[Bug target/64027] inefficient handling of msp430 byte operands

[pab at pabigot dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64027

--- Comment #1 from Peter A. Bigot <pab at pabigot dot com> ---
The following program:

int request();
int release();
unsigned char execute (unsigned char arg);
unsigned char safe_execute (unsigned char arg)
{
  int rc;
  unsigned char rs = 0;

  rc = request();
  if (0 == rc) {
    do {
      rs = execute(arg);
    } while(0);
    (void)release();
  }
  return rs;
}

when compiled with msp430-elf-gcc -Os produces:

00000000 <safe_execute>:
   0:   0a 15           pushm   #1,     r10     ;16-bit words
   2:   4a 4c           mov.b   r12,    r10     ;
   4:   3a f0 ff 00     and     #255,   r10     ;#0x00ff
   8:   b0 12 00 00     call    #0              ;
                        a: R_MSP430X_ABS16      request
   c:   0c 93           cmp     #0,     r12     ;r3 As==00
   e:   00 20           jnz     $+2             ;abs 0x10
                        e: R_MSP430X_10_PCREL   .L3
  10:   4c 4a           mov.b   r10,    r12     ;
  12:   b0 12 00 00     call    #0              ;
                        14: R_MSP430X_ABS16     execute
  16:   4a 4c           mov.b   r12,    r10     ;
  18:   3a f0 ff 00     and     #255,   r10     ;#0x00ff
  1c:   b0 12 00 00     call    #0              ;
                        1e: R_MSP430X_ABS16     release
  20:   30 40 00 00     br      #0x0000         ;
                        22: R_MSP430X_ABS16     .L2
00000024 <.L3>:
  24:   0a 43           clr     r10             ;
00000026 <.L2>:
  26:   4c 4a           mov.b   r10,    r12     ;
  28:   0a 17           popm    #1,     r10     ;16-bit words
  2a:   30 41           ret

Under mspgcc this produces:

00000000 <safe_execute>:
   0:   0b 12           push    r11
   2:   4b 4f           mov.b   r15,    r11
   4:   b0 12 00 00     call    #0x0000
   8:   0f 93           tst     r15
   a:   07 20           jnz     $+16            ;abs 0x1a
   c:   4f 4b           mov.b   r11,    r15
   e:   b0 12 00 00     call    #0x0000
  12:   4b 4f           mov.b   r15,    r11
  14:   b0 12 00 00     call    #0x0000
  18:   01 3c           jmp     $+4             ;abs 0x1c
  1a:   4b 43           clr.b   r11
  1c:   4f 4b           mov.b   r11,    r15
  1e:   3b 41           pop     r11
  20:   30 41           ret

There are two things to be fixed here.  

First, in this code:

   2:   4a 4c           mov.b   r12,    r10     ;
   4:   3a f0 ff 00     and     #255,   r10     ;#0x00ff

the second instruction is unnecessary, because the mov.b guarantees the
upper half of r10 will be zero.  In general the msp430 target does not
appear to take full advantage of the semantics of byte operands in
MSP430.  8 bytes would be saved by eliminating this instruction in two
places.

Second, this sequence generated by msp430-elf:
        CALL    #release
        BR      #.L2
.L3:
        MOV.W   #0, R10
.L2:

should use a "jmp" instruction instead of a "br", saving two bytes.