[Libre-soc-bugs] [Bug 1229] fosdem2024 llvm simple-v

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=1229

--- Comment #7 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #4)
> (In reply to Jacob Lifshay from comment #1)
> > some points:
> > the register keyword won't be going away completely
> 
> whew
> 
> >
> > register long a asm("r3");
> > asm("addi r3, r3, 1" : "+r"(a));  // increments a
> 
> what i envision is that this:
>    register long a[10] asm ("*r3")

I think we should focus on not requiring specifying exact registers, since imo
that's like half the benefit of compilers -- you don't have to figure out where
everything goes, since that's really hard for humans. e.g. it took me
*hours/days* to figure out where all the registers should go for divmod, and
that's a short function!

> 
> literally traslates to
>    setvl maxvl=10

I think we should avoid requiring the programmer to specify where all the
setvls go, instead we just rely on vector types to track that information, and
the compiler can then insert setvl instructions where necessary. additionally,
compiler optimizations tend to work much better when it doesn't have to worry
about a bunch of global state (VL), and can just reintroduce (insert setvl ops)
the global state at the end after doing the optimizations.

> 
> and marks r3 as vector, such that sv.addi works as expected.
> following on from that...

I think it should just mark `a` as a vector, not r3, since a has the type
`register long[10]` which tells the compiler that it's a vector with MAXVL=10,
so the compiler can assign it to any convenient spot or even optimize it out
completely rather than being forced to keep it in r3 and key all scalar/vector
decisions off of whether r3 is mentioned or not.

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