:53 PM, Hans-Peter Nilsson <hp@bitrange.com> wrote:
> On Tue, 2 Aug 2011, Richard Guenther wrote:
>> On Tue, Aug 2, 2011 at 2:06 PM, Mikael Pettersson <mikpe@it.uu.se> wrote:
>> > Michael Walle writes:
>> > ?>
>> > ?> Hi,
>> > ?>
>> > ?> > To confirm that try -fno-tree-ter.
>> > ?>
>> > ?> "lm32-gcc -O1 -fno-tree-ter -S -c test.c" generates the following working
>> > ?> assembly code:
>> > ?>
>> > ?> f2:
>> > ?> ? ? ?addi ? ? sp, sp, -4
>> > ?> ? ? ?sw ? ? ? (sp+4), ra
>> > ?> ? ? ?addi ? ? r2, r0, 10
>> > ?> ? ? ?calli ? ?__ashrsi3
>> > ?> ? ? ?addi ? ? r8, r0, 10
>> > ?> ? ? ?scall
>> > ?> ? ? ?lw ? ? ? ra, (sp+4)
>> > ?> ? ? ?addi ? ? sp, sp, 4
>> > ?> ? ? ?b ? ? ? ?ra
>> >
>> > -fno-tree-ter also unbreaks the ARM test case in PR48863 comment #4.
>>
>> It's of course only a workaround, not a real fix as nothing prevents
>> other optimizers from performing the re-scheduling TER does.
>>
>> I suggest to amend the documentation for local call-clobbered register
>> variables to say that the only valid sequence using them is from a
>> non-inlinable function that contains only direct initializations of the
>> register variables from constants or parameters.
>
> I'd be ok with that, FWIW; I see the problem with keeping the
> scheduling of operations in a working order (yuck) and I don't
> see how else to keep it working ...except perhaps make gcc flag
> functions with register asms as non-inlinable, maybe even flag
> down any of the dangerous re-scheduling?
But then can't people use a pure assembler stub instead? Without
inlining there isn't much benefit left from writing
void f1(int arg)
{
register int a1 asm("r8") = 10;
register int a2 asm("r1") = arg;
asm("scall" : : "r"(a1), "r"(a2));
}
instead of
f1:
mov r8, 10
mov r1, rX
scall
ret
in a .s file no? I doubt much prologue/epilogue is needed.
Or even write
void f1(int arg)
{
asm("mov r8, %0; mov r1 %1; scall;" : : "g"(a1), "g"(a2) : "r8", "r1");
}
which should be inlinable again (yes, in inlined for not optimally
register-allocated, but compared to the non-inline routine?).
Richard.
> Maybe I can do that with some hand-holding?
>
>> Or go one step further and deprecate local register variables alltogether
>> (they IMHO don't make much sense, and rather the targets should provide
>> a way to properly constrain asm inputs and outputs).
>
> They do make sense when implementing e.g. system calls, and
> they're documented to work as discussed. ?(I almost regret
> making that happen, though.) ?Fortunately such functions are
> small, and not relatively much helped by inlining (it's a
> *syscall*; much more happening beyond the call than is affected
> by inlining some parameter initialization). ?Sure, new targets
> are much better off by implementing that through other means,
> but preferably intrinsic functions to asms.
>
> brgds, H-P