From b319715456c08097f2cd936da309ed6e8aec2843 Mon Sep 17 00:00:00 2001
From: Wesley Wiser <wwiser@gmail.com>
Date: Thu, 22 Nov 2018 12:56:15 -0500
Subject: [PATCH 1/2] Disable the self-profiler unless the `-Z self-profile`
 flag is set

Related to #51648
---
 src/librustc/session/mod.rs | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/librustc/session/mod.rs b/src/librustc/session/mod.rs
index d688d93b80842..1187c53305d1c 100644
--- a/src/librustc/session/mod.rs
+++ b/src/librustc/session/mod.rs
@@ -826,8 +826,10 @@ impl Session {
     }
 
     pub fn profiler<F: FnOnce(&mut SelfProfiler) -> ()>(&self, f: F) {
-        let mut profiler = self.self_profiling.borrow_mut();
-        f(&mut profiler);
+        if self.opts.debugging_opts.self_profile {
+            let mut profiler = self.self_profiling.borrow_mut();
+            f(&mut profiler);
+        }
     }
 
     pub fn print_profiler_results(&self) {

From dce1c4530e2707c338fe56b26a36797377f11514 Mon Sep 17 00:00:00 2001
From: Wesley Wiser <wwiser@gmail.com>
Date: Thu, 22 Nov 2018 13:05:25 -0500
Subject: [PATCH 2/2] [Windows] Work around non-monotonic clocks in the
 self-profiler

On Windows, the high-resolution timestamp api doesn't seem to always be
monotonic. This can cause panics when the self-profiler uses the
`Instant` api to find elapsed time.

Work around this by detecting the case where now is less than the start
time and just use 0 elapsed ticks as the measurement.

Fixes #51648
---
 src/librustc/util/profiling.rs | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/src/librustc/util/profiling.rs b/src/librustc/util/profiling.rs
index 37073b6e82080..1f05043976491 100644
--- a/src/librustc/util/profiling.rs
+++ b/src/librustc/util/profiling.rs
@@ -12,7 +12,7 @@ use session::config::Options;
 
 use std::fs;
 use std::io::{self, StdoutLock, Write};
-use std::time::Instant;
+use std::time::{Duration, Instant};
 
 macro_rules! define_categories {
     ($($name:ident,)*) => {
@@ -197,7 +197,20 @@ impl SelfProfiler {
     }
 
     fn stop_timer(&mut self) -> u64 {
-        let elapsed = self.current_timer.elapsed();
+        let elapsed = if cfg!(windows) {
+            // On Windows, timers don't always appear to be monotonic (see #51648)
+            // which can lead to panics when calculating elapsed time.
+            // Work around this by testing to see if the current time is less than
+            // our recorded time, and if it is, just returning 0.
+            let now = Instant::now();
+            if self.current_timer >= now {
+                Duration::new(0, 0)
+            } else {
+                self.current_timer.elapsed()
+            }
+        } else {
+            self.current_timer.elapsed()
+        };
 
         self.current_timer = Instant::now();