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

[Andrew Haley <aph at redhat dot com>]

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.

Andrew.