From 3d5a1e330fb48c5f64b078cf755cbb44ee73343e Mon Sep 17 00:00:00 2001
From: Simon Vandel Sillesen <simon.vandel@gmail.com>
Date: Sun, 15 Nov 2020 22:34:50 +0100
Subject: [PATCH] Only go through the body if something can be optimized

---
 compiler/rustc_mir/src/transform/instcombine.rs | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_mir/src/transform/instcombine.rs b/compiler/rustc_mir/src/transform/instcombine.rs
index b36633fbfb3..04ea2bb542e 100644
--- a/compiler/rustc_mir/src/transform/instcombine.rs
+++ b/compiler/rustc_mir/src/transform/instcombine.rs
@@ -29,8 +29,10 @@ impl<'tcx> MirPass<'tcx> for InstCombine {
             optimization_finder.optimizations
         };
 
-        // Then carry out those optimizations.
-        MutVisitor::visit_body(&mut InstCombineVisitor { optimizations, tcx }, body);
+        if !optimizations.is_empty() {
+            // Then carry out those optimizations.
+            MutVisitor::visit_body(&mut InstCombineVisitor { optimizations, tcx }, body);
+        }
     }
 }
 
@@ -296,3 +298,12 @@ struct OptimizationList<'tcx> {
     unneeded_equality_comparison: FxHashMap<Location, Operand<'tcx>>,
     unneeded_deref: FxHashMap<Location, Place<'tcx>>,
 }
+
+impl<'tcx> OptimizationList<'tcx> {
+    fn is_empty(&self) -> bool {
+        self.and_stars.is_empty()
+            && self.arrays_lengths.is_empty()
+            && self.unneeded_equality_comparison.is_empty()
+            && self.unneeded_deref.is_empty()
+    }
+}