Re: peephole or expand optimization for fixing poor code generated in conversion from QImode to PSImode pointer?

[Jeff Law <law@redhat.com>]

On 9/23/19 5:56 AM, Jozef Lawrynowicz wrote:
> For msp430-elf in the large memory model there are PSImode (20-bit) pointers.
> POINTERS_EXTEND_UNSIGNED == 1 and "char" is unsigned by default.
> 
> We get poor code generated for the following code snippet:
> 
> const int table[2] = {1, 2};
> 
> int
> foo (char i)
> {
>   return table[i];
> }
> 
> The RTL generated by expand uses two insns to convert "i" to a register's
> natural mode; there is a sign extension which would be unnecessary if the first
> instruction had a PSImode register as the lvalue:
> 
> (insn 2 4 3 2 (set (reg/v:HI 25 [ i ])
>         (zero_extend:HI (reg:QI 12 R12 [ i ])))
>      (nil))
> .....
> (insn 7 6 8 2 (set (reg:PSI 28)
>         (subreg:PSI (sign_extend:SI (reg/v:HI 25 [ i ])) 0))
>      (nil))
> 
> All we really need is:
> 
> (insn (set (reg:PSI 28 [ i ])
>         (zero_extend:PSI (reg:QI 12 R12 [ i ])))
>      (nil))
> 
> The zero extend is implicit with byte sized register operations, so this would
> result in assembly such as:
>   MOV.B R12, R12
> 
> My question is whether this is the type of thing that should be handled with a
> peephole optimization or if it is worth trying to fix the initial RTL generated
> by expand (or in a later RTL pass e.g. combine)?
Ideally it'd be fixed earlier, but that can be painful due to the way
promotions work.

I certainly recall doing some combiner patterns to catch the more
egregious codegen issues on the mn102 port (similar, but with 24bit
pointers).  I think I mostly focused on stuff that showed up with
integer loop indices and their annoying promotion to ptrdiff_t when used
for array indexing.

I'd bet if you extracted mn10200.md out of the archive the patterns
would be obvious ;-)

In this specific case I'd think a combiner pattern ought to do the right
thing except for the fact that you're probably dealing with hard
registers which combine no longer handles as aggressively.

jeff