From 5744d97d59b1e41a3056ee1d35774c31ec0d8ea4 Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Thu, 17 May 2018 08:13:39 +0000
Subject: [PATCH] firmware: adjust backtrace addresses correctly.

---
 artiq/firmware/libunwind_backtrace/lib.rs |  4 +---
 artiq/firmware/runtime/main.rs            |  4 +++-
 artiq/firmware/runtime/profiler.rs        | 11 +++++++++--
 3 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/artiq/firmware/libunwind_backtrace/lib.rs b/artiq/firmware/libunwind_backtrace/lib.rs
index d1c71cf45..22c55a628 100644
--- a/artiq/firmware/libunwind_backtrace/lib.rs
+++ b/artiq/firmware/libunwind_backtrace/lib.rs
@@ -32,9 +32,7 @@ pub fn backtrace<F>(f: F) -> Result<(), uw::_Unwind_Reason_Code>
                 trace_context.prev_sp = cur_sp;
             }
 
-            // GetIP gives us the return address, i.e. the address after the delay slot,
-            // but we're interested in the call instruction.
-            (trace_context.step_fn)(uw::_Unwind_GetIP(context) - 2 * 4);
+            (trace_context.step_fn)(uw::_Unwind_GetIP(context));
             uw::_URC_NO_REASON
         }
     }
diff --git a/artiq/firmware/runtime/main.rs b/artiq/firmware/runtime/main.rs
index 087f69167..f0bef4108 100644
--- a/artiq/firmware/runtime/main.rs
+++ b/artiq/firmware/runtime/main.rs
@@ -313,7 +313,9 @@ pub extern fn panic_fmt(args: core::fmt::Arguments, file: &'static str,
     println!("backtrace for software version {}:",
              include_str!(concat!(env!("OUT_DIR"), "/git-describe")));
     let _ = unwind_backtrace::backtrace(|ip| {
-        println!("{:#08x}", ip);
+        // Backtrace gives us the return address, i.e. the address after the delay slot,
+        // but we're interested in the call instruction.
+        println!("{:#08x}", ip - 2 * 4);
     });
 
     if config::read_str("panic_reset", |r| r == Ok("1")) {
diff --git a/artiq/firmware/runtime/profiler.rs b/artiq/firmware/runtime/profiler.rs
index 9765100cc..ac5097dfe 100644
--- a/artiq/firmware/runtime/profiler.rs
+++ b/artiq/firmware/runtime/profiler.rs
@@ -201,7 +201,7 @@ mod imp {
     const SKIP_FRAMES: i32 = 3;
 
     #[inline(always)] // make the top of backtrace predictable
-    fn record(profile: &mut Profile, pc: usize) -> Result<(), ()> {
+    fn record(profile: &mut Profile, exn_pc: usize) -> Result<(), ()> {
         let mut result = Ok(());
         let mut frame = -SKIP_FRAMES;
 
@@ -212,6 +212,12 @@ mod imp {
         if profile.has_edges() {
             let mut prev_pc = 0;
             let _ = backtrace(|pc| {
+                // Backtrace gives us the return address, i.e. the address after the delay slot,
+                // but we're interested in the call instruction, *except* when going through
+                // the frame directly below the exception frame, which has the address that's
+                // being executed.
+                let pc = if pc != exn_pc { pc - 2 * 4 } else { pc };
+
                 if frame == 0 {
                     result = result.and_then(|()|
                         profile.record_hit(Address::new(pc)));
@@ -221,6 +227,7 @@ mod imp {
                         profile.record_edge(Address::new(pc),
                                             Address::new(prev_pc)));
                 }
+
                 prev_pc = pc;
                 frame += 1;
             });
@@ -229,7 +236,7 @@ mod imp {
         // If we couldn't get anything useful out of a backtrace, at least
         // record a hit at the exception PC.
         if frame <= 0 {
-            result = profile.record_hit(Address::new(pc));
+            result = profile.record_hit(Address::new(exn_pc));
         }
 
         result