compiler thoughts

Monday February 26, 2007

interesting bug

While working on building OpenSolaris with gcc4/gccfss I encountered one interesting bug which can be demonstrated by the following simple C program:

void foo (long long *);

void bar (long a, long long b)
{
   typeof (a+b) c;
   foo (&c);
}

compiled with gcc 3.4 it will print the warning:

$ gcc3 -c simple.c -m64
simple.c: In function `bar':
simple.c:6: warning: passing arg 1 of `foo' from incompatible pointer type

whereas gcc 4.x will compile this testcase without any warnings.

The reason for that is GCC 3.x consider 'long + long_long' expression as 'long' type in 64-bit mode which is incorrect.

Why would I care ?

There are multiple places in OpenSolaris where it needs to print 64-bit value and it is done with '%llx' printf specifier. Due to this gcc 3.4 bug the Solaris sources were using '%lx' specifier to print expressions involving long and longlong types and now our changes to compile ON with gcc 4.x would break ON build with /usr/sfw/bin/gcc 3.4 unless the fix can be backported into "csl-sol210-3_4" Solaris specific gcc branch.

All in all, Solaris seems to be overdue to upgrade to some newer version of GCC.