[cfe-dev] A question on register allocation

[Renato Golin via cfe-dev]

On 5 October 2015 at 01:48, James Y Knight via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> I don't think it'd actually be terribly difficult: every target already has
> a list of target-specific reserved registers that are setup by
> TargetRegisterInfo::getReservedRegs(const MachineFunction &). Merging that
> builtin set with an set of "extra" reserved registers attached to a
> MachineFunction seems like it'd be easy enough.

It's not about the implementation, but the side effects. There has
been enough discussions on this topic already as a quick search on the
mailing list will show. One may be able to persuade people to accept
it, but this argument above is the first that everyone uses when the
threads start (I have used that, too, myself), and it never works.

So, unless someone comes with a rock solid investigation on all the
side effects on the multiple register allocators and the side effects
on code generation and low-level optimizations for all affected
targets, people will look away. :)


> Of note for the use-case given here: there are no callee-saved xmm registers
> in the sysV ABI. So, unless you re-compile *everything* linked into your
> binary with the reserved register (including libc, dynamic linker, compiler
> runtime support library, etc.) and validate that no asm code in any of those
> clobbers it either, you can't be guaranteed that the value will always be
> preserved -- even if clang has this feature.

Precisely. :)


> On Oct 4, 2015, at 2:06 PM, mats petersson via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> Your problem here is that you also need to "remove" your chosen register
> from the list of registers the compiler may use for other purposes. As far
> as I understand, there's no infrastructure to "remove this register ALWAYS",
> so you'll most likely need to do a fair bit of changes - I'm not familiar
> with this particular bit of code, but it's obviously an ugly/difficult
> enough solution that for the "make Linux compile with Clang" was solved by
> "remove use of global registers", except for "stack pointer" (Stack pointer
> is OK, since compiler will not use that register for anything other than "as
> stack pointer").

Precisely++. :)

cheers,
--renato