From dbd7226d299ac23647975068bea1168cf56e0feb Mon Sep 17 00:00:00 2001
From: Wesley Wiser <wwiser@gmail.com>
Date: Thu, 17 Sep 2020 09:29:39 -0400
Subject: [PATCH] [mir-opt] Disable the `ConsideredEqual` logic in
 SimplifyBranchSame opt

The logic is currently broken and we need to disable it to fix a beta
regression (see #76803)
---
 .../rustc_mir/src/transform/simplify_try.rs   | 11 ++++++--
 .../simplify_arm.id.SimplifyBranchSame.diff   | 27 +++++++++----------
 .../ui/mir/issue-76803-branches-not-same.rs   | 19 +++++++++++++
 3 files changed, 40 insertions(+), 17 deletions(-)
 create mode 100644 src/test/ui/mir/issue-76803-branches-not-same.rs

diff --git a/compiler/rustc_mir/src/transform/simplify_try.rs b/compiler/rustc_mir/src/transform/simplify_try.rs
index 76a60c45575b8..9a11d92724020 100644
--- a/compiler/rustc_mir/src/transform/simplify_try.rs
+++ b/compiler/rustc_mir/src/transform/simplify_try.rs
@@ -612,7 +612,7 @@ impl<'a, 'tcx> SimplifyBranchSameOptimizationFinder<'a, 'tcx> {
                     && bb_l.terminator().kind == bb_r.terminator().kind;
                     let statement_check = || {
                         bb_l.statements.iter().zip(&bb_r.statements).try_fold(StatementEquality::TrivialEqual, |acc,(l,r)| {
-                            let stmt_equality = self.statement_equality(*adt_matched_on, &l, bb_l_idx, &r, bb_r_idx);
+                            let stmt_equality = self.statement_equality(*adt_matched_on, &l, bb_l_idx, &r, bb_r_idx, self.tcx.sess.opts.debugging_opts.mir_opt_level);
                             if matches!(stmt_equality, StatementEquality::NotEqual) {
                                 // short circuit
                                 None
@@ -672,6 +672,7 @@ impl<'a, 'tcx> SimplifyBranchSameOptimizationFinder<'a, 'tcx> {
         x_bb_idx: BasicBlock,
         y: &Statement<'tcx>,
         y_bb_idx: BasicBlock,
+        mir_opt_level: usize,
     ) -> StatementEquality {
         let helper = |rhs: &Rvalue<'tcx>,
                       place: &Place<'tcx>,
@@ -690,7 +691,13 @@ impl<'a, 'tcx> SimplifyBranchSameOptimizationFinder<'a, 'tcx> {
 
             match rhs {
                 Rvalue::Use(operand) if operand.place() == Some(adt_matched_on) => {
-                    StatementEquality::ConsideredEqual(side_to_choose)
+                    // FIXME(76803): This logic is currently broken because it does not take into
+                    // account the current discriminant value.
+                    if mir_opt_level > 2 {
+                        StatementEquality::ConsideredEqual(side_to_choose)
+                    } else {
+                        StatementEquality::NotEqual
+                    }
                 }
                 _ => {
                     trace!(
diff --git a/src/test/mir-opt/simplify_arm.id.SimplifyBranchSame.diff b/src/test/mir-opt/simplify_arm.id.SimplifyBranchSame.diff
index 81a0e6ba0b4ee..06f359da2e70d 100644
--- a/src/test/mir-opt/simplify_arm.id.SimplifyBranchSame.diff
+++ b/src/test/mir-opt/simplify_arm.id.SimplifyBranchSame.diff
@@ -13,27 +13,24 @@
   
       bb0: {
           _2 = discriminant(_1);           // scope 0 at $DIR/simplify-arm.rs:11:9: 11:16
--         switchInt(move _2) -> [0_isize: bb1, 1_isize: bb3, otherwise: bb2]; // scope 0 at $DIR/simplify-arm.rs:11:9: 11:16
-+         goto -> bb1;                     // scope 0 at $DIR/simplify-arm.rs:11:9: 11:16
+          switchInt(move _2) -> [0_isize: bb1, 1_isize: bb3, otherwise: bb2]; // scope 0 at $DIR/simplify-arm.rs:11:9: 11:16
       }
   
       bb1: {
--         discriminant(_0) = 0;            // scope 0 at $DIR/simplify-arm.rs:12:17: 12:21
--         goto -> bb4;                     // scope 0 at $DIR/simplify-arm.rs:10:5: 13:6
--     }
-- 
--     bb2: {
--         unreachable;                     // scope 0 at $DIR/simplify-arm.rs:10:11: 10:12
--     }
-- 
--     bb3: {
+          discriminant(_0) = 0;            // scope 0 at $DIR/simplify-arm.rs:12:17: 12:21
+          goto -> bb4;                     // scope 0 at $DIR/simplify-arm.rs:10:5: 13:6
+      }
+  
+      bb2: {
+          unreachable;                     // scope 0 at $DIR/simplify-arm.rs:10:11: 10:12
+      }
+  
+      bb3: {
           _0 = move _1;                    // scope 1 at $DIR/simplify-arm.rs:11:20: 11:27
--         goto -> bb4;                     // scope 0 at $DIR/simplify-arm.rs:10:5: 13:6
-+         goto -> bb2;                     // scope 0 at $DIR/simplify-arm.rs:10:5: 13:6
+          goto -> bb4;                     // scope 0 at $DIR/simplify-arm.rs:10:5: 13:6
       }
   
--     bb4: {
-+     bb2: {
+      bb4: {
           return;                          // scope 0 at $DIR/simplify-arm.rs:14:2: 14:2
       }
   }
diff --git a/src/test/ui/mir/issue-76803-branches-not-same.rs b/src/test/ui/mir/issue-76803-branches-not-same.rs
new file mode 100644
index 0000000000000..a6a5762200548
--- /dev/null
+++ b/src/test/ui/mir/issue-76803-branches-not-same.rs
@@ -0,0 +1,19 @@
+// run-pass
+
+#[derive(Debug, Eq, PartialEq)]
+pub enum Type {
+    A,
+    B,
+}
+
+
+pub fn encode(v: Type) -> Type {
+    match v {
+        Type::A => Type::B,
+        _ => v,
+    }
+}
+
+fn main() {
+  assert_eq!(Type::B, encode(Type::A));
+}