Re: [PATCH] rtl-optimization: ppc backend generates unnecessary signed extension.

[Jeff Law via Gcc-patches]

On 3/23/23 10:29, Peter Bergner wrote:

     https://gcc.gnu.org/PR41742

These are not addressed in the trunk patch, because int c is not initialized
with registers and for this reason we cannot eliminate them. If we initialize
int c then zero extension goes away.

I'm sorry that I don't know how REE works.  Why can't it optimize this?
I see in the REE dump:

(insn 20 18 22 3 (set (reg:DI 4 4)
                       (zero_extend:DI (reg:QI 4 4 [orig:120 cD.3556+3 ] [120]))) 
"pr41742.c":6:41 8 {zero_extendqidi2} (nil))
(call_insn 22 20 41 3 (parallel [
             (set (reg:DI 3 3)
                  (call (mem:SI (symbol_ref:DI ("memset") [flags 0x41]  
<function_decl 0x7fff925f8400 __builtin_memset>) [0 memsetD.1196 S4 A8])
                     (const_int 64 [0x40])))
             (use (const_int 0 [0]))
             (clobber (reg:DI 96 lr)) ...

Is there a reason why REE cannot see that our (reg:QI 4) is a param register
and thus due to our ABI, already correctly sign/zero extended?
I don't think REE has ever considered exploiting ABI constraints. 
Handling that might be a notable improvement on various targets.  It'd 
be a great place to do some experimentation.

jeff