Add notes for the Cranelift project meeting on 16.05.2022 (#4158)

Copyright (c) 2022, Arm Limited.

meetings/cranelift/2022/cranelift-05-16.md

@@ -9,12 +9,93 @@
     1. Please help take notes.
     1. Thanks!
 1. Announcements
-    1. _Submit a PR to add your announcement here_
+    1. no announcements
 1. Other agenda items
-    1. _Submit a PR to add your item here_
+    1. no fixed agenda
 
 ## Notes
 
 ### Attendees
 
+* Alexa VanHattum (avanhatt)
+* Andrew Brown (abrown)
+* Anton Kirilov (akirilov-arm)
+* Benjamin Bouvier (bnjbvr)
+* bjorn3
+* Chris Fallin (cfallin)
+* Johnnie Birch (jlb6740)
+* Nick Fitzgerald (fitzgen)
+* Ulrich Weigand (uweigand)
+
 ### Notes
+
+* regalloc2 regressions
+    * Ulrich Weigand: rebased the PR migrating call and return instructions to ISLE for s390x
+    * Ulrich Weigand: on top of regalloc2 and noticed regressions in the generated code, i.e.
+    * Ulrich Weigand: lower quality results
+    * Ulrich Weigand: old implementation would move directly into the output register
+    * Ulrich Weigand: the new one creates a new vreg that is aliased to the output register
+    * Chris Fallin: would like to have a look, is there a public branch to check out
+    * Chris Fallin: don't expect extra vregs and moves
+    * Ulrich Weigand: also, the register allocator uses callee-saved registers instead of
+    * Ulrich Weigand: caller-saved ones
+    * Ulrich Weigand: the second issue is that with the previous register allocator there was
+    * Ulrich Weigand: a way to influence the order in which registers are preferred
+    * Ulrich Weigand: s390x uses load and store multiple that accept ranges of registers and that
+    * Ulrich Weigand: become inefficient if successive registers are not used
+    * Chris Fallin: random choice of registers is a deliberate design decision that led to
+    * Chris Fallin: improvements on x86-64; is the issue that non-contiguous ranges are used?
+    * Ulrich Weigand: yes, the code saves and restores unnecessary registers to fill in the gaps
+* Status updates
+    * Nick Fitzgerald: no Cranelift updates
+    * Chris Fallin: regalloc2 changes, many ISLE-related things - improvements such as if-let,
+    * Chris Fallin: changes to the build system (no checked-in generated source code), various small
+    * Chris Fallin: fixes; the major project right now is working on the middle end, e.g. machine-
+    * Chris Fallin: independent optimizations, starting with alias analysis; the hope is that by
+    * Chris Fallin: using ISLE we will enable interesting optimizations such as fusion, while
+    * Chris Fallin: making verification of the middle end easier
+    * Alexa VanHattum: working on ISLE verification, have a PR not to inline internal constructors,
+    * Alexa VanHattum: also looking into the ISA semantics of Arm and x86 to include one of them
+    * Alexa VanHattum: into the verification process
+    * Benjamin Bouvier: working on the incremental cache idea that has been discussed before, have a
+    * Benjamin Bouvier: heavy use case for hot reload - makes things much faster; the next step is
+    * Benjamin Bouvier: to open a GitHub issue for discussion; the implementation will need a
+    * Benjamin Bouvier: key-value store to keep compilation artifacts
+    * Ulrich Weigand: fixes for ISA feature flag handling and various logic errors in the bitwise
+    * Ulrich Weigand: operation implementations
+    * Andrew Brown: working mostly on the shared linear memory implementation in Wasmtime, which is
+    * Andrew Brown: expected to have some impact on Cranelift
+    * Johnnie Birch: started working with ISLE by implementing square root operations, looking
+    * Johnnie Birch: forward to do more
+    * bjorn3: looking into implementing exception handling (`eh_cleanup` branch on Wasmtime fork),
+    * bjorn3: but a bit stuck at how to deal with caller-saved registers
+    * Anton Kirilov: working on migrating `bitselect`, `vselect` and `splat` to ISLE; after the
+    * Anton Kirilov: associated RFC has been accepted, has finalized the forward-edge CFI
+    * Anton Kirilov: implementation, which is now ready for review
+* sightglass discussion
+    * Andrew Brown: Docker had been introduced to sightglass to facilitate reproducible builds
+    * Andrew Brown: recently a PR on the handling of build metadata that failed CI testing led to
+    * Andrew Brown: the idea of getting rid of Docker
+    * Nick Fitzgerald: Docker is frequently a source of inconvenience
+    * Chris Fallin: Docker is also Linux-specific, which might be a problem in the future if we
+    * Chris Fallin: decide to support other platforms
+    * Johnnie Birch: another problematic use case - internal framework to run stuff everywhere, e.g.
+    * Johnnie Birch: in the cloud; it uses Docker within Docker, which has also caused trouble
+    * Andrew Brown: so let's remove Docker, but would be the replacement - something to encode a
+    * Andrew Brown: sequence of commands in an OS-agnostic way?
+    * Chris Fallin: what are we using Docker exactly for?
+    * Andrew Brown: cloning repositories and `cargo build`, but there are knobs for configuratrion
+    * Andrew Brown: parameters, so that it is possible to use your repository, for example
+    * Nick Fitzgerald: there are so many different ways to build something, so we should push back
+    * Nick Fitzgerald: on supporting everything, just the things we really need
+    * Nick Fitzgerald: How many knobs are there? If it is just a commit ID, then it is fine, but
+    * Nick Fitzgerald: adding support for more than that might open a can of worms
+    * Andrew Brown: repository location, commit ID, build flags
+    * Nick Fitzgerald: other engines have other settings - Make flags, `configure` flags, etc.
+    * Chris Fallin: environment variables as well
+    * Nick Fitzgerald: is there a use case for using non-default build flags?
+    * Andrew Brown: it is not huge
+    * Nick Fitzgerald: the feeling is that sightglass should be simpler than it is right now
+    * Nick Fitzgerald: currently we usually compare either two branches or compare commits over time
+    * Nick Fitzgerald: on the other hand, we should record enough metadata, so that a suitably
+    * Nick Fitzgerald: motivated individual could reproduce the environment manually