[PATCH] [mips] Always clobber $1 for MIPS inline asm.
Eric Christopher
echristo at gmail.com
Tue Jan 6 12:15:33 PST 2015
> 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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