[llvm-commits] [llvm] r77989 - in /llvm/trunk: lib/CodeGen/LowerSubregs.cpp lib/CodeGen/MachineInstr.cpp test/CodeGen/ARM/2009-08-02-RegScavengerAssert-Neon.ll

[Evan Cheng]

On Aug 4, 2009, at 12:01 PM, Jakob Stoklund Olesen wrote:

>
> On 04/08/2009, at 19.31, Evan Cheng wrote:
>> I am very hesitant to change the semantics. That will break a lot of
>> assumptions in the register allocator. I think the current approach  
>> of
>> having subreg lowering pass insert the right instructions to model
>> "kill" is enough. Is it not?
>
> You are probably right. I am still struggling to completely understand
> the semantics.
>
> We will almost certainly need to insert fake instructions that only
> change flags. I am using IMPLICIT_DEF for that now, but it should
> probably be replaced by a new ALTER_LIFE or something.

I suggest KILL. It's easy to understand.

>
> [...]
>
>>>> Undef operands are "don't care". :-) It would have been cleaner to
>>>> have an Undef machineoperand rather than a register machineoperand
>>>> that's undef. But we still need to assign registers to them so the
>>>> instructions can be translated to machine code.
>
> Ok, so if I understand you correctly, the actual register doesn't
> really matter. It is only there so we produce valid machine code and
> the only restriction is that the register class must be correct. And
> of course there are two-address constraints.

Yep.

>
> If that is correct, we could write an undef operand as %UNDEF. Like
> this:
>
>     %R0<def> = INSERT_SUBREG %UNDEF, %R0L<kill>, 1

Right.

>
> This makes sense to me. Note that in this case %R0 must be dead. Not
> because it is referenced by an <undef>, but because it is not killed
> before being defined.

Yep.

>
> MachineInstr::addRegisterKilled() should not remove the <undef> flag.
> It should completely skip the undef operand, just like the
> regscavenger does.

Ok.

>
> The flags <kill> and <undef> should not be allowed at the same time.
> It is nonsensical.

Yes, that would be nice because it makes the instructions easier to  
read. I thought about changing the machine instruction printer to just  
print <undef> instead of %r0<undef>.


>
> Am I on the right track?

Very much so! Thanks for working on this.

Evan

>
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