Strange GDB Behavior on Cortex-M4

[Sal Ammoniac]

I was debugging some Cortex-M4 assembly code tonight when I noticed that my code would run fine when I single-stepped it using GDB, but crashed with a Usage Fault when I just let it run.

I traced the usage faults down to some "BX  r0" instructions. I had forgotten to set bit 0 (the Thumb bit) in r0, and the branch caused a usage fault for trying to change modes to "ARM" mode, which Cortex-M MCUs don't support. After I made sure all instances of "BX r0" followed code to set the Thumb bit, my code ran fine.

The question is: why did it run at all when single-stepping? Why didn't the first "BX r0" it executed when r0 bit 0 was set to 0 cause a usage fault?

[mubes]

Running in RAM or Flash? GDB, by default, inserts soft breakpoints for RAM based code and may be giving you some special love when it finds a BX with bit 0 clear....that might be a red herring but in general there's nothing that says that GDB might not be processing the BX internally and setting bit0...I think it needs to perform _some_ processing of the BX destination to know where to put the next BKPT and it's quite possible it doesn't actually *perform* the BX, just updates LR etc; Its source code perusal time if that kind of thing keeps you awake at night :-) Let us know what you find though, 'cos that is the kind of knowledge its useful to keep in your back pocket.

DAVE

[Sal Ammoniac]

Running in RAM or Flash? GDB, by default, inserts soft breakpoints for RAM based code and may be giving you some special love when it finds a BX with bit 0 clear....that might be a red herring but in general there's nothing that says that GDB might not be processing the BX internally and setting bit0...I think it needs to perform _some_ processing of the BX destination to know where to put the next BKPT and it's quite possible it doesn't actually *perform* the BX, just updates LR etc; Its source code perusal time if that kind of thing keeps you awake at night :-) Let us know what you find though, 'cos that is the kind of knowledge its useful to keep in your back pocket.

Code is running from Flash. I don't know why GDB does this, but it's a bad thing because it can mask errors in code that it shouldn't mask. If code is incorrect and will generate a fault, then I want to know about it. I don't want GDB "fixing" it for me. 

[mubes]

Yup. Not nessesarily defending it...but I suspect it's just updating LR rather than executing the BX.  You have no idea how many times I've got burned by that bit0....