Re: Inline Assembly: Assembling ARM/Thumb-16 ADDS or ADD with status update

[Andrew Haley <aph at redhat dot com>]

On 23/06/11 00:41, Jeffrey Walton wrote:
> On Tue, Jun 21, 2011 at 4:50 AM, Andrew Haley <aph@redhat.com> wrote:
>> On 21/06/11 06:21, Jeffrey Walton wrote:
>>> Hi All,
>>>
>>> ARMv6, -mthumb, and Apple's AS (1.38, probably modified).
>>>
>>> I've got a routine to add two unsigned ints and then check the carry
>>> flag in the CPSR. To perform the add, I use the Thumb-16 ADD
>>> instruction (ADDS is not available).
>>>
>>> __asm__ volatile
>>> (
>>>  "ldr  r4, %[xa]  ;"  // R4 = a
>>>  "ldr  r5, %[xb]  ;"  // R5 = b
>>>  "add  r6, r4, r5  ;"  // R6 = R4 + R5, set status
>>>  "bcc  1f      ;"  // jump if carry is clear
>>>  "mov  r5, #0    ;"  // set overflow
>>>  "str  r5, %[xc]  ;"  // write it to memory
>>>  "1:        ;"  // jump label
>>>  "str  r6, %[xr]  ;"  // result = R6
>>>  : [xr] "=m" (result), [xc] "=m" (no_carry)
>>>  : [xa] "m" (a), [xb] "m" (b)
>>>  : "r4", "r5", "r6"
>>>  );
>>>
>>> According to  'ARM® Developer Suite, v1.2', Section 2.2.8, page 2-9,
>>> the status flags are set with ADD under Thumb-16:
>>>
>>>    The conditional branch instruction is the only Thumb
>>>    instruction that can be executed conditionally on the
>>>    value of the ALU status flags in the CPSR. All data
>>>    processing instructions update these flags, except
>>>    when one or more high registers are specified as
>>>    operands to the MOV or ADD instructions. In these
>>>    cases the flags cannot be updated.
>>>
>>> Unfortunately, the status flags are not being updated.
>>
>> Yes they are, as you'll see when you step through.
>>
>> This is a logic failure: your asm declares that it writes no_carry, but
>> it only does so if carry is set.  Your asm must also write to no_carry
>> when carry is clear.  Then your asm will work.
> Thanks Andrew. My bad - what you did not see was the C language
> prologue which initialized no_carry.

That does not help at all.  Your asm declares that it writes no_carry,
so it must do so in both cases.  The compiler can drop the initialization
to no_carry in the C language prologue because it has no effect.

Incidentally, this is a useful lesson: when you post sample code,
you must post a complete example that other people can run.

Andrew.