[Bug c/23544] New: behavour by expression with unsigned variables in if-statement is not like expected

[Bug c/23544] New: behavour by expression with unsigned variables in if-statement is not like expected

[tobias dot leutwein at bosch dot com]

void main(void)
{
    unsigned short v1, v2, e1;

    v1 = 6;
    v2 = 8;

    if ( v1 - v2 < 2 )
    {
	/* this is not the expected behaviour */
        e1 = v1 - v2;
    }
}

I would expect that v1-v2 in the if statement is 65534. It seems to be that
v1-v2 in the if statement gets (-2) and so the condition is true.


Compiling: V:/tmp/compiler-fehler/src/main.c
cd V:/tmp/compiler-fehler/makeout/obj; \
V:/Programme/MinGW/bin/gcc -v -c -g                      -std=gnu99 -IV:/tmp/com
piler-fehler/src -DPROJECT_H="<project.h>" -DSTDTYPE_H="<stdtype.h>" -D__AVR_ATm
ega32__=1 -fno-common -fverbose-asm -O0 -Wall -Wbad-function-cast -Wcast-align -
Winline -Wmissing-declarations -Wmissing-noreturn -Wmissing-prototypes -Wnested-
externs -Wno-comment -Wno-unused-function -Wpacked -Wpointer-arith -Wstrict-prot
otypes  -Wp,-M,-MP,-MT,V:/tmp/compiler-fehler/makeout/obj/main.o,-MF,V:/tmp/comp
iler-fehler/makeout/dep/main.d V:/tmp/compiler-fehler/src/main.c -o main.o;
Reading specs from v:/Programme/MinGW/bin/../lib/gcc/mingw32/3.4.2/specs
Configured with: ../gcc/configure --with-gcc --with-gnu-ld --with-gnu-as --host=
mingw32 --target=mingw32 --prefix=/mingw --enable-threads --disable-nls --enable
-languages=c,c++,f77,ada,objc,java --disable-win32-registry --disable-shared --e
nable-sjlj-exceptions --enable-libgcj --disable-java-awt --without-x --enable-ja
va-gc=boehm --disable-libgcj-debug --enable-interpreter --enable-hash-synchroniz
ation --enable-libstdcxx-debug
Thread model: win32
gcc version 3.4.2 (mingw-special)
 v:/Programme/MinGW/bin/../libexec/gcc/mingw32/3.4.2/cc1.exe -quiet -v -IV:/tmp/
compiler-fehler/src -iprefix v:\Programme\MinGW\bin\../lib/gcc/mingw32/3.4.2/ -D
PROJECT_H=<project.h> -DSTDTYPE_H=<stdtype.h> -D__AVR_ATmega32__=1 -M -MP -MT V:
/tmp/compiler-fehler/makeout/obj/main.o -MF V:/tmp/compiler-fehler/makeout/dep/m
ain.d V:/tmp/compiler-fehler/src/main.c -quiet -dumpbase main.c -auxbase-strip m
ain.o -g -O0 -Wall -Wbad-function-cast -Wcast-align -Winline -Wmissing-declarati
ons -Wmissing-noreturn -Wmissing-prototypes -Wnested-externs -Wno-comment -Wno-u
nused-function -Wpacked -Wpointer-arith -Wstrict-prototypes -std=gnu99 -version
-fno-common -fverbose-asm -o c:\TEMP/ccwzaaaa.s
ignoring nonexistent directory "v:/Programme/MinGW/bin/../lib/gcc/mingw32/3.4.2/
../../../../mingw32/include"
ignoring nonexistent directory "/mingw/include"
ignoring nonexistent directory "/mingw/include"
ignoring nonexistent directory "/mingw/lib/gcc/mingw32/3.4.2/include"
ignoring nonexistent directory "/mingw/mingw32/include"
ignoring nonexistent directory "/mingw/include"
#include "..." search starts here:
#include <...> search starts here:
 V:/tmp/compiler-fehler/src
 v:/Programme/MinGW/bin/../lib/gcc/mingw32/3.4.2/../../../../include
 v:/Programme/MinGW/bin/../lib/gcc/mingw32/3.4.2/include
End of search list.
GNU C version 3.4.2 (mingw-special) (mingw32)
        compiled by GNU C version 3.4.2 (mingw-special).
GGC heuristics: --param ggc-min-expand=99 --param ggc-min-heapsize=131001
V:/tmp/compiler-fehler/src/main.c:2: warning: return type of 'main' is not `int'

 v:/Programme/MinGW/bin/../lib/gcc/mingw32/3.4.2/../../../../mingw32/bin/as.exe
-o main.o c:\TEMP/ccwzaaaa.s

-- 
           Summary: behavour by expression with unsigned variables in if-
                    statement is not like expected
           Product: gcc
           Version: 3.4.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tobias dot leutwein at bosch dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-mingw32
  GCC host triplet: i686-pc-mingw32
GCC target triplet: i686-pc-mingw32


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23544

[Bug c/23544] behavour by expression with unsigned variables in if-statement is not like expected

[tobias dot leutwein at bosch dot com]

------- Additional Comments From tobias dot leutwein at bosch dot com  2005-08-24 10:54 -------
Example is wrong.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23544