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Re: [avr-gcc-list] Strange compile behaviour

From: Wouter van Gulik
Subject: Re: [avr-gcc-list] Strange compile behaviour
Date: Tue, 02 Feb 2010 13:06:34 +0100
User-agent: Thunderbird (Windows/20090812)

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.

If you are using interrupts than you might (by accident) be waiting for the moment that no interrupts are being executed. And thus no extra stack usage during your function execution.

The code size increment is due to a smart optimization. You force app_TXIncomming to be zero before continuing, gcc notes it and does not do a multiply that saves some pointer calculation and therefore space. So your loop is doing something after all ;)

This also indicates app_TXIncomming as not being volatile? Or a gcc optimizer bug.



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
Web: www.wirelessvalue.nl <http://www.wirelessvalue.nl>

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