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

[Jeffrey Walton]

On Wed, Jun 22, 2011 at 7:41 PM, Jeffrey Walton <noloader@gmail.com> 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.
>
> I believe the problem is the ADD on Thumb-16 does not set the flags
> (despite the ARM manual cited - perhaps it applies to Thumb-2). When I
> followed the ADD with a CMN, the flags were set.
Finished with the knob turning, and wanted to post results in case
someone else stumbles across this.

According to the Thumb-16 Quick Reference Guide [1], the ADDS
instruction should be available. This appears to be a bug in the
assembler (ie, rejecting a valid sentence of the language). Unless of
course, spurious rejections are OK and expected....

To work around AS's unwillingness to assemble certain mnemonics, one
can emit the bytes via a directive. For example, in the following code
the `.short 0x1809` is actually `ADDS R1, R1, R0`.

Jeff

int add_u32(u32t a, u32t b, u32t* r)
{
    volatile int no_carry;
    volatile u32t result;
	
    __asm__ volatile
    (
        // Per the ARM ABI, A and B are in R0 and R1.

        "mov    r3, #1    ;"	// R3 = no_carry = TRUE
	
        // "adds    r1, r1, r0  ;" // Nope - assembler error
        ".short 0x1809    ;"    // Same as ADDS R1, R1, R0
        "bcc    1f			;"
        "mov	r3, #0		;"	// R3 = no_carry = FALSE
	
        "1:				;"
        "str    r1, %[xr]	;"	// result = R1
        "str    r3, %[xc]	;"	// no_carry = R3
	
	 : [xr] "=m" (result), [xc] "=m" (no_carry)
	 :
	 : "r1", "r3"
    );

    if(r)
        *r = result;

    return no_carry;
}