a77322c16f
Add `Ord::cmp` for primitives as a `BinOp` in MIR
Update: most of this OP was written months ago. See https://github.com/rust-lang/rust/pull/118310#issuecomment-2016940014 below for where we got to recently that made it ready for review.
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There are dozens of reasonable ways to implement `Ord::cmp` for integers using comparison, bit-ops, and branches. Those differences are irrelevant at the rust level, however, so we can make things better by adding `BinOp::Cmp` at the MIR level:
1. Exactly how to implement it is left up to the backends, so LLVM can use whatever pattern its optimizer best recognizes and cranelift can use whichever pattern codegens the fastest.
2. By not inlining those details for every use of `cmp`, we drastically reduce the amount of MIR generated for `derive`d `PartialOrd`, while also making it more amenable to MIR-level optimizations.
Having extremely careful `if` ordering to μoptimize resource usage on broadwell (#63767) is great, but it really feels to me like libcore is the wrong place to put that logic. Similarly, using subtraction [tricks](https://graphics.stanford.edu/~seander/bithacks.html#CopyIntegerSign) (#105840) is arguably even nicer, but depends on the optimizer understanding it (https://github.com/llvm/llvm-project/issues/73417) to be practical. Or maybe [bitor is better than add](https://discourse.llvm.org/t/representing-in-ir/67369/2?u=scottmcm)? But maybe only on a future version that [has `or disjoint` support](https://discourse.llvm.org/t/rfc-add-or-disjoint-flag/75036?u=scottmcm)? And just because one of those forms happens to be good for LLVM, there's no guarantee that it'd be the same form that GCC or Cranelift would rather see -- especially given their very different optimizers. Not to mention that if LLVM gets a spaceship intrinsic -- [which it should](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Suboptimal.20inlining.20in.20std.20function.20.60binary_search.60/near/404250586) -- we'll need at least a rustc intrinsic to be able to call it.
As for simplifying it in Rust, we now regularly inline `{integer}::partial_cmp`, but it's quite a large amount of IR. The best way to see that is with
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Cargo.toml | ||
README.md | ||
rust-toolchain.toml |
This crate is regularly synced with its mirror in the rustc repo at compiler/rustc_smir
.
We use git subtree
for this to preserve commits and allow the rustc repo to
edit these crates without having to touch this repo. This keeps the crates compiling
while allowing us to independently work on them here. The effort of keeping them in
sync is pushed entirely onto us, without affecting rustc workflows negatively.
This may change in the future, but changes to policy should only be done via a
compiler team MCP.
Instructions for working on this crate locally
Since the crate is the same in the rustc repo and here, the dependencies on rustc_* crates will only either work here or there, but never in both places at the same time. Thus we use optional dependencies on the rustc_* crates, requiring local development to use
cargo build --no-default-features -Zavoid-dev-deps
in order to compile successfully.
Instructions for syncing
Updating this repository
In the rustc repo, execute
git subtree push --prefix=compiler/rustc_smir url_to_your_fork_of_project_stable_mir some_feature_branch
and then open a PR of your some_feature_branch
against https://github.com/rust-lang/project-stable-mir
Updating the rustc library
First we need to bump our stack limit, as the rustc repo otherwise quickly hits that:
ulimit -s 60000
Maximum function recursion depth (1000) reached
Then we need to disable dash
as the default shell for sh scripts, as otherwise we run into a
hard limit of a recursion depth of 1000:
sudo dpkg-reconfigure dash
and then select No
to disable dash.
Patching your git worktree
The regular git worktree does not scale to repos of the size of the rustc repo.
So download the git-subtree.sh
from https://github.com/gitgitgadget/git/pull/493/files and run
sudo cp --backup /path/to/patched/git-subtree.sh /usr/lib/git-core/git-subtree
sudo chmod --reference=/usr/lib/git-core/git-subtree~ /usr/lib/git-core/git-subtree
sudo chown --reference=/usr/lib/git-core/git-subtree~ /usr/lib/git-core/git-subtree
Actually doing a sync
In the rustc repo, execute
git subtree pull --prefix=compiler/rustc_smir https://github.com/rust-lang/project-stable-mir smir
Note: only ever sync to rustc from the project-stable-mir's smir
branch. Do not sync with your own forks.
Then open a PR against rustc just like a regular PR.
Stable MIR Design
The stable-mir will follow a similar approach to proc-macro2. It’s implementation will eventually be broken down into two main crates:
stable_mir
: Public crate, to be published on crates.io, which will contain the stable data structure as well as proxy APIs to make calls to the compiler.rustc_smir
: The compiler crate that will translate from internal MIR to SMIR. This crate will also implement APIs that will be invoked by stable-mir to query the compiler for more information.
This will help tools to communicate with the rust compiler via stable APIs. Tools will depend on
stable_mir
crate, which will invoke the compiler using APIs defined in rustc_smir
. I.e.:
┌──────────────────────────────────┐ ┌──────────────────────────────────┐
│ External Tool ┌──────────┐ │ │ ┌──────────┐ Rust Compiler │
│ │ │ │ │ │ │ │
│ │stable_mir| │ │ │rustc_smir│ │
│ │ │ ├──────────►| │ │ │
│ │ │ │◄──────────┤ │ │ │
│ │ │ │ │ │ │ │
│ │ │ │ │ │ │ │
│ └──────────┘ │ │ └──────────┘ │
└──────────────────────────────────┘ └──────────────────────────────────┘
More details can be found here: https://hackmd.io/XhnYHKKuR6-LChhobvlT-g?view
For now, the code for these two crates are in separate modules of this crate.
The modules have the same name for simplicity. We also have a third module,
rustc_internal
which will expose APIs and definitions that allow users to
gather information from internal MIR constructs that haven't been exposed in
the stable_mir
module.