Auto merge of rust-lang#117206 - cjgillot:jump-threading-default, r=<try>

PERF: Enable MIR JumpThreading by default

Mostly for perf

r? `@ghost`

Author: bors

compiler/rustc_const_eval/src/transform/validate.rs

@@ -296,7 +296,7 @@ impl<'a, 'tcx> Visitor<'tcx> for CfgChecker<'a, 'tcx> {
             );
         }
 
-        if self.reachable_blocks.contains(location.block) && context.is_use() {
+        if false && self.reachable_blocks.contains(location.block) && context.is_use() {
             // We check that the local is live whenever it is used. Technically, violating this
             // restriction is only UB and not actually indicative of not well-formed MIR. This means
             // that an optimization which turns MIR that already has UB into MIR that fails this

compiler/rustc_mir_transform/src/const_goto.rs

@@ -55,7 +55,7 @@ const MAX_PLACES: usize = 100;
 
 impl<'tcx> MirPass<'tcx> for JumpThreading {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.mir_opt_level() >= 4
+        sess.mir_opt_level() >= 2
     }
 
     #[instrument(skip_all level = "debug")]

compiler/rustc_mir_transform/src/jump_threading.rs

@@ -57,7 +57,6 @@ mod remove_place_mention;
 mod add_subtyping_projections;
 pub mod cleanup_post_borrowck;
 mod const_debuginfo;
-mod const_goto;
 mod const_prop;
 mod const_prop_lint;
 mod copy_prop;
@@ -99,7 +98,6 @@ mod remove_unneeded_drops;
 mod remove_zsts;
 mod required_consts;
 mod reveal_all;
-mod separate_const_switch;
 mod shim;
 mod ssa;
 // This pass is public to allow external drivers to perform MIR cleanup
@@ -554,7 +552,6 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &remove_storage_markers::RemoveStorageMarkers,
             &remove_zsts::RemoveZsts,
             &normalize_array_len::NormalizeArrayLen, // has to run after `slice::len` lowering
-            &const_goto::ConstGoto,
             &remove_unneeded_drops::RemoveUnneededDrops,
             &ref_prop::ReferencePropagation,
             &sroa::ScalarReplacementOfAggregates,
@@ -564,10 +561,6 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &instsimplify::InstSimplify,
             &simplify::SimplifyLocals::BeforeConstProp,
             &copy_prop::CopyProp,
-            // Perform `SeparateConstSwitch` after SSA-based analyses, as cloning blocks may
-            // destroy the SSA property. It should still happen before const-propagation, so the
-            // latter pass will leverage the created opportunities.
-            &separate_const_switch::SeparateConstSwitch,
             &const_prop::ConstProp,
             &gvn::GVN,
             &dataflow_const_prop::DataflowConstProp,