On 23/06/11 00:41, Jeffrey Walton 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. 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.
