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Bug#: 29808
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Status: RESOLVED
Resolution: INVALID
Assigned To: Not yet assigned to anyone <unassigned@gcc.gnu.org>
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Reporter: bero@arklinux.org
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Description:   Last confirmed: Opened: 2006-11-12 09:05 
The following code (extracted from the Linux kernel) fails to compile with
optimization enabled:

static void load_fs(unsigned short sel)
{
        asm("mov %0, %%fs" : : "g"(sel));
}

int main(int argc, char **argv)
{
        load_fs(0);
}


[bero@dhcppc1 arklinux]$ gcc test.c
[bero@dhcppc1 arklinux]$ gcc -O1 test.c
/tmp/ccSpA5Rm.s: Assembler messages:
/tmp/ccSpA5Rm.s:13: Error: suffix or operands invalid for `mov'

gcc is 4.2 SVN revision 118519 (20061106).

------- Comment #1 From Segher Boessenkool 2006-11-12 11:10 ------- 
Not a bug in GCC but in your code; "g" says immediate values
are allowed, while this asm insn only takes registers (or 16-bit
memory).

------- Comment #2 From Richard Guenther 2006-11-12 13:26 ------- 
invalid

------- Comment #3 From bero@arklinux.org 2006-11-12 13:40 ------- 
If the code is invalid, the fact that it compiles with -O0 is probably a bug...

------- Comment #4 From Segher Boessenkool 2006-11-12 14:01 ------- 
> If the code is invalid, the fact that it compiles with -O0 is probably a bug...

No, GCC cannot in general detect whether your asm code is buggy.
The assembler however can detect many asm bugs, as it did in this
case.

The reason it worked with -O0 is that with -O0 you get different
generated code (namely, very inefficient code).

------- Comment #5 From Andrew Pinski 2006-11-12 17:15 ------- 
(In reply to comment #3)
> If the code is invalid, the fact that it compiles with -O0 is probably a bug...
No it is not really a bug that it compiles at -O0 either becuase "g" means
"r"+"i" so the register allocator in the -O0 case is selecting "r" while in the
optimize case is selecting "i".
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