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[avr-gcc-list] Strange compile behaviour

From: Han Hoekstra
Subject: [avr-gcc-list] Strange compile behaviour
Date: Tue, 2 Feb 2010 11:35:35 +0100

Hello all,


I hope someone can help me with a strange problem I have.


What I see is that there is a problem with my stack, I check if the 2 bytes at __bss_end and __bss_end + 1 have a specific value, which I put in them, so I can detect stack overflows in my application. I have a piece of code that will generate a stack problem depending on if I enable or disable a while loop in that piece of code.


The code is :


while (app_TXIncomming != 0) {

}     /* while */

                        configMsg = &app_RS232_TX_Buffers [app_TXIncomming];

                        if (!configMsg->BufferFull) {

                              /* There is free space, so create the message */

                              /* We will send NoConfig_ind messages to the base CPU and the

                               * base Extender, because we don't know what is in the

                               * basestation */

                              app_GenAddress.NetworkID = NETWORK_ID_BROADCAST;

                              app_GenAddress.DeviceID  = DEVICE_ID_BROADCAST;

                              switch (stepStateNoConfig) {

                                    case app_NOCONFIGSTEP_BASECPU:

                                          app_GenAddress.DeviceType = DEVTYPE_WS_BASE_CPU;

                                          stepStateNoConfig         = app_NOCONFIGSTEP_EXTENDER;


                                    case app_NOCONFIGSTEP_EXTENDER:

                                          app_GenAddress.DeviceType = DEVTYPE_WS_EXTENDER;

                                          stepStateNoConfig         = app_NOCONFIGSTEP_BASECPU;


                              }     /* switch */


                              //WISP_Put_Flags (configMsg, 0);

                              WISP_Put_Flags (configMsg, 0);


This code is part of a larger function.

A little explanation of the code:

configMsg                     pointer to the same type as the array app_RS232_TX_Buffers

app_TXIncomming          the index of the next buffer that I can use to create a message that must be transmitted via the RS-232 port.

app_GenAddress           a structure that can contain an address as we  use it


What I see no is that when the while loop at the beginning is compiled into the code, everything is running OK, but when I comment it out, so it is not in de code, the stack check fails, indicating that __bss_end and __bss_end + 1 no longer have the specific test values.


Below is the part of the .lst file for both cases.


With commented out while loop:


585                                           lds r24,app_TXIncomming

 586                                          mul r24,r16

 587 012a 84E0                         movw r30,r0

 588 012c 0E94 0000                 clr r1

 589 0130 00C0                         subi r30,lo8(-(app_RS232_TX_Buffers))

 590                                          sbci r31,hi8(-(app_RS232_TX_Buffers))

 592                  .LM51:

 593 0132 8091 0000                  ldd r24,Z+1

 594 0136 809F                          tst r24

 595 0138 F001                          brne .L20

 597 013c E050             .LM52:

 598 013e F040                          sts app_GenAddress,__zero_reg__

 600                  .LM53:

 601 0140 8181                          sts (app_GenAddress+2)+1,__zero_reg__

 602 0142 8823                          sts app_GenAddress+2,__zero_reg__

 604                  .LM54:

 605                                          tst r17

 606 0146 1092 0000                  brne .L45

 608                  .LM55:

 609 014a 1092 0000                  sts app_GenAddress+1,r15

 610 014e 1092 0000                  ldi r17,lo8(1)

 611                  .L21:

 613 0152 1123              .LM56:

 614 0154 01F4                          movw r24,r30

 615                                          ldi r22,lo8(0)

 616                                          call WISP_Put_Flags


With while loop in place:


 583 0126 0E94 0000     .LM50:

 584 012a 00C0                         lds r24,app_RS232_TX_Buffers+1

 585                                          tst r24

 586 012c 8091 0000                  brne .L21

 588                  .LM51:

 589                                          sts app_GenAddress,__zero_reg__

 591 0132 01F4              .LM52:

 592                                          sts (app_GenAddress+2)+1,__zero_reg__

 593                                          sts app_GenAddress+2,__zero_reg__

 595 0138 8823              .LM53:

 596 013a 01F4                          tst r17

 597                                          brne .L47

 599 013c 1092 0000      .LM54:

 600                                          sts app_GenAddress+1,r16

 601                                          ldi r17,lo8(1)

 602 0140 1092 0000      .L22:

 604                  .LM55:

 605                                          ldi r24,lo8(app_RS232_TX_Buffers)

 606 0148 1123                          ldi r25,hi8(app_RS232_TX_Buffers)

 607 014a 01F4                          ldi r22,lo8(0)

 608                                          call WISP_Put_Flags


When I leave the while loop in the code, the code is smaller than without the while loop, which seems strange, because the while loop has no effect on the actual code.


I am using the latest WinAVR with AVR-GCC 4.3.3 at the moment, but I started with the previous version AVR-GCC 4.3.2, which did the exact same thing.

Optimisation is set to size, I am using an ATMega168. Optimisations that I use are :

-mcall-prologues, -ffunction-sections, -fdata-sections, -fno-free-loop-optimize, -fno-inline, -fno-tree-scev-cprop, -fpack-struct

This seems to give me the smallest code, which is what I need, because I am almost at the limit of code space.


The most simple solution would be to just keep the while loop in the code, but since it does nothing, it is a strange piece of code to keep in my application and what other parts of the code would have similar problems and would also require something like this ?!


Does anyone know what is going wrong here ?






Han Hoekstra

Software Engineer

Wireless Value B.V.
Waanderweg 30a
7812 HZ  Emmen
Tel: +31-591-633200
Fax: +31-591-631197


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