Auto merge of #142915 - cjgillot:dest-prop-default, r=saethlin

Enable DestinationPropagation by default

This PR proposes to perform destination propagation on MIR. Most of the pass was fully rewritten by `@JakobDegen` in #96451.

This pass is quite heavy, as it needs to perform and save the results of a full liveness dataflow analysis. This accounts for ~50% of the pass' runtime.

Perf sees a few decent savings in later llvm passes, but also sizeable régressions when there are no savings to balance this pass' runtime.

Author: bors

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -153,15 +153,7 @@ pub(super) struct DestinationPropagation;
 
 impl<'tcx> crate::MirPass<'tcx> for DestinationPropagation {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        // For now, only run at MIR opt level 3. Two things need to be changed before this can be
-        // turned on by default:
-        //  1. Because of the overeager removal of storage statements, this can cause stack space
-        //     regressions. This opt is not the place to fix this though, it's a more general
-        //     problem in MIR.
-        //  2. Despite being an overall perf improvement, this still causes a 30% regression in
-        //     keccak. We can temporarily fix this by bounding function size, but in the long term
-        //     we should fix this by being smarter about invalidating analysis results.
-        sess.mir_opt_level() >= 3
+        sess.mir_opt_level() >= 2
     }
 
     #[tracing::instrument(level = "trace", skip(self, tcx, body))]

compiler/rustc_mir_transform/src/lib.rs

@@ -156,7 +156,6 @@ declare_passes! {
     mod match_branches : MatchBranchSimplification;
     mod mentioned_items : MentionedItems;
     mod multiple_return_terminators : MultipleReturnTerminators;
-    mod nrvo : RenameReturnPlace;
     mod post_drop_elaboration : CheckLiveDrops;
     mod prettify : ReorderBasicBlocks, ReorderLocals;
     mod promote_consts : PromoteTemps;
@@ -715,15 +714,14 @@ pub(crate) fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'
             &jump_threading::JumpThreading,
             &early_otherwise_branch::EarlyOtherwiseBranch,
             &simplify_comparison_integral::SimplifyComparisonIntegral,
-            &dest_prop::DestinationPropagation,
             &o1(simplify_branches::SimplifyConstCondition::Final),
             &o1(remove_noop_landing_pads::RemoveNoopLandingPads),
             &o1(simplify::SimplifyCfg::Final),
             // After the last SimplifyCfg, because this wants one-block functions.
             &strip_debuginfo::StripDebugInfo,
             &copy_prop::CopyProp,
             &dead_store_elimination::DeadStoreElimination::Final,
-            &nrvo::RenameReturnPlace,
+            &dest_prop::DestinationPropagation,
             &simplify::SimplifyLocals::Final,
             &multiple_return_terminators::MultipleReturnTerminators,
             &large_enums::EnumSizeOpt { discrepancy: 128 },

compiler/rustc_mir_transform/src/nrvo.rs

@@ -1,5 +1,5 @@
-- // MIR for `nrvo` before RenameReturnPlace
-+ // MIR for `nrvo` after RenameReturnPlace
+- // MIR for `nrvo` before DestinationPropagation
++ // MIR for `nrvo` after DestinationPropagation
 
   fn nrvo(_1: for<'a> fn(&'a mut [u8; 1024])) -> [u8; 1024] {
       debug init => _1;
@@ -10,32 +10,33 @@
       let mut _5: &mut [u8; 1024];
       let mut _6: &mut [u8; 1024];
       scope 1 {
--         debug buf => _2;
-+         debug buf => _0;
+          debug buf => _2;
       }
 
       bb0: {
--         StorageLive(_2);
--         _2 = [const 0_u8; 1024];
-+         _0 = [const 0_u8; 1024];
+          StorageLive(_2);
+          _2 = [const 0_u8; 1024];
           StorageLive(_3);
-          StorageLive(_4);
-          _4 = copy _1;
+-         StorageLive(_4);
+-         _4 = copy _1;
++         nop;
++         nop;
           StorageLive(_5);
           StorageLive(_6);
--         _6 = &mut _2;
-+         _6 = &mut _0;
+          _6 = &mut _2;
           _5 = &mut (*_6);
-          _3 = move _4(move _5) -> [return: bb1, unwind unreachable];
+-         _3 = move _4(move _5) -> [return: bb1, unwind unreachable];
++         _3 = move _1(move _5) -> [return: bb1, unwind unreachable];
       }
 
       bb1: {
           StorageDead(_5);
-          StorageDead(_4);
+-         StorageDead(_4);
++         nop;
           StorageDead(_6);
           StorageDead(_3);
--         _0 = copy _2;
--         StorageDead(_2);
+          _0 = copy _2;
+          StorageDead(_2);
           return;
       }
   }

tests/mir-opt/nrvo_simple.nrvo.RenameReturnPlace.panic-abort.diff renamed to tests/mir-opt/dest-prop/nrvo_borrowed.nrvo.DestinationPropagation.panic-abort.diff

@@ -1,5 +1,5 @@
-- // MIR for `nrvo` before RenameReturnPlace
-+ // MIR for `nrvo` after RenameReturnPlace
+- // MIR for `nrvo` before DestinationPropagation
++ // MIR for `nrvo` after DestinationPropagation
 
   fn nrvo(_1: for<'a> fn(&'a mut [u8; 1024])) -> [u8; 1024] {
       debug init => _1;
@@ -10,32 +10,33 @@
       let mut _5: &mut [u8; 1024];
       let mut _6: &mut [u8; 1024];
       scope 1 {
--         debug buf => _2;
-+         debug buf => _0;
+          debug buf => _2;
       }
 
       bb0: {
--         StorageLive(_2);
--         _2 = [const 0_u8; 1024];
-+         _0 = [const 0_u8; 1024];
+          StorageLive(_2);
+          _2 = [const 0_u8; 1024];
           StorageLive(_3);
-          StorageLive(_4);
-          _4 = copy _1;
+-         StorageLive(_4);
+-         _4 = copy _1;
++         nop;
++         nop;
           StorageLive(_5);
           StorageLive(_6);
--         _6 = &mut _2;
-+         _6 = &mut _0;
+          _6 = &mut _2;
           _5 = &mut (*_6);
-          _3 = move _4(move _5) -> [return: bb1, unwind continue];
+-         _3 = move _4(move _5) -> [return: bb1, unwind continue];
++         _3 = move _1(move _5) -> [return: bb1, unwind continue];
       }
 
       bb1: {
           StorageDead(_5);
-          StorageDead(_4);
+-         StorageDead(_4);
++         nop;
           StorageDead(_6);
           StorageDead(_3);
--         _0 = copy _2;
--         StorageDead(_2);
+          _0 = copy _2;
+          StorageDead(_2);
           return;
       }
   }

tests/mir-opt/nrvo_simple.nrvo.RenameReturnPlace.panic-unwind.diff renamed to tests/mir-opt/dest-prop/nrvo_borrowed.nrvo.DestinationPropagation.panic-unwind.diff

@@ -1,8 +1,8 @@
 // skip-filecheck
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
-//@ test-mir-pass: RenameReturnPlace
+//@ test-mir-pass: DestinationPropagation
 
-// EMIT_MIR nrvo_simple.nrvo.RenameReturnPlace.diff
+// EMIT_MIR nrvo_borrowed.nrvo.DestinationPropagation.diff
 fn nrvo(init: fn(&mut [u8; 1024])) -> [u8; 1024] {
     let mut buf = [0; 1024];
     init(&mut buf);