[Bug rtl-optimization/90706] [10/11/12/13 Regression] Useless code generated for stack / register operations on AVR

[gjl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90706

--- Comment #23 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Created attachment 55130
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55130&action=edit
Test case for -Os -mmcu=attiny40

As it appears, this bug is not fixed completely.  For the -mmcu=avrtiny
architecture, there is still bloat for even the smallest test cases like:

$ avr-gcc bloat.c -mmcu=attiny40 -Os -S

char func3 (char c)
{
    return 1 + c;
}

"GCC: (GNU) 14.0.0 20230520 (experimental)" compiles this to:

func3:
        push r28                 ;  22  [c=4 l=1]  pushqi1/0
        push r29                 ;  23  [c=4 l=1]  pushqi1/0
        push __tmp_reg__         ;  27  [c=4 l=1]  *addhi3_sp
        in r28,__SP_L__  ;  38  [c=4 l=2]  *movhi/7
        in r29,__SP_H__
/* prologue: function */
/* frame size = 1 */
/* stack size = 3 */
        mov r20,r24      ;  18  [c=4 l=1]  movqi_insn/0
        subi r20,lo8(-(1))       ;  19  [c=4 l=1]  *addqi3/1
        mov r24,r20      ;  21  [c=4 l=1]  movqi_insn/0
/* epilogue start */
        pop __tmp_reg__  ;  33  [c=4 l=1]  *addhi3_sp
        pop r29          ;  34  [c=4 l=1]  popqi
        pop r28          ;  35  [c=4 l=1]  popqi
        ret              ;  36  [c=0 l=1]  return_from_epilogue

For reference, avr-gcc v8 generates for this function:

func3:
/* prologue: function */
/* frame size = 0 */
/* stack size = 0 */
.L__stack_usage = 0
        subi r24,lo8(-(1))       ;  6   [c=4 l=1]  addqi3/1
/* epilogue start */
        ret              ;  17  [c=0 l=1]  return