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From: "wwieser at gmx dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/52474] Regression: AVR-GCC: arithmetics produce completely wrong result
Date: Wed, 14 Mar 2012 08:30:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52474

--- Comment #3 from wwieser at gmx dot de 2012-03-14 01:19:59 UTC ---
OK, so talking to the debian maintainer, the GCC version I'm using is based
upon patches available here: 

http://distribute.atmel.no/tools/opensource/avr-gcc/

Here is a test case. I'm unsure how to make a test case for code to be executed
on a microcontroller because I need to make some assumptions about the board so
that the result can be shown to the user somehow (flash a LED, whatever...)

So, here is a test case which I believe to reproduce the bug. Please test this
against the newest version of avr-gcc. 

Compile with: 

bash# avr-gcc -W -Wall -O2 -mmcu=atxmega192a3 f1.c f2.c -o f.elf

We need 2 files (f1.c, f2.c) to prevent optimization of the whole computation. 
So, it is important to keep both files separate!

----------------<File: f1.c >------------------------------------
typedef signed short int int16;
typedef signed long int int32;

extern void ERROR(int16 v);
extern void SUCCESS(int16 v);
extern int16 QUERY(void);

int main()
{
    int16 slope = 1025 + QUERY();
    int16 v = (int16)( (int32)(-800) * slope / 512L ) ;

    // This will compute: v=31166 instead of the correct v=-1601. 
    if(v>0) ERROR(v); else SUCCESS(v);

    return(0);
}
-------------------------------------------------------------------

----------------<File: f2.c >------------------------------------
typedef signed short int int16;

void ERROR(int16 v)
{
    // fill in code that signals ERROR to the user or debugger breakpoint. 
}

void SUCCESS(int16 v)
{
    // fill in code that signals SUCCESS to the user or debug breakpoint. 
}


int16 QUERY(void)
{
    return(0);
}
--------------------------------------------------------------