[Bug inline-asm/17346] New: linux kernel: handwritten offsetof break inline asm

["matz at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

The linux kernel has a file asm-context.c, which is used to get hold 
of several offsets inside assembler.  For this inline asm is used to 
place the offsets directly into the assembler output, which later is parsed. 
A testcase is this: 
------------------------------------- 
struct A { int a, b; }; 
void foo(void) 
{ __asm volatile ("%0" : : "i" ((long)&((struct A*)0)->b)); } 
------------------------------------- 
 
The problem is, that the expression (T*)0->b is not thought to be constant 
anymore, and is placed inside a register.  The "i" constraints require a 
constant of course, so we get this error message when compiling the above: 
  foo.c:3: warning: asm operand 0 probably doesn't match constraints 
  foo.c:3: error: impossible constraint in `asm' 
 
Using __builtin_offsetof would fix this particular instance, but I think 
that the expression really should be taken as constant by the optimizers.

-- 
           Summary: linux kernel: handwritten offsetof break inline asm
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: inline-asm
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: matz at suse dot de
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i686-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17346