Auto merge of rust-lang#115105 - cjgillot:dest-prop-default, r=oli-obk

Enable DestinationPropagation by default.

~~Based on rust-lang#115291

This PR proposes to enable the destination propagation pass by default.
This pass is meant to reduce the amount of copies present in MIR.

At the same time, this PR removes the `RenameReturnPlace` pass, as it is currently unsound.
`DestinationPropagation` is not limited to `_0`, but does not handle borrowed locals.

Author: bors

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -160,7 +160,7 @@ impl<'tcx> MirPass<'tcx> for DestinationPropagation {
         //  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
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {

compiler/rustc_mir_transform/src/lib.rs

@@ -88,7 +88,6 @@ mod match_branches;
 mod mentioned_items;
 mod multiple_return_terminators;
 mod normalize_array_len;
-mod nrvo;
 mod prettify;
 mod promote_consts;
 mod ref_prop;
@@ -608,13 +607,12 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &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),
             &copy_prop::CopyProp,
             &dead_store_elimination::DeadStoreElimination::Final,
-            &nrvo::RenameReturnPlace,
+            &dest_prop::DestinationPropagation,
             &simplify::SimplifyLocals::Final,
             &multiple_return_terminators::MultipleReturnTerminators,
             &deduplicate_blocks::DeduplicateBlocks,

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 = _1;
+-         StorageLive(_4);
+-         _4 = _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 = _2;
--         StorageDead(_2);
+          _0 = _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 = _1;
+-         StorageLive(_4);
+-         _4 = _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 = _2;
--         StorageDead(_2);
+          _0 = _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);