Stack problem in THUMB mode with GCC 4.1.0

[moved from tracker item] I am using CrossWorks 1.6 build 3 (=> GCC codesourcery version 4.1.0) and FreeRTOS 4.1.1. I am compiling my project in THUMB mode. The problem occurs in small functions without local variables. The following code is generated (interleaved view of the crossworks debugger): 01: StatModeInfo *GetStatModeInfo(void) 02: { 03: B580 push {r7, lr} 04: AF02 add r7, sp, #0x008 05: return( &g_StatModeInfo ); 06: 4B03 ldr r3, [pc, #0x00c] 07: }//StatModeInfo *GetStatModeInfo(void) 08: 1C18 adds r0, r3, #0x0 09: 46BD mov sp, r7 10: B082 sub sp, #0x008 11: BC80 pop {r7} 12: BC02 pop {r1} 13: 4708 bx r1 When a timer interrupt (context switch) occurs after the execution of line 9 but before line 10 then the context of the current task will be copied to the stack. But the SP is wrong at that moment! The return address will be overwritten with some register contents. The problem does not occur in ARM mode (at least I have not yet seen it). And it does not occur with GCC 3.4.4 (comes with CrossWorks 1.5 build 2). Maybe this is not a bug and should be treated as an incompatibility between FreeRTOS 4.1.1 and codesourcery GCC 4.1.0?? René

Stack problem in THUMB mode with GCC 4.1.0

Thanks for the information.  Can you provide the compiler switches you were using when this code was compiled (optimisation in particular).  Thanks. Regards.

Stack problem in THUMB mode with GCC 4.1.0

On line #4, why is 8 added to the stack pointer.  Should this not be subtracted for a negative growing stack?

Stack problem in THUMB mode with GCC 4.1.0

Optimization is off. The compiler switches for the file containing the mentioned function GetStatModeInfo() are: -mlittle-endian -Wmissing-prototypes -Wstrict-prototypes -Wimplicit-function-declaration -Wunused-variable -gdwarf-2 -march=armv4t -mthumb -mthumb-interwork -mlittle-endian -fno-builtin -msoft-float -mfpu=vfp I have already got a hint from CrossWorks helpdesk. With compiler switch -fomit-frame-pointers the code works. But I don’t understand completely the meaning of this switch. I am not sure about the consequences this switch has for the generated code and so I am not sure if it does solve the problem in any case. René

Stack problem in THUMB mode with GCC 4.1.0

Because 8 bytes have already been pushed onto the stack, and it is recording the stack position for when the function exits. Regards.

Stack problem in THUMB mode with GCC 4.1.0

There was some talk in this forum before with using -fomit-frame-pointers with WinARM.  I think this is a bug in GCC.  Any interrupt code that switches to use system mode will have this problem.  It is not just FreeRTOS but all the ST standard interrupt stubs do this.

Stack problem in THUMB mode with GCC 4.1.0

Having discussed this with people that know a lot more about GCC than me the consensus is that this is a bug in GCC, of which users need to be aware. Regards.

Stack problem in THUMB mode with GCC 4.1.0

-fomit-frame-pointers does the trick, but unfortunately messes up the ability for the debugger (Rowley CrossWorks) to properly unwind the stack. GCC 4.1.1 does not fix the problem. So I am curious to know what people are doing. The choices seem to be to use ARM mode (I haven’t yet verified that this even works) or live with a crippled debuggin environment. Are there other suggestions?

Stack problem in THUMB mode with GCC 4.1.0

Ah. What I was forgetting was that FreeRTOS could be ‘fixed’. The patch is quite inocuous (little overhead and still works in the absence of the GCC bug). The bug is that in Thumb mode GCC can emit code that, on the return from functions with certain prototypes, will briefly have the stack pointer located 8 bytes into the actual stack. If a context switch occurs here, the task registers are saved on the stack, thus clobbering the valid data there. So the fix is to always skip 8 stack bytes before pushing the task context. I know this is not ideal, but it is the best alternative for me right now. In the portSAVE_CONTEXT() macro of portmacro.h, just subtract 8 from the stack pointer (actually in R0 at the point of interest), just before the return address is pushed. Some of the surrounding dressing has been removed in the following snippet: /*gb – step over possible GCC stacked items (GCC bug)*/ "SUB    R0, R0, #8   …. /* Push the return address onto the stack. */ "STMDB    R0!, {LR}    …. I hope it is never more than 8 bytes! My test hasn’t been all thorough. But I don’t think I have broken anything.

Stack problem in THUMB mode with GCC 4.1.0

Thanks for the fix but should be aware this is a problem with any code that uses the user stack from an ISR.  Any bought software might do this.  I think this is a big bug in GCC that needs fixing.