From dc5feeb1fa2aa45b5a879feff526aad736a63b74 Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Thu, 2 Dec 2021 14:38:04 -0800
Subject: [PATCH] Don't "simplify" during optimizations if optimizations are
 disabled

---
 compiler/rustc_mir_transform/src/lib.rs       | 20 +++++++++++-----
 .../rustc_mir_transform/src/pass_manager.rs   | 23 +++++++++++++++++++
 2 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index 793e46687d2..a7e003a55b4 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -35,7 +35,7 @@ use rustc_span::{Span, Symbol};
 #[macro_use]
 mod pass_manager;
 
-use pass_manager::{self as pm, Lint, MirLint};
+use pass_manager::{self as pm, Lint, MirLint, WithMinOptLevel};
 
 mod abort_unwinding_calls;
 mod add_call_guards;
@@ -438,6 +438,10 @@ fn run_post_borrowck_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tc
 }
 
 fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+    fn o1<T>(x: T) -> WithMinOptLevel<T> {
+        WithMinOptLevel(1, x)
+    }
+
     // Lowering generator control-flow and variables has to happen before we do anything else
     // to them. We run some optimizations before that, because they may be harder to do on the state
     // machine than on MIR with async primitives.
@@ -450,7 +454,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &normalize_array_len::NormalizeArrayLen, // has to run after `slice::len` lowering
             &unreachable_prop::UnreachablePropagation,
             &uninhabited_enum_branching::UninhabitedEnumBranching,
-            &simplify::SimplifyCfg::new("after-uninhabited-enum-branching"),
+            &o1(simplify::SimplifyCfg::new("after-uninhabited-enum-branching")),
             &inline::Inline,
             &generator::StateTransform,
         ],
@@ -472,17 +476,21 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &multiple_return_terminators::MultipleReturnTerminators,
             &instcombine::InstCombine,
             &separate_const_switch::SeparateConstSwitch,
+            //
             // FIXME(#70073): This pass is responsible for both optimization as well as some lints.
             &const_prop::ConstProp,
-            &simplify_branches::SimplifyBranches::new("after-const-prop"),
+            //
+            // FIXME: The old pass manager ran this only at mir-opt-level >= 1, but
+            // const-prop runs unconditionally. Should this run unconditionally as well?
+            &o1(simplify_branches::SimplifyConstCondition::new("after-const-prop")),
             &early_otherwise_branch::EarlyOtherwiseBranch,
             &simplify_comparison_integral::SimplifyComparisonIntegral,
             &simplify_try::SimplifyArmIdentity,
             &simplify_try::SimplifyBranchSame,
             &dest_prop::DestinationPropagation,
-            &simplify_branches::SimplifyBranches::new("final"),
-            &remove_noop_landing_pads::RemoveNoopLandingPads,
-            &simplify::SimplifyCfg::new("final"),
+            &o1(simplify_branches::SimplifyConstCondition::new("final")),
+            &o1(remove_noop_landing_pads::RemoveNoopLandingPads),
+            &o1(simplify::SimplifyCfg::new("final")),
             &nrvo::RenameReturnPlace,
             &const_debuginfo::ConstDebugInfo,
             &simplify::SimplifyLocals,
diff --git a/compiler/rustc_mir_transform/src/pass_manager.rs b/compiler/rustc_mir_transform/src/pass_manager.rs
index ea6f82dfc06..729b8cae5e4 100644
--- a/compiler/rustc_mir_transform/src/pass_manager.rs
+++ b/compiler/rustc_mir_transform/src/pass_manager.rs
@@ -49,6 +49,29 @@ where
     }
 }
 
+pub struct WithMinOptLevel<T>(pub u32, pub T);
+
+impl<T> MirPass<'tcx> for WithMinOptLevel<T>
+where
+    T: MirPass<'tcx>,
+{
+    fn name(&self) -> Cow<'_, str> {
+        self.1.name()
+    }
+
+    fn is_enabled(&self, sess: &Session) -> bool {
+        sess.mir_opt_level() >= self.0 as usize
+    }
+
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        self.1.run_pass(tcx, body)
+    }
+
+    fn phase_change(&self) -> Option<MirPhase> {
+        self.1.phase_change()
+    }
+}
+
 pub fn run_passes(tcx: TyCtxt<'tcx>, body: &'mir mut Body<'tcx>, passes: &[&dyn MirPass<'tcx>]) {
     let start_phase = body.phase;
     let mut cnt = 0;