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

[Eric Christopher]

On Wed Jan 07 2015 at 8:40:55 AM Daniel Sanders <Daniel.Sanders at imgtec.com>
wrote:

> > From: Eric Christopher [echristo at gmail.com]
> > Sent: 06 January 2015 20:15
> > To: Toma Tabacu; Daniel Sanders
> > Cc: cfe-commits at cs.uiuc.edu
> > Subject: Re: [PATCH] [mips] Always clobber $1 for MIPS inline asm.
> >
> > > 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.
> >
> > Totally is, but blindly adding clobbers is much the same, just a more
> > focused hammer. I'm not against it, mostly that the original discussion
> > didn't say any of these things.
>
> Agreed. It's currently a compromise between precision and difficulty of
> implementation.
>
> > > 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
> >
> > No, it's been known for a while that the use of macro instructions isn't
> a
> > good idea in the compiler. It's why we switched the gcc port away from
> > using them as well.
>
> Out of curiosity, when did the switch happen? I thought it was emitting
> macros last time I looked (probably around June) but I could be mistaken.
>

2001/2002? At one point it emitted things like la, etc.

-eric


>
> > > 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.
> >
> > Agreed. Please do make sure that the places where you need to do this are
> > commented though. It's pretty poorly commented at the moment.
> >
> > -eric
>
> Toma: Could you add a suitable comment to getClobbers()?
>
> > http://reviews.llvm.org/D6638
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