bug around inline asm, or expected?

[Luke <luke at neoclaw dot net>]

I hit an odd problem recently while working with some inline assembly 
(x86).  I'm not sure if it's a bug, or if it is expected behavior.  I 
managed to repro it in a small compilable file:

#include <iostream>

//Compares expectedValue with the value at pInt, and if they are the 
same, sets pInt to newValue and returns true with outResultValue set to 
newValue.  If they are different then pInt is unaffected, and returns 
false with outResultValue set to the value that was found at pInt.
inline bool AtomicCompareExchange(volatile unsigned int *pInt, unsigned 
int expectedValue, unsigned int newValue, volatile unsigned int 
&outResultValue)
{
  unsigned int changed=0;
  unsigned int rval;
  asm(
          "lock cmpxchgl %4, (%2);"
          "jnz AtomCmpExch_Diff%=;"
          "movl %4, %%eax;"
          "incl %1;"
          "AtomCmpExch_Diff%=:;"
          "movl %%eax, %0;"
      :"=g"(rval), "=g"(changed)
      :"r"(pInt), "a"(expectedValue), "r"(newValue)
      :"memory", "cc");

  outResultValue=rval;
  return changed!=0;
}

int main()
{
  unsigned int value=0x100;
  unsigned int ignored;
  bool changed=AtomicCompareExchange(&value, 0x200, 0x300, ignored);
    if (changed) std::cout<<"Returned true, but should have returned 
false.\n";
  else std::cout<<"Returned false as expected.\n";
    return 0;
}

With optimization off it works fine.  Running g++ with -O1 or -O2 
though, and it hits a problem: that the inline function always returns 
true, when it should in this case return false.  I actually did find a 
"solution" to it.  Declaring the "unsigned int changed=0;" as volatile 
prevents the problem from happening.  However, it seems a bit odd that 
that's necessary, seeing as it's given as an output parameter in the asm 
declaration.

Here's the output of g++ -v:
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v 
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr 
--enable-shared --with-system-zlib --libexecdir=/usr/lib 
--without-included-gettext --enable-threads=posix --enable-nls 
--with-gxx-include-dir=/usr/include/c++/4.1.3 --program-suffix=-4.1 
--enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug 
--enable-mpfr --with-tune=i686 --enable-checking=release i486-linux-gnu
Thread model: posix
gcc version 4.1.3 20070629 (prerelease) (Debian 4.1.2-13)