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

[Eric Christopher]

> 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.

> 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.

> 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

> http://reviews.llvm.org/D6638

> 

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