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

[Jeffrey Walton <noloader at gmail dot com>]

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.

Off topic: I find that x86/x64 can sometimes benefit form inline
assembly. But the same does not appear to be true with ARM: you should
do it in C, or do it in ASM, but don't mix C with inline ASM.

Jeff