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ARM Compiler creates a bug in code - how to fix?

AlexRast
Note: This was originally posted on 17th September 2009 at http://forums.arm.com

I wouldn't have thought ARMCC could create an explicit bug, but it seems here it is.

In this case, what it's doing is placing an item on the stack in a position which will certainly corrupt the stack.

Looking through the disassembly for the offending function, I can see that the first line (as expected) is

PUSH     {r2-r4,r6-r11,lr}

But then later in the code, and with NO other stack operations in the meanwhile (nor any function calls or other branches out of the current function context) it has

ADD      r2,r0,r4,LSL #2
LDR      r0,|L1.3728|
STR      r2,[sp,#4]

As I understand it, since the stack is full-descending, this means that the value of r2 currently in the stack will get corrupted, because the STR instruction will store the (changed) current r2 value in the position occupied by the original r2. The function in concern is declared

void receivePacket(void)

so it's not expecting to return values in r2 or take arguments in r2.

Why would armcc do something so glaringly illegal? Is there something I can do to fix this?

As an aside, I'll mention that this bad stacking is actually occuring in the setup for a subroutine call, that happens a few lines lower down. The routine in concern uses r1, r2 and r3 for its arguments and returns in r0 and r1. As it happens, the value that it's stacking from r2 is actually the address that will take the return value in r0 - really, what the compiler *should* do, I think, is place the address in some register rx (x > 3) then do an STR r0, [rx, #0] on return.
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  • Arm Employee Badge
    Note: This was originally posted on 18th September 2009 at http://forums.arm.com

    Paraphrasing Isogen,

    Your function is using a shed load of registers, plus two words of stack;
    the PUSH provides the space for the two words of stack space without
    the need for an addition "SUB SP"; presumably your code is optimised
    to reduce code size, rather than execution time.

    hth
    s.
Reply
  • Arm Employee Badge
    Note: This was originally posted on 18th September 2009 at http://forums.arm.com

    Paraphrasing Isogen,

    Your function is using a shed load of registers, plus two words of stack;
    the PUSH provides the space for the two words of stack space without
    the need for an addition "SUB SP"; presumably your code is optimised
    to reduce code size, rather than execution time.

    hth
    s.
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