mir-opt: add BoolChainOpt pass scaffold to fold pure bool && chains

[gustavo89587]

Add initial scaffolding for a MIR optimization pass that detects chains
of && operators over pure boolean expressions and prepares to fold them
into BitAnd BinOps, eliminating phi nodes that LLVM cannot optimize away.

Motivation

When a && b && c && d is lowered to MIR, each operand gets its own
BasicBlock with a SwitchInt terminator. This produces phi nodes in
LLVM IR that mem2reg and instcombine cannot eliminate, because the
short-circuit CFG structure prevents LLVM from proving side-effect-freedom.

Clang compiles equivalent C++ to a flat sequence of and i1 instructions
that LLVM can vectorize with SSE2/AVX2. Rust produces a chain of branches.

What this PR does

• Adds compiler/rustc_mir_transform/src/bool_chain_opt.rs with the pass
  skeleton: detects SwitchInt blocks over pure bool locals with no
  side-effecting statements
• Registers the pass after InstSimplify::AfterSimplifyCfg at mir-opt level 2
• Adds tests/mir-opt/bool_chain_opt.rs as the test fixture

The rewrite logic (emitting BinOp(BitAnd, ...)) will follow in the next
commit once the detection is reviewed and confirmed correct.

Related

Fixes #83623

r? @nikic

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

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  • df6ba80

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[TIMING:end] tool::ToolBuild { build_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, tool: "tidy", path: "src/tools/tidy", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 12.959
[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_mir_transform/src/bool_chain_opt.rs:1:
 //! Folds chains of `&&` over pure boolean expressions into `BitAnd` BinOps,
 //! eliminating the phi nodes that LLVM cannot optimize away.
 
-use crate::MirPass;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, TyCtxt};
+
+use crate::MirPass;
 
 pub(crate) struct BoolChainOpt;
 
Diff in /checkout/compiler/rustc_mir_transform/src/lib.rs:751:
             // optimizations. This invalidates CFG caches, so avoid putting between
             // `ReferencePropagation` and `GVN` which both use the dominator tree.
             &instsimplify::InstSimplify::AfterSimplifyCfg,
-                &bool_chain_opt::BoolChainOpt,
+            &bool_chain_opt::BoolChainOpt,
             // After `InstSimplify-after-simplifycfg` with `-Zub_checks=false`, simplify
             // ```
             // _13 = const false;
fmt: checked 6913 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:00:50

[dianqk]

I can’t see what transformations this pass performs.

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[traviscross]

@gustavo89587: Please tell us about what steps you took to self-review and validate the correctness of this PR and your process for creating it.