Auto merge of #112001 - saethlin:enable-matchbranchsimplification, r=cjgillot

Enable MatchBranchSimplification

This pass is one of the small number of benefits from `-Zmir-opt-level=3` that has motivated rustc_codegen_cranelift to use it:

https://github.com/rust-lang/rust/blob/19ed0aade60e1c1038fe40554bcd9d01b717effa/compiler/rustc_codegen_cranelift/build_system/build_sysroot.rs#L244-L246

Cranelift's motivation for this is _runtime_ performance improvements in debug builds. Lifting this pass all the way to `-Zmir-opt-level=1` seems to come without significant perf overhead, so that's what I'm suggesting here.

Author: bors

compiler/rustc_mir_transform/src/match_branches.rs

@@ -41,7 +41,7 @@ pub struct MatchBranchSimplification;
 
 impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.mir_opt_level() >= 3
+        sess.mir_opt_level() >= 1
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
@@ -62,7 +62,12 @@ impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
                     ..
                 } if targets.iter().len() == 1 => {
                     let (value, target) = targets.iter().next().unwrap();
-                    if target == targets.otherwise() {
+                    // We require that this block and the two possible target blocks all be
+                    // distinct.
+                    if target == targets.otherwise()
+                        || bb_idx == target
+                        || bb_idx == targets.otherwise()
+                    {
                         continue;
                     }
                     (discr, value, target, targets.otherwise())

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -116,6 +116,7 @@ impl<'a, 'tcx> FulfillmentContext<'tcx> {
 }
 
 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
+    #[inline]
     fn register_predicate_obligation(
         &mut self,
         infcx: &InferCtxt<'tcx>,

tests/mir-opt/simplify_locals_fixedpoint.foo.SimplifyLocals-final.diff

@@ -8,8 +8,6 @@
       let mut _3: std::option::Option<T>;  // in scope 0 at $DIR/simplify_locals_fixedpoint.rs:+1:51: +1:68
       let mut _4: isize;                   // in scope 0 at $DIR/simplify_locals_fixedpoint.rs:+1:22: +1:26
       let mut _5: isize;                   // in scope 0 at $DIR/simplify_locals_fixedpoint.rs:+1:13: +1:20
--     let mut _7: bool;                    // in scope 0 at $DIR/simplify_locals_fixedpoint.rs:+2:12: +2:20
--     let mut _8: u8;                      // in scope 0 at $DIR/simplify_locals_fixedpoint.rs:+2:12: +2:13
       scope 1 {
           debug a => _6;                   // in scope 1 at $DIR/simplify_locals_fixedpoint.rs:+1:18: +1:19
           let _6: u8;                      // in scope 1 at $DIR/simplify_locals_fixedpoint.rs:+1:18: +1:19
@@ -34,10 +32,9 @@
       }
 
       bb2: {
+          StorageLive(_6);                 // scope 1 at $DIR/simplify_locals_fixedpoint.rs:+1:18: +1:19
           _6 = (((_1.0: std::option::Option<u8>) as Some).0: u8); // scope 1 at $DIR/simplify_locals_fixedpoint.rs:+1:18: +1:19
--         StorageLive(_7);                 // scope 1 at $DIR/simplify_locals_fixedpoint.rs:+2:12: +2:20
--         _7 = Gt(_6, const 42_u8);        // scope 1 at $DIR/simplify_locals_fixedpoint.rs:+2:12: +2:20
--         StorageDead(_7);                 // scope 1 at $DIR/simplify_locals_fixedpoint.rs:+4:9: +4:10
+          StorageDead(_6);                 // scope 0 at $DIR/simplify_locals_fixedpoint.rs:+5:5: +5:6
           goto -> bb3;                     // scope 0 at $DIR/simplify_locals_fixedpoint.rs:+1:5: +5:6
       }
 

tests/mir-opt/switch_to_self.rs

@@ -0,0 +1,21 @@
+// Test that MatchBranchSimplification doesn't ICE on a SwitchInt where
+// one of the targets is the block that the SwitchInt terminates.
+#![crate_type = "lib"]
+#![feature(core_intrinsics, custom_mir)]
+use std::intrinsics::mir::*;
+
+// EMIT_MIR switch_to_self.test.MatchBranchSimplification.diff
+#[custom_mir(dialect = "runtime", phase = "post-cleanup")]
+pub fn test(x: bool) {
+    mir!(
+        {
+            Goto(bb0)
+        }
+        bb0 = {
+            match x { false => bb0, _ => bb1 }
+        }
+        bb1 = {
+            match x { false => bb0, _ => bb1 }
+        }
+    )
+}

tests/mir-opt/switch_to_self.test.MatchBranchSimplification.diff

@@ -0,0 +1,19 @@
+- // MIR for `test` before MatchBranchSimplification
++ // MIR for `test` after MatchBranchSimplification
+  
+  fn test(_1: bool) -> () {
+      let mut _0: ();                      // return place in scope 0 at $DIR/switch_to_self.rs:+0:22: +0:22
+  
+      bb0: {
+          goto -> bb1;                     // scope 0 at $DIR/switch_to_self.rs:+3:13: +3:22
+      }
+  
+      bb1: {
+          switchInt(_1) -> [0: bb1, otherwise: bb2]; // scope 0 at $DIR/switch_to_self.rs:+6:13: +6:47
+      }
+  
+      bb2: {
+          switchInt(_1) -> [0: bb1, otherwise: bb2]; // scope 0 at $DIR/switch_to_self.rs:+9:13: +9:47
+      }
+  }
+