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Auto merge of rust-lang#110569 - saethlin:mir-pass-cooperation, r=cjg…
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…illot

Deduplicate unreachable blocks, for real this time

In rust-lang#106428 (in particular rust-lang@41eda69) we noticed that inlining `unreachable_unchecked` can produce duplicate unreachable blocks. So we improved two MIR optimizations: `SimplifyCfg` was given a simplify to deduplicate unreachable blocks, then `InstCombine` was given a combiner to deduplicate switch targets that point at the same block. The problem is that change doesn't actually work.

Our current pass order is
```
SimplifyCfg (does nothing relevant to this situation)
Inline (produces multiple unreachable blocks)
InstCombine (doesn't do anything here, oops)
SimplifyCfg (produces the duplicate SwitchTargets that InstCombine is looking for)
```

So in here, I have factored out the specific function from `InstCombine` and placed it inside the simplify that produces the case it is looking for. This should ensure that it runs in the scenario it was designed for.

Fixes rust-lang#110551
r? `@cjgillot`
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bors committed Apr 21, 2023
2 parents 4096619 + 8ec49ad commit 4a03f14
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Showing 5 changed files with 61 additions and 19 deletions.
21 changes: 3 additions & 18 deletions compiler/rustc_mir_transform/src/instcombine.rs
Original file line number Diff line number Diff line change
@@ -1,11 +1,9 @@
//! Performs various peephole optimizations.

use crate::simplify::combine_duplicate_switch_targets;
use crate::MirPass;
use rustc_hir::Mutability;
use rustc_middle::mir::{
BinOp, Body, CastKind, Constant, ConstantKind, LocalDecls, Operand, Place, ProjectionElem,
Rvalue, SourceInfo, Statement, StatementKind, SwitchTargets, Terminator, TerminatorKind, UnOp,
};
use rustc_middle::mir::*;
use rustc_middle::ty::layout::ValidityRequirement;
use rustc_middle::ty::util::IntTypeExt;
use rustc_middle::ty::{self, ParamEnv, SubstsRef, Ty, TyCtxt};
Expand Down Expand Up @@ -46,7 +44,7 @@ impl<'tcx> MirPass<'tcx> for InstCombine {
&mut block.terminator.as_mut().unwrap(),
&mut block.statements,
);
ctx.combine_duplicate_switch_targets(&mut block.terminator.as_mut().unwrap());
combine_duplicate_switch_targets(block.terminator.as_mut().unwrap());
}
}
}
Expand Down Expand Up @@ -264,19 +262,6 @@ impl<'tcx> InstCombineContext<'tcx, '_> {
terminator.kind = TerminatorKind::Goto { target: destination_block };
}

fn combine_duplicate_switch_targets(&self, terminator: &mut Terminator<'tcx>) {
let TerminatorKind::SwitchInt { targets, .. } = &mut terminator.kind
else { return };

let otherwise = targets.otherwise();
if targets.iter().any(|t| t.1 == otherwise) {
*targets = SwitchTargets::new(
targets.iter().filter(|t| t.1 != otherwise),
targets.otherwise(),
);
}
}

fn combine_intrinsic_assert(
&self,
terminator: &mut Terminator<'tcx>,
Expand Down
14 changes: 14 additions & 0 deletions compiler/rustc_mir_transform/src/simplify.rs
Original file line number Diff line number Diff line change
Expand Up @@ -278,6 +278,18 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
}
}

pub fn combine_duplicate_switch_targets(terminator: &mut Terminator<'_>) {
if let TerminatorKind::SwitchInt { targets, .. } = &mut terminator.kind {
let otherwise = targets.otherwise();
if targets.iter().any(|t| t.1 == otherwise) {
*targets = SwitchTargets::new(
targets.iter().filter(|t| t.1 != otherwise),
targets.otherwise(),
);
}
}
}

pub fn remove_duplicate_unreachable_blocks<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
struct OptApplier<'tcx> {
tcx: TyCtxt<'tcx>,
Expand All @@ -298,6 +310,8 @@ pub fn remove_duplicate_unreachable_blocks<'tcx>(tcx: TyCtxt<'tcx>, body: &mut B
}
}

combine_duplicate_switch_targets(terminator);

self.super_terminator(terminator, location);
}
}
Expand Down
Original file line number Diff line number Diff line change
Expand Up @@ -34,7 +34,7 @@
- // + literal: Const { ty: unsafe fn(Option<T>) -> T {Option::<T>::unwrap_unchecked}, val: Value(<ZST>) }
+ StorageLive(_3); // scope 0 at $DIR/unwrap_unchecked.rs:+1:9: +1:27
+ _4 = discriminant(_2); // scope 1 at $SRC_DIR/core/src/option.rs:LL:COL
+ switchInt(move _4) -> [0: bb1, 1: bb2, otherwise: bb1]; // scope 1 at $SRC_DIR/core/src/option.rs:LL:COL
+ switchInt(move _4) -> [1: bb2, otherwise: bb1]; // scope 1 at $SRC_DIR/core/src/option.rs:LL:COL
}

bb1: {
Expand Down
16 changes: 16 additions & 0 deletions tests/mir-opt/instcombine_duplicate_switch_targets_e2e.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,16 @@
// compile-flags: -Zmir-opt-level=2 -Zinline-mir
// ignore-debug: standard library debug assertions add a panic that breaks this optimization
#![crate_type = "lib"]

pub enum Thing {
A,
B,
}

// EMIT_MIR instcombine_duplicate_switch_targets_e2e.ub_if_b.PreCodegen.after.mir
pub unsafe fn ub_if_b(t: Thing) -> Thing {
match t {
Thing::A => t,
Thing::B => std::hint::unreachable_unchecked(),
}
}
Original file line number Diff line number Diff line change
@@ -0,0 +1,27 @@
// MIR for `ub_if_b` after PreCodegen

fn ub_if_b(_1: Thing) -> Thing {
debug t => _1; // in scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+0:23: +0:24
let mut _0: Thing; // return place in scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+0:36: +0:41
let mut _2: isize; // in scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+2:9: +2:17
scope 1 (inlined unreachable_unchecked) { // at $DIR/instcombine_duplicate_switch_targets_e2e.rs:14:21: 14:55
scope 2 {
scope 3 (inlined unreachable_unchecked::runtime) { // at $SRC_DIR/core/src/intrinsics.rs:LL:COL
}
}
}

bb0: {
_2 = discriminant(_1); // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+1:11: +1:12
switchInt(move _2) -> [0: bb2, otherwise: bb1]; // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+1:5: +1:12
}

bb1: {
unreachable; // scope 2 at $SRC_DIR/core/src/intrinsics.rs:LL:COL
}

bb2: {
_0 = move _1; // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+2:21: +2:22
return; // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+5:2: +5:2
}
}

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