[PATCH] [mips] Always clobber $1 for MIPS inline asm.

[Daniel Sanders]

> From: Eric Christopher [echristo at gmail.com]
> Sent: 06 January 2015 00:00
> To: Toma Tabacu; Daniel Sanders
> Cc: cfe-commits at cs.uiuc.edu
> Subject: Re: [PATCH] [mips] Always clobber $1 for MIPS inline asm.
> 
> In http://reviews.llvm.org/D6638#103013, @dsanders wrote:
> 
> > > > On Thu Dec 18 2014 at 1:21:53 PM Daniel Sanders <Daniel.Sanders at imgtec.com<mailto:Daniel.Sanders at imgtec.com>> wrote:
> >  > > For the record, gcc does use $at for code generation, take a look at mips_print_operand for the @ symbol and then look at all of the cases it's being used.
> >  > That's correct. However, LLVM currently doesn't use $at for codegen and quite a bit of effort has been put in to making that the case. I'd like to retain this advantage over GCC if I can.
> > >  That's not what the original mail said and there's no real reason to avoid using $at. I'm not sure I understand this rationale.
> >
> > I see where we're confusing you, Toma and I aren't being precise enough. In GCC, $at is a reserved register. As you say, it does use $a
> > in generated code but as far as I know, it only does so as a very short lived temporary. In LLVM, $at is an allocatable register and is
> > freely used in all code generation.
> 
> Yes...

> > > > In GCC inline assembly, $at is still a reserved register and users have taken advantage of this. In LLVM, $at needs to be clobbered by inline
> > > > assembly so that the users usage of $at doesn't break the compiler generated code.
> > > The point I was making is that the rationale for this patch is completely mistaken. i.e. gcc does use $at for code
> > >  generation so unless gcc is adding an implicit clobber of $at in inline asm then it's not going to be correct.
> > GCC goes further than an implicit clobber. It has promised not to keep any values in $at.
> 
> The idea behind the gcc usage is that anything that needs an explicit relocation or was a macro instruction before
> the demacroifying (yay, made up words) uses $at. We can definitely avoid using $at for anything in llvm, just pull it
> out of the register allocation pool?
> 
> I'm not sure where you're going with this?
> 
> -eric

We could stop using $at but why is that beneficial? Why does universally reserving a register to support an (uncommon) construct make more sense to you than having that construct properly indicate that it may clobber a specific register? The former sounds like a sledgehammer to me.

The work pre-dates me but as far as I can tell the developers of the Mips port deliberately decided not to reserve $at and emit code in '.set noat' mode. Presumably this was to see how much of gcc's $at usage is essential to the correct operation of the output code. It turns out that the answer is 'almost none'. In LLVM's CodeGen, $at isn't particularly special. The few instructions/macros/relocations that need a temporary register to be $at describe this with an implicit def of AT. Branches like B are a good example of this (it's worth pointing out that most of the time we don't really need to clobber $at since the branch target will be in range). Where the temporary doesn't really need to be $at, we just ask the register allocator to give us any register. The hard work to avoid reserving $at is already done and I don't see a good reason to change the original decision.