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.
