Spill registers before scanning the stack.

From the change:

    // xtensa has more registers than an instruction can address. The 16 that
    // can be addressed are called the "window". When a function is called or
    // returns the window rotates. This allows for more efficient function calls
    // because ram doesn't need to be used. It's only used if the window wraps
    // around onto itself. At that point values are "spilled" to empty spots in
    // the stack that were set aside. When the window rotates back around (on
    // function return), the values are restored into the register from ram.

    // So, in order to read the values in the stack scan we must make sure all
    // of the register values we care about have been spilled to RAM. Luckily,
    // there is a HAL call to do it. There is a bit of a race condition here
    // because the register value could change after it's been restored but that
    // is unlikely to happen with a heap pointer while we do a GC.

Fixes #2907

ports/esp32s2/mphalport.c

@@ -37,6 +37,24 @@ void mp_hal_delay_us(mp_uint_t delay) {
     mp_hal_delay_ms(delay / 1000);
 }
 
-mp_uint_t cpu_get_regs_and_sp(mp_uint_t *regs) {
+// This is provided by the esp-idf/components/xtensa/esp32s2/libhal.a binary
+// blob.
+extern void xthal_window_spill(void);
+
+mp_uint_t cpu_get_regs_and_sp(mp_uint_t *regs, uint8_t reg_count) {
+    // xtensa has more registers than an instruction can address. The 16 that
+    // can be addressed are called the "window". When a function is called or
+    // returns the window rotates. This allows for more efficient function calls
+    // because ram doesn't need to be used. It's only used if the window wraps
+    // around onto itself. At that point values are "spilled" to empty spots in
+    // the stack that were set aside. When the window rotates back around (on
+    // function return), the values are restored into the register from ram.
+
+    // So, in order to read the values in the stack scan we must make sure all
+    // of the register values we care about have been spilled to RAM. Luckily,
+    // there is a HAL call to do it. There is a bit of a race condition here
+    // because the register value could change after it's been restored but that
+    // is unlikely to happen with a heap pointer while we do a GC.
+    xthal_window_spill();
     return (mp_uint_t) __builtin_frame_address(0);
 }

ports/esp32s2/tools/decode_backtrace.py

@@ -0,0 +1,23 @@
+"""Simple script that translates "Backtrace:" lines from the ESP output to files
+   and line numbers.
+
+   Run with: python3 tools/decode_backtrace.py <board>
+
+   Enter the backtrace line at the "? " prompt. CTRL-C to exit the script.
+   """
+
+import subprocess
+import sys
+
+board = sys.argv[1]
+print(board)
+
+while True:
+    addresses = input("? ")
+    if addresses.startswith("Backtrace:"):
+        addresses = addresses[len("Backtrace:"):]
+    addresses = addresses.strip().split()
+    addresses = [address.split(":")[0] for address in addresses]
+    print('got', addresses)
+    subprocess.run(["xtensa-esp32s2-elf-addr2line",
+                    "-e", "build-{}/firmware.elf".format(board)] + addresses)