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131: Allow GDB to unwind HardFault callstacks r=therealprof a=adamgreen When I currently request GDB to dump a hard fault stack, I see something like this: ``` (gdb) bt #0 UserHardFault_ (ef=0x10001fb8) at /depots/cortex-m-rt/src/lib.rs:537 #1 0x08003fe6 in HardFault () Backtrace stopped: previous frame identical to this frame (corrupt stack?) ``` GDB can't unwind past HardFault since the current implementation of this function overwrites the Link Register (LR) value. This change pushes LR and R0 (to maintain 8-byte stack alignment) to the stack before transferring execution to UserHardFault(). After this change, I see a callstack like this from GDB: ``` (gdb) bt #0 UserHardFault_ (ef=0x10001fb0) at /depots/cortex-m-rt/src/lib.rs:537 #1 0x08003fe8 in HardFault () #2 <signal handler called> #3 0x08002820 in core::ptr::read_volatile (src=0x48001800) at libcore/ptr.rs:472 #4 0x080001a2 in main () at src/07-registers/src/main.rs:14 ``` Notes: * This code uses 8 more stack bytes. * Increases the size of the HardFault handler by 2 narrow instructions or 4 bytes. This could be decreased to 2 bytes by removing the pop since UserHardFault() doesn't currently return but it just looks too odd for me to do as an initial attempt. Co-authored-by: Adam Green <adamgreen@users.noreply.github.com>
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going to wait a bit on this one because
#[used]just landed