On Wed, Jun 22, 2011 at 7:41 PM, Jeffrey Walton <noloader@xxxxxxxxx> wrote:
> On Tue, Jun 21, 2011 at 4:50 AM, Andrew Haley <aph@xxxxxxxxxx> 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;
}
