Basic implementation of backtraces on panic

agb/.cargo/config.toml

@@ -6,9 +6,17 @@ build-std-features = ["compiler-builtins-mem"]
 target = "thumbv4t-none-eabi"
 
 [target.thumbv4t-none-eabi]
-rustflags = ["-Clink-arg=-Tgba.ld", "-Ctarget-cpu=arm7tdmi"]
+rustflags = [
+    "-Clink-arg=-Tgba.ld",
+    "-Ctarget-cpu=arm7tdmi",
+    "-Cforce-frame-pointers=yes",
+]
 runner = "mgba-test-runner"
 
 [target.armv4t-none-eabi]
-rustflags = ["-Clink-arg=-Tgba.ld", "-Ctarget-cpu=arm7tdmi"]
+rustflags = [
+    "-Clink-arg=-Tgba.ld",
+    "-Ctarget-cpu=arm7tdmi",
+    "-Cforce-frame-pointers=yes",
+]
 runner = "mgba-test-runner"

agb/src/backtrace.rs

@@ -0,0 +1,86 @@
+use core::{arch::asm, ops::Index};
+
+use alloc::vec::Vec;
+
+// only works for code compiled as THUMB
+#[repr(C)]
+#[derive(Clone, Default, Debug)]
+struct Context {
+    registers: [u32; 11],
+}
+
+pub struct Frame {
+    pub address: u32,
+}
+
+#[allow(unused)]
+enum Register {
+    R0,
+    R1,
+    R2,
+    R3,
+    R4,
+    R5,
+    R6,
+    FP,
+    SP,
+    LR,
+    PC,
+}
+
+impl Index<Register> for Context {
+    type Output = u32;
+
+    fn index(&self, index: Register) -> &Self::Output {
+        &self.registers[index as usize]
+    }
+}
+
+#[inline(never)]
+pub(crate) fn unwind_exception() -> Vec<Frame> {
+    let mut context = Context::default();
+
+    unsafe {
+        let context_ptr = (&mut context) as *mut _;
+
+        asm!(
+            "
+            str r0, [r0, #0x00]
+            str r1, [r0, #0x04]
+            str r2, [r0, #0x08]
+            str r3, [r0, #0x0C]
+            str r4, [r0, #0x10]
+            str r5, [r0, #0x14]
+            str r6, [r0, #0x18]
+            str r7, [r0, #0x1C]
+            mov r7, sp
+            str r7, [r0, #0x20]
+            mov r7, lr
+            str r7, [r0, #0x24]
+            mov r7, pc
+            str r7, [r0, #0x28]
+            ldr r7, [r0, #0x1C]
+            ",
+            in("r0") context_ptr
+        );
+    }
+
+    let mut frame_pointer = context[Register::FP];
+
+    let mut frames = Vec::new();
+
+    loop {
+        let sp = unsafe { *(frame_pointer as *const u32) };
+        let lr = unsafe { *((frame_pointer as *const u32).add(1)) };
+
+        if sp == 0 {
+            break;
+        }
+
+        frames.push(Frame { address: lr });
+
+        frame_pointer = sp;
+    }
+
+    frames
+}

agb/src/lib.rs

@@ -150,6 +150,7 @@ extern crate alloc;
 mod agb_alloc;
 
 mod agbabi;
+mod backtrace;
 mod bitarray;
 /// Implements everything relating to things that are displayed on screen.
 pub mod display;
@@ -317,11 +318,19 @@ pub mod test_runner {
 
     #[panic_handler]
     fn panic_implementation(info: &core::panic::PanicInfo) -> ! {
+        let frames = backtrace::unwind_exception();
+
         if let Some(mut mgba) = mgba::Mgba::new() {
-            mgba.print(format_args!("[failed]"), mgba::DebugLevel::Error)
-                .unwrap();
-            mgba.print(format_args!("Error: {info}"), mgba::DebugLevel::Fatal)
-                .unwrap();
+            let _ = mgba.print(format_args!("[failed]"), mgba::DebugLevel::Error);
+
+            for frame in frames {
+                let _ = mgba.print(
+                    format_args!("{:#08x}", frame.address),
+                    mgba::DebugLevel::Error,
+                );
+            }
+
+            let _ = mgba.print(format_args!("Error: {info}"), mgba::DebugLevel::Fatal);
         }
 
         loop {}