[Bug target/81357] Extra mov for zero extend of add

["wilco at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81357

--- Comment #11 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Qing Zhao from comment #10)
> the following is my conclusion on this bug based on previous discussion and
> study, for this testing case:
> 
>   1. due to the fact that  "mov" and "uxtw" are the same instruction, the
> assembly generated by -O1 and -O2 are exactly the same except 
>      A. the order of the instructions (this is due to the instruction
> scheduling applied in -O2). 
>      B. the registers used in difference instructions. 
> 
>   2. I agree with Wilco's comments in comment 7:
> "The compiler believes there are 2 distinct values so it uses 2 registers
> irrespectively of the order"
> 
> i.e, for the testing case:
> 
>  1 unsigned long long d;
>  2 unsigned int test1(unsigned int fParm)
>  3 {
>  4   d = fParm + 1;
>  5   return fParm + 1;
>  6 }
> 
> at line 4 and 5,  the result of fparm + 1 has two different usages:
> 	* one is at line 4,  convert to unsigned long long first,  and then assign
> to the global d;
> 	* the other is at line 5, directly return as the return result of the
> routine. 
> 
> the compiler has to use 2 different registers for these two different
> values. 
> 
> So, I think that the compiler does NOT do anything wrong for this testing
> case.
> the additional "mov" or "uxtw" instruction that is claimed in comment 1
> actually is necessary and should NOT be deleted. 
> 
> I think that this bug could be closed as not a bug.

Well it is not wrong, just non-optimal. It is possible to use a single register
here but it means teaching GCC that these values are identical, which is
non-trivial as it likely affects various places in the mid-end (this issue is
target-independent).