From e200a4a0d21473de9cc8ff817a0e4eba304e7e69 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomasz=20Mi=C4=85sko?= <tomasz.miasko@gmail.com>
Date: Wed, 21 Oct 2020 00:00:00 +0000
Subject: [PATCH] Disable "optimization to avoid load of address" in
 InstCombine

---
 compiler/rustc_mir/src/transform/instcombine.rs              | 5 +++++
 src/test/mir-opt/const_prop/ref_deref.main.ConstProp.diff    | 2 +-
 .../mir-opt/const_prop/ref_deref_project.main.ConstProp.diff | 2 +-
 src/test/mir-opt/inst_combine_deref.rs                       | 2 +-
 4 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/compiler/rustc_mir/src/transform/instcombine.rs b/compiler/rustc_mir/src/transform/instcombine.rs
index 1a8e281d417..c0b3d5ff18b 100644
--- a/compiler/rustc_mir/src/transform/instcombine.rs
+++ b/compiler/rustc_mir/src/transform/instcombine.rs
@@ -119,6 +119,11 @@ impl OptimizationFinder<'b, 'tcx> {
     }
 
     fn find_deref_of_address(&mut self, rvalue: &Rvalue<'tcx>, location: Location) -> Option<()> {
+        // FIXME(#78192): This optimization can result in unsoundness.
+        if !self.tcx.sess.opts.debugging_opts.unsound_mir_opts {
+            return None;
+        }
+
         // Look for the sequence
         //
         // _2 = &_1;
diff --git a/src/test/mir-opt/const_prop/ref_deref.main.ConstProp.diff b/src/test/mir-opt/const_prop/ref_deref.main.ConstProp.diff
index feef65f52eb..4fd1b8b2276 100644
--- a/src/test/mir-opt/const_prop/ref_deref.main.ConstProp.diff
+++ b/src/test/mir-opt/const_prop/ref_deref.main.ConstProp.diff
@@ -19,7 +19,7 @@
                                            // + span: $DIR/ref_deref.rs:5:6: 5:10
                                            // + literal: Const { ty: &i32, val: Unevaluated(WithOptConstParam { did: DefId(0:3 ~ ref_deref[317d]::main), const_param_did: None }, [], Some(promoted[0])) }
           _2 = _4;                         // scope 0 at $DIR/ref_deref.rs:5:6: 5:10
--         _1 = (*_4);                      // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
+-         _1 = (*_2);                      // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
 +         _1 = const 4_i32;                // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
           StorageDead(_2);                 // scope 0 at $DIR/ref_deref.rs:5:10: 5:11
           StorageDead(_1);                 // scope 0 at $DIR/ref_deref.rs:5:10: 5:11
diff --git a/src/test/mir-opt/const_prop/ref_deref_project.main.ConstProp.diff b/src/test/mir-opt/const_prop/ref_deref_project.main.ConstProp.diff
index 7ec0751263f..812c7c97718 100644
--- a/src/test/mir-opt/const_prop/ref_deref_project.main.ConstProp.diff
+++ b/src/test/mir-opt/const_prop/ref_deref_project.main.ConstProp.diff
@@ -19,7 +19,7 @@
                                            // + span: $DIR/ref_deref_project.rs:5:6: 5:17
                                            // + literal: Const { ty: &(i32, i32), val: Unevaluated(WithOptConstParam { did: DefId(0:3 ~ ref_deref_project[317d]::main), const_param_did: None }, [], Some(promoted[0])) }
           _2 = &((*_4).1: i32);            // scope 0 at $DIR/ref_deref_project.rs:5:6: 5:17
-          _1 = ((*_4).1: i32);             // scope 0 at $DIR/ref_deref_project.rs:5:5: 5:17
+          _1 = (*_2);                      // scope 0 at $DIR/ref_deref_project.rs:5:5: 5:17
           StorageDead(_2);                 // scope 0 at $DIR/ref_deref_project.rs:5:17: 5:18
           StorageDead(_1);                 // scope 0 at $DIR/ref_deref_project.rs:5:17: 5:18
           _0 = const ();                   // scope 0 at $DIR/ref_deref_project.rs:4:11: 6:2
diff --git a/src/test/mir-opt/inst_combine_deref.rs b/src/test/mir-opt/inst_combine_deref.rs
index 3be8c2f3ac7..78361c33660 100644
--- a/src/test/mir-opt/inst_combine_deref.rs
+++ b/src/test/mir-opt/inst_combine_deref.rs
@@ -1,4 +1,4 @@
-// compile-flags: -O
+// compile-flags: -O -Zunsound-mir-opts
 // EMIT_MIR inst_combine_deref.simple_opt.InstCombine.diff
 fn simple_opt() -> u64 {
     let x = 5;