From 5b99523de9cb362a2328829959618aef0becb38e Mon Sep 17 00:00:00 2001
From: Ariel Ben-Yehuda <ariel.byd@gmail.com>
Date: Tue, 1 Aug 2017 00:10:46 +0300
Subject: [PATCH] rustc_mir::transform::simplify - remove nops first

Removing nops can allow more basic blocks to be merged, but merging
basic blocks can't allow for more nops to be removed, so we should
remove nops first.

This doesn't matter *that* much, because normally we run SimplifyCfg
several times, but there's no reason not to do it.
---
 src/librustc_mir/transform/simplify.rs |  4 ++--
 src/test/mir-opt/basic_assignment.rs   | 22 ++++++++--------------
 2 files changed, 10 insertions(+), 16 deletions(-)

diff --git a/src/librustc_mir/transform/simplify.rs b/src/librustc_mir/transform/simplify.rs
index d5b79c0d1c382..a1d56ccd874a1 100644
--- a/src/librustc_mir/transform/simplify.rs
+++ b/src/librustc_mir/transform/simplify.rs
@@ -105,6 +105,8 @@ impl<'a, 'tcx: 'a> CfgSimplifier<'a, 'tcx> {
     }
 
     pub fn simplify(mut self) {
+        self.strip_nops();
+
         loop {
             let mut changed = false;
 
@@ -141,8 +143,6 @@ impl<'a, 'tcx: 'a> CfgSimplifier<'a, 'tcx> {
 
             if !changed { break }
         }
-
-        self.strip_nops()
     }
 
     // Collapse a goto chain starting from `start`
diff --git a/src/test/mir-opt/basic_assignment.rs b/src/test/mir-opt/basic_assignment.rs
index 6afc344ced847..d3bf7f68785d5 100644
--- a/src/test/mir-opt/basic_assignment.rs
+++ b/src/test/mir-opt/basic_assignment.rs
@@ -50,10 +50,10 @@ fn main() {
 //         StorageLive(_5);
 //         StorageLive(_6);
 //         _6 = _4;
-//         replace(_5 <- _6) -> [return: bb1, unwind: bb7];
+//         replace(_5 <- _6) -> [return: bb1, unwind: bb5];
 //     }
 //     bb1: {
-//         drop(_6) -> [return: bb8, unwind: bb5];
+//         drop(_6) -> [return: bb6, unwind: bb4];
 //     }
 //     bb2: {
 //         resume;
@@ -62,27 +62,21 @@ fn main() {
 //         drop(_4) -> bb2;
 //     }
 //     bb4: {
-//         goto -> bb3;
+//         drop(_5) -> bb3;
 //     }
 //     bb5: {
-//         drop(_5) -> bb4;
+//         drop(_6) -> bb4;
 //     }
 //     bb6: {
-//         goto -> bb5;
-//     }
-//     bb7: {
-//         drop(_6) -> bb6;
-//     }
-//     bb8: {
 //         StorageDead(_6);
 //         _0 = ();
-//         drop(_5) -> [return: bb9, unwind: bb3];
+//         drop(_5) -> [return: bb7, unwind: bb3];
 //     }
-//     bb9: {
+//     bb7: {
 //         StorageDead(_5);
-//         drop(_4) -> bb10;
+//         drop(_4) -> bb8;
 //     }
-//     bb10: {
+//     bb8: {
 //         StorageDead(_4);
 //         StorageDead(_2);
 //         StorageDead(_1);