Auto merge of #36306 - nagisa:mir-local-cleanup, r=eddyb

A way to remove otherwise unused locals from MIR

There is a certain amount of desire for a pass which cleans up the provably unused variables (no assignments or reads). There has been an implementation of such pass by @scottcarr, and another (two!) implementations by me in my own dataflow efforts.

PR like https://github.com/rust-lang/rust/pull/35916 proves that this pass is useful even on its own, which is why I cherry-picked it out from my dataflow effort.

@nikomatsakis previously expressed concerns over this pass not seeming to be very cheap to run and therefore unsuitable for regular cleanup duties. Turns out, regular cleanup of local declarations is not at all necessary, at least now, because majority of passes simply do not (or should not) care about them. That’s why it is viable to only run this pass once (perhaps a few more times in the future?) per function, right before translation.

r? @eddyb or @nikomatsakis

1 files changed, 12 insertions, 9 deletions

diff --git a/src/librustc_driver/driver.rs b/src/librustc_driver/driver.rs
index da1d5ad2c4a..e33428a3e96 100644
--- a/src/librustc_driver/driver.rs
+++ b/src/librustc_driver/driver.rs
@@ -917,17 +917,19 @@ pub fn phase_3_run_analysis_passes<'tcx, F, R>(sess: &'tcx Session,
              "MIR dump",
              || mir::mir_map::build_mir_for_crate(tcx));
 
-        time(time_passes, "MIR passes", || {
+        time(time_passes, "MIR cleanup and validation", || {
             let mut passes = sess.mir_passes.borrow_mut();
-            // Push all the built-in passes.
+            // Push all the built-in validation passes.
+            // NB: if you’re adding an *optimisation* it ought to go to another set of passes
+            // in stage 4 below.
             passes.push_hook(box mir::transform::dump_mir::DumpMir);
-            passes.push_pass(box mir::transform::simplify_cfg::SimplifyCfg::new("initial"));
+            passes.push_pass(box mir::transform::simplify::SimplifyCfg::new("initial"));
             passes.push_pass(
                 box mir::transform::qualify_consts::QualifyAndPromoteConstants::default());
             passes.push_pass(box mir::transform::type_check::TypeckMir);
             passes.push_pass(
                 box mir::transform::simplify_branches::SimplifyBranches::new("initial"));
-            passes.push_pass(box mir::transform::simplify_cfg::SimplifyCfg::new("qualify-consts"));
+            passes.push_pass(box mir::transform::simplify::SimplifyCfg::new("qualify-consts"));
             // And run everything.
             passes.run_passes(tcx);
         });
@@ -989,13 +991,13 @@ pub fn phase_4_translate_to_llvm<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
          "resolving dependency formats",
          || dependency_format::calculate(&tcx.sess));
 
-    // Run the passes that transform the MIR into a more suitable for translation
-    // to LLVM code.
-    time(time_passes, "Prepare MIR codegen passes", || {
+    // Run the passes that transform the MIR into a more suitable form for translation to LLVM
+    // code.
+    time(time_passes, "MIR optimisations", || {
         let mut passes = ::rustc::mir::transform::Passes::new();
         passes.push_hook(box mir::transform::dump_mir::DumpMir);
         passes.push_pass(box mir::transform::no_landing_pads::NoLandingPads);
-        passes.push_pass(box mir::transform::simplify_cfg::SimplifyCfg::new("no-landing-pads"));
+        passes.push_pass(box mir::transform::simplify::SimplifyCfg::new("no-landing-pads"));
 
         // From here on out, regions are gone.
         passes.push_pass(box mir::transform::erase_regions::EraseRegions);
@@ -1003,13 +1005,14 @@ pub fn phase_4_translate_to_llvm<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         passes.push_pass(box mir::transform::add_call_guards::AddCallGuards);
         passes.push_pass(box borrowck::ElaborateDrops);
         passes.push_pass(box mir::transform::no_landing_pads::NoLandingPads);
-        passes.push_pass(box mir::transform::simplify_cfg::SimplifyCfg::new("elaborate-drops"));
+        passes.push_pass(box mir::transform::simplify::SimplifyCfg::new("elaborate-drops"));
 
         // No lifetime analysis based on borrowing can be done from here on out.
         passes.push_pass(box mir::transform::instcombine::InstCombine::new());
         passes.push_pass(box mir::transform::deaggregator::Deaggregator);
         passes.push_pass(box mir::transform::copy_prop::CopyPropagation);
 
+        passes.push_pass(box mir::transform::simplify::SimplifyLocals);
         passes.push_pass(box mir::transform::add_call_guards::AddCallGuards);
         passes.push_pass(box mir::transform::dump_mir::Marker("PreTrans"));