TMS320F28027: Compiler adding blank address in a struct

dev123

Part Number: TMS320F28027

Hello,

The problem arise only using an unsigned long in a struct and only with a struct size greater than a specific threashold ( 9? )

Compilers tested

16.9.10 LTS
20.2.5 LTS

example:

in the working code, outsideVar has address: 0x00008306
This is correct, since the previus variable w3 has address: 0x00008305

In the code with bug, outsideVar has address: 0x00008320
This is not correct, since the previus variable w10 has address: 0x0000831E
So compier let 0x0000831F blank. However you can see this address in the variable "all", so this behavior can be dangerus for memcopy routine

Code working fine

typedef union _TMP
{
    struct
    {
        union
        {
            struct
            {
                unsigned short w0;
                unsigned short w1;
                unsigned short w2;
                unsigned short w3;
            } EXPANDED;
            struct
            {
                unsigned long w0_w1;
                unsigned short w2_w3[2];
            } COMPRESSED;
            unsigned short all[4];
        } SUBSYSTEM;
        unsigned short outsideVar;
    }expand;
    unsigned short all[5];
} TMP;

Code with bug:

typedef union _TMP
{
    struct
    {
        union
        {
            struct
            {
                unsigned short w0;
                unsigned short w1;
                unsigned short w2;
                unsigned short w3;
                unsigned short w4;
                unsigned short w5;
                unsigned short w6;
                unsigned short w7;
                unsigned short w8;
                unsigned short w9;
                unsigned short w10;
            } EXPANDED;
            struct
            {
                unsigned long w0_w1;
                unsigned short w2_w18[9];
            } COMPRESSED;
            unsigned short all[11];
        } SUBSYSTEM;
        unsigned short outsideVar;
    }expand;
    unsigned short all[12];
} TMP;

over 1 year ago

0 Omer Amir over 1 year ago

TI__Mastermind 30185 points

Hello Nicola,

To clarify, do both compiled codes function the same? I'm curious why the first situation would be okay, considering outsideVar and all[4] are pointing to the exact same address, whereas in the compiled code with a 'bug', they don't overlap. Is this intentional? How are you defining a variable/its fields with this struct? It's difficult for me to understand the issue since I don't see how this struct gets used (i.e. are all the fields supposed to be pointing to the same array/variable addresses?). Please provide some example code on how you're using the struct.

0 dev123 over 1 year ago in reply to Omer Amir

Prodigy 110 points

Hi Omer, this is a running example. I don't use this struct, it's just wrote to be tested

typedef union _TMP
{
    struct
    {
        union
        {
            struct
            {
                unsigned short w0;
                unsigned short w1;
                unsigned short w2;
                unsigned short w3;
                unsigned short w4;
                unsigned short w5;
                unsigned short w6;
                unsigned short w7;
                unsigned short w8;
                unsigned short w9;
                unsigned short w10;
            } EXPANDED;
            struct
            {
                unsigned long w0_w1;
                unsigned short w2_w10[9];
            } COMPRESSED;
            unsigned short all[11];
        } SUBSYSTEM;
        unsigned short outsideVar;
    }expand;
    unsigned short all[12];
} TMP;

TMP test;

while (1)
{
    test.expand.SUBSYSTEM.EXPANDED.w0 = 0;
    test.expand.SUBSYSTEM.EXPANDED.w1 = 1;
    test.expand.SUBSYSTEM.EXPANDED.w2 = 2;
    test.expand.SUBSYSTEM.EXPANDED.w3 = 3;
    test.expand.SUBSYSTEM.EXPANDED.w4 = 4;
    test.expand.SUBSYSTEM.EXPANDED.w5 = 5;
    test.expand.SUBSYSTEM.EXPANDED.w6 = 6;
    test.expand.SUBSYSTEM.EXPANDED.w7 = 7;
    test.expand.SUBSYSTEM.EXPANDED.w8 = 8;
    test.expand.SUBSYSTEM.EXPANDED.w9 = 9;
    test.expand.SUBSYSTEM.EXPANDED.w10 = 10;
    test.expand.outsideVar = 11;
}

Please note that if I change

unsigned long w0_w1;

unsigned short w0_w1[2];

everything works fine

0 Genatco over 1 year ago in reply to dev123

Guru 55913 points

Hi dev123 - Omer,

There is some oddness around arrays and longs. The C2000 compiler guide Optimizing-C gives clues, by default bytes are 16 bit and seemingly w0_w1[2] is actually 32 bit long. Oddly longs do not behave well in C lib arrays data often has random breaks. Reported similar behavior some time ago casting 32 bits into functions using arrays as storage is a bit touchy. Imagine the same is true of nested structs with arrays in your test code.

/cfs-file/__key/communityserver-discussions-components-files/171/TI_2D00_C2000-Optimizing-C_2C00_C_2B002B00_-Compilers-spru514y.pdf

+1 George Mock over 1 year ago in reply to Genatco

TI__Guru**** 249860 points

The compiler is working as designed. For a general introduction to the concepts involved, please see this FAQ (not from TI).

Here are some facts about the C2000 architecture that are relevant.

Each address corresponds to a 16-bit word, and not an 8-bit byte
The type long is 32-bits wide, and must be a aligned on a 2-word boundary
The type short is 16-bits wide, and has no alignment requirement

This structure ...

            struct
            {
                unsigned long w0_w1;
                unsigned short w2_w18[9];
            } COMPRESSED;

... contains a long. Thus, it must be aligned to a 2-word boundary, and must contain an even number of words. The total size of the members in this struct is 11 words. Thus, a word of padding is added to the end for a total length of 12 words. This alignment and padding requirement propagates out, and causes the struct member outsideVar to always be on an even address.

dev123 said:
if I change
Fullscreen
1
unsigned long w0_w1;
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
unsigned long w0_w1; 
to
Fullscreen
1
unsigned short w0_w1[2];
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
unsigned short w0_w1[2];
everything works fine

Because you removed the long, the padding and alignment are not needed.

Thanks and regards,

-George

C2000™︎ microcontrollers

C2000 microcontrollers forum

TMS320F28027: Compiler adding blank address in a struct