[mc] error: ambiguous instructions require an explicit suffix (could be 'btw', 'btl', or 'btq')

[edwintorok]

Bugzilla Link	9264
Resolution	FIXED
Resolved on	Feb 19, 2011 15:06
Version	trunk
OS	Linux
Blocks	#4440

Extended Description

clang -integrated-as first error with Linux kernel:
/home/edwin/builds/linux-2.6/arch/x86/include/asm/bitops.h:319:15: error: ambiguous instructions require an explicit suffix
(could be 'btw', 'btl', or 'btq')
asm volatile("bt %2,%1\n\t"
^
:1:2: note: instantiated into assembly here
bt $0,cpu_all_bits
^

asm volatile("bt %2,%1\n\t"
"sbb %0,%0"
: "=r" (oldbit)
: "m" (*(unsigned long *)addr), "Ir" (nr));

[edwintorok]

Also:
/home/edwin/builds/linux-2.6/arch/x86/include/asm/bitops.h:68:16: error: ambiguous instructions require an explicit suffix (could be 'btsw', 'btsl', or 'btsq')
asm volatile(LOCK_PREFIX "bts %1,%0"
^
In file included from arch/x86/ia32/ia32_signal.c:11:
In file included from include/linux/sched.h:54:
In file included from include/linux/kernel.h:17:
In file included from include/linux/bitops.h:22:
In file included from /home/edwin/builds/linux-2.6/arch/x86/include/asm/bitops.h:16:
/home/edwin/builds/linux-2.6/arch/x86/include/asm/alternative.h:43:21: note: instantiated from:
#define LOCK_PREFIX ""
^
:1:2: note: instantiated into assembly here
bts $2,-8136(%rax)
^
arch/x86/kernel/process_64.c:83:6: error: ambiguous instructions require an explicit suffix (could be 'btrw', 'btrl', or 'btrq')
if (x86_test_and_clear_bit_percpu(0, is_idle) == 0)
^

[edwintorok]

In this case (bit test of memory operand), the suffix only determines the maximum range of the bit index (i.e. if it is modulo 16,32, or 64-bits), since it is accessing memory.

The GNU assembler always chooses "btl" in this case:

GNU assembler turns this:
.global cpu_all_bits
foo:
bt $1, cpu_all_bits
bts $1, cpu_all_bits

Into this:
0000000000000000 :
0: 0f ba 24 25 00 00 00 btl $0x1,0x0
7: 00 01
9: 0f ba 2c 25 00 00 00 btsl $0x1,0x0
10: 00 01

I think "mc" should also choose "l" in this case, and not give an error, if one wants a specific bt* variant, they can write btw/btl/btq and choose it.

[lattner]

Fair enough, implemented in r126047.