On Tue, 22 Nov 2022 13:50:28 PST (-0800), jeffreya...@gmail.com wrote:
On 11/22/22 08:29, Palmer Dabbelt wrote:
On Tue, 22 Nov 2022 07:20:15 PST (-0800), jeffreya...@gmail.com wrote:
On 11/20/22 18:36, Kito Cheng wrote:
So the idea here is just to define the extension so that it gets
defined
in the ISA strings and passed through to the assembler, right?
That will also define arch test marco:
https://github.com/riscv-non-isa/riscv-c-api-doc/blob/master/riscv-c-api.md#architecture-extension-test-macro
Sorry I should have been clearer and included the test macro(s) as well.
So a better summary would be that while it doesn't change the codegen
behavior in the compiler, it does provide the mechanisms to pass along
isa strings to other tools such as the assembler and signal via the test
macros that this extension is available.
IMO the important bit here is that we're not adding any compatibility
flags, like we did when fence.i was removed from the ISA. That's fine
as long as we never remove these instructions from the base ISA in the
software, but that's what's suggested by Andrew in the post.
Right. IIUC these instructions were never supposed to be in the base
ISA, but, in effect, snuck through. We're retro-actively adding them as
an extension, at least in terms of ISA strings & test macros. We're
currently (forever?) going to allow them in the assembler without
strictly requiring the extension be on.
That'd the the idea.
It's a super weird one, but there's a bunch of cases in RISC-V where
we're told to just ignore words in the ISA manual. Definitely a trap
for users (and we already had some Linux folks get bit by the counter
changes here), but that's just how RISC-V works.
Mistakes happen. The key is to adjust for them as best as we can.
I'd lean towards a stricter enforcement, bringing these
instructions/extension in line with how we handle the others. It'd
potentially mean source incompatibilities that would need to be fixed,
but they shouldn't be difficult and we're still early enough in the game
that we *could* take that route. Andrew's position is more
accommodating of existing code and while I may not 100% agree with his
position, I understand it.
So while I'd lean towards a stricter checking, I can live with this
approach. I wouldn't mind hearing from Kito, Philipp and others though.
That's the sort of thing we've traditionally done: essentially just read
the actual words in the PDF and produce implementations that match
those, tagging versions when things change (the fence.i stuff is a good
example). After some amount of time we can then move the default spec
version over to the new one. That's a little bit of churn for users,
but it shouldn't be all that bad.
IMO that's the sane way to go, I'd certainly expect to be able to read
the words in the PDFs and go implement things according to them. It's
pretty clearly not what the ISA folks want, though.
There's also the secondary issue of getting ISA strings to match between
the various bits of the software stack that uses them. We're trying to
move away from ISA strings as a stable uABI in Linux for exactly this
reason, but ISA strings have already ended up all over the place so
there's only so much we can do.
