[Bug c++/81147] [avr] C++ for -mmcu=atmega2560 doesn't do NRVO or RVO when class inherits from empty base

[felipe at expertisesolutions dot com.br]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81147

--- Comment #8 from Felipe Magno de Almeida <felipe at expertisesolutions dot com.br> ---
With GCC 7.2.0 I'm still not seeing the same results as you are. The following
is the assembly result, using the same .cpp file and the same command line:

        .section        .text._Z3abcv,"ax",@progbits
.global _Z3abcv
        .type   _Z3abcv, @function
_Z3abcv:
        push r28         ;       ;  52  pushqi1/1       [length = 1]
        push r29         ;       ;  53  pushqi1/1       [length = 1]
        in r28,__SP_L__  ;       ;  54  *movhi/8        [length = 2]
        in r29,__SP_H__  ; 
        sbiw r28,15      ; ,     ;  55  *addhi3/3       [length = 1]
        in __tmp_reg__,__SREG__  ;  56  movhi_sp_r/3    [length = 5]
        cli
        out __SP_H__,r29         ; ,
        out __SREG__,__tmp_reg__
        out __SP_L__,r28         ; ,
/* prologue: function */
/* frame size = 15 */
/* stack size = 17 */
.L__stack_usage = 17
        ldi r24,lo8(32)  ;  D.1843,      ;  6   movqi_insn/2    [length = 1]
 ;  test.cpp:19:   seq(L l, R r) : l(l), r(r) {}
        std Y+7,__zero_reg__     ;  D.2011.l,    ;  37  movqi_insn/3    [length
= 1]
        ldi r25,lo8(97)  ;  D.1843,      ;  42  movqi_insn/2    [length = 1]
        std Y+8,r25      ;  D.2011.l, D.1843     ;  38  movqi_insn/3    [length
= 1]
        std Y+9,r24      ;  D.2011.l, D.1843     ;  39  movqi_insn/3    [length
= 1]
 ;  test.cpp:25:   return { l, r};
        std Y+10,r24     ;  MEM[(struct seq *)&D.2011 + 4B], D.1843      ;  14 
movqi_insn/3    [length = 1]
        ldi r24,lo8(5)   ;  tmp47,       ;  15  movqi_insn/2    [length = 1]
        movw r30,r28     ; ,     ;  45  *movhi/1        [length = 1]
        adiw r30,6       ; ,     ;  16  *addhi3/3       [length = 1]
        movw r26,r28     ; ,     ;  46  *movhi/1        [length = 1]
        adiw r26,1       ; ,     ;  17  *addhi3/3       [length = 1]
        0:       ;  18  movmem_qi       [length = 4]
        ld r0,Z+         ; 
        st X+,r0         ; 
        dec r24  ;  tmp47
        brne 0b
        ldi r24,lo8(5)   ;  tmp48,       ;  19  movqi_insn/2    [length = 1]
        movw r30,r28     ; ,     ;  47  *movhi/1        [length = 1]
        adiw r30,1       ; ,     ;  20  *addhi3/3       [length = 1]
        movw r26,r28     ; ,     ;  48  *movhi/1        [length = 1]
        adiw r26,6       ; ,     ;  21  *addhi3/3       [length = 1]
        0:       ;  22  movmem_qi       [length = 4]
        ld r0,Z+         ; 
        st X+,r0         ; 
        dec r24  ;  tmp48
        brne 0b
 ;  test.cpp:35:          )
        ldi r24,lo8(5)   ;  tmp49,       ;  23  movqi_insn/2    [length = 1]
        movw r30,r28     ; ,     ;  49  *movhi/1        [length = 1]
        adiw r30,6       ; ,     ;  24  *addhi3/3       [length = 1]
        movw r26,r28     ; ,     ;  50  *movhi/1        [length = 1]
        adiw r26,11      ; ,     ;  25  *addhi3/3       [length = 1]
        0:       ;  26  movmem_qi       [length = 4]
        ld r0,Z+         ; 
        st X+,r0         ; 
        dec r24  ;  tmp49
        brne 0b
 ;  test.cpp:37:        foo(rule);
        movw r24,r28     ; ,     ;  51  *movhi/1        [length = 1]
        adiw r24,11      ; ,     ;  28  addhi3_clobber/1        [length = 1]
        call _Z3fooI3seqIS0_I1AS1_ES1_EEvRT_     ;       ;  29  call_insn/2    
[length = 2]
/* epilogue start */
 ;  test.cpp:38: }
        adiw r28,15      ; ,     ;  59  *addhi3/3       [length = 1]
        in __tmp_reg__,__SREG__  ;  60  movhi_sp_r/3    [length = 5]
        cli
        out __SP_H__,r29         ; ,
        out __SREG__,__tmp_reg__
        out __SP_L__,r28         ; ,
        pop r29  ;       ;  61  popqi   [length = 1]
        pop r28  ;       ;  62  popqi   [length = 1]
        ret      ;  63  return_from_epilogue    [length = 1]
        .size   _Z3abcv, .-_Z3abcv
        .ident  "GCC: (GNU) 7.2.0"

And this is the result if seq no longer inherits from seq_base (which is an
empty class):

        .section        .text._Z3abcv,"ax",@progbits
.global _Z3abcv
        .type   _Z3abcv, @function
_Z3abcv:
        push r28         ;       ;  19  pushqi1/1       [length = 1]
        push r29         ;       ;  20  pushqi1/1       [length = 1]
         ; SP -= 3       ;       ;  24  *addhi3_sp      [length = 1]
        rcall .
        in r28,__SP_L__  ;       ;  25  *movhi/8        [length = 2]
        in r29,__SP_H__  ; 
/* prologue: function */
/* frame size = 3 */
/* stack size = 5 */
.L__stack_usage = 5
 ;  test.cpp:35:          )
        ldi r24,lo8(97)  ;  tmp42,       ;  5   movqi_insn/2    [length = 1]
        std Y+1,r24      ;  MEM[(struct seq *)&rule], tmp42      ;  6  
movqi_insn/3    [length = 1]
        ldi r24,lo8(32)  ;  tmp43,       ;  7   movqi_insn/2    [length = 1]
        std Y+2,r24      ;  MEM[(struct seq *)&rule + 1B], tmp43         ;  8  
movqi_insn/3    [length = 1]
        std Y+3,r24      ;  MEM[(struct seq *)&rule + 2B], tmp43         ;  10 
movqi_insn/3    [length = 1]
 ;  test.cpp:37:        foo(rule);
        movw r24,r28     ; ,     ;  18  *movhi/1        [length = 1]
        adiw r24,1       ; ,     ;  11  *addhi3/3       [length = 1]
        call _Z3fooI3seqIS0_I1AS1_ES1_EEvRT_     ;       ;  12  call_insn/2    
[length = 2]
/* epilogue start */
 ;  test.cpp:38: }
         ; SP += 3       ;       ;  30  *addhi3_sp      [length = 3]
        pop __tmp_reg__
        pop __tmp_reg__
        pop __tmp_reg__
        pop r29  ;       ;  31  popqi   [length = 1]
        pop r28  ;       ;  32  popqi   [length = 1]
        ret      ;  33  return_from_epilogue    [length = 1]
        .size   _Z3abcv, .-_Z3abcv
        .ident  "GCC: (GNU) 7.2.0"

Which seems to indicate that in avr backend, when a class inherits from another
(be that trivial or not), named return value optimization no longer kicks in
and all return types gets copied.