Coldfire Toolchain packed structure bug version 4.1.30 and newer

[Malnar, Tom]

We recently ugraded our tools to version 4.1.30 for the coldfire from
3.4.  

Our processor is a MCF5475VR266.  

We noticed a new bug in the assembler code produced when dereferencing a
packed structure.   

 

1. The bug occurs in gcc and g++. 

2. The bug occurs when using:  #pramga pack(1)  or  __attribute__
((__packed__,aligned(1))) directives

3. The code was built using the following command line options:

 

gcc -g -Wall -o obj/main.o -c main.c

 

4. The assembly code was dumped using:

 

m68k-linux-gnu-objdump -lS src/testCode/obj/main.o

 

5. The code used in our example

 

#include <stdio.h>

 

struct TestStruct {

    unsigned short var1;

};

 

int main() {

    struct TestStruct varStruct;

    struct TestStruct *pVarStruct = &varStruct;

    varStruct.var1 = 1;

    pVarStruct->var1 = 2;

    return 0;

}

 

6. The good assembly code produced:

 

src/testCode/obj/main.o:     file format elf32-m68k

 

Disassembly of section .text:

 

00000000 <main>:

main():

main.c:7

struct TestStruct {

    unsigned short var1;

};

 

int main() {

   0:   4e56 fff8       linkw %fp,#-8

main.c:9

    volatile TestStruct varStruct;

    volatile TestStruct *pVarStruct = &varStruct;

   4:   41ee fffa       lea %fp@(-6),%a0

   8:   2d48 fffc       movel %a0,%fp@(-4)

main.c:10

    varStruct.var1 = 1;

   c:   7001            moveq #1,%d0

   e:   3d40 fffa       movew %d0,%fp@(-6)

main.c:11

    pVarStruct->var1 = 2;

  12:   206e fffc       moveal %fp@(-4),%a0

  16:   30bc 0002       movew #2,%a0@

main.c:12

    return 0;

  1a:   4280            clrl %d0

main.c:13

}

  1c:   4e5e            unlk %fp

1e:   4e75            rts

 

 

7.  Code that exhibits the issue:

 

#include <stdio.h>

 

struct __attribute__ ((__packed__,aligned(1))) TestStruct {

    unsigned short var1;

};

 

int main() {

    struct TestStruct varStruct;

    struct TestStruct *pVarStruct = &varStruct;

    varStruct.var1 = 1;

    pVarStruct->var1 = 2;

    return 0;

}

 

8.  The assembly created by the source above. Problem: the deference of
the unsigned short variable and move is done in two 1 byte moves, plus
there also seems to be a lot of extra assembly instructions.  

 

src/testCode/obj/main.o:     file format elf32-m68k

 

Disassembly of section .text:

 

00000000 <main>:

main():

main.c:7

struct __attribute__ ((__packed__,aligned(1))) TestStruct {

    unsigned short var1;

};

 

int main() {

   0:   4e56 fff8       linkw %fp,#-8

main.c:9

    TestStruct varStruct;

    TestStruct *pVarStruct = &varStruct;

   4:   41ee fffa       lea %fp@(-6),%a0

   8:   2d48 fffc       movel %a0,%fp@(-4)

main.c:10

    varStruct.var1 = 1;

   c:   7001            moveq #1,%d0

   e:   3d40 fffa       movew %d0,%fp@(-6)

main.c:11

    pVarStruct->var1 = 2;

  12:   206e fffc       moveal %fp@(-4),%a0

  16:   1010            moveb %a0@,%d0

  18:   4281            clrl %d1

  1a:   c081            andl %d1,%d0

  1c:   1000            moveb %d0,%d0

  1e:   1080            moveb %d0,%a0@

  20:   1028 0001       moveb %a0@(1),%d0

  24:   4281            clrl %d1

  26:   c081            andl %d1,%d0

  28:   1000            moveb %d0,%d0

  2a:   a541            mov3ql #2,%d1

  2c:   8081            orl %d1,%d0

  2e:   1000            moveb %d0,%d0

  30:   1140 0001       moveb %d0,%a0@(1)

main.c:12

    return 0;

  34:   4280            clrl %d0

main.c:13

}

  36:   4e5e            unlk %fp

38:   4e75            rts

 

 

9. One point to add.  If we create an unsigned short pointer and assign
it to pVarStruct->var1 the assembly code produced when dereferencing the
unsigned short pointer is correct. 

 

 

Does anyone have any suggestions on some things we could try?  Will this
issue be addressed in a future tool chain release?

 

 

Thanks.

 

- Tom

 

 

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