Hi
Normally gcc generates well optimized code, but
sometimes i wunder how gcc can do simple things
so complicated.
Here is an example,
uint16_t genGetTickCount(void)
{ return (((uint16_t) uxTickCount.HighByte) << 8) | (uint16_t)
(uxTickCount.LowByte) ; }
generates
00000768 <genGetTickCount>:
768: 80 91 1d 01 lds r24, 0x011D
76c: 20 91 1c 01 lds r18, 0x011C
770: 99 27 eor r25, r25
772: 98 2f mov r25, r24
774: 88 27 eor r24, r24
776: 33 27 eor r19, r19
778: 82 2b or r24, r18
77a: 93 2b or r25, r19
77c: 08 95 ret
whereas it could have been 12 bytes (!) shorter:
00000768 <genGetTickCount>:
768: 80 91 1d 01 lds r25, 0x011D
76c: 20 91 1c 01 lds r24, 0x011C
770: 08 95 ret
Is there a way to write the methode defined above in C to make the
generate this assembly? Some special combine function maybe?
Further, i dont know how much intelligence you may expect from the
compiler, but for example, first cleaning r25, and directly afterwards
filling it with r24 seems really a waste of effort. By direct inspection,
thus _without_ any knowledge what is going on, this code could be
reduced in the following simple steps (ignore line numbers):
00000768 <genGetTickCount>:
768: 80 91 1d 01 lds r24, 0x011D
76c: 20 91 1c 01 lds r18, 0x011C
770: 99 27 eor r25, r25 //remove this, since it directly overwritten
afterwards
772: 98 2f mov r25, r24
774: 88 27 eor r24, r24
776: 33 27 eor r19, r19
778: 82 2b or r24, r18
77a: 93 2b or r25, r19 //remove this since "or" with zero does not
change the value of r25
77c: 08 95 ret
00000768 <genGetTickCount>:
768: 80 91 1d 01 lds r24, 0x011D
76c: 20 91 1c 01 lds r18, 0x011C
772: 98 2f mov r25, r24
774: 88 27 eor r24, r24
776: 33 27 eor r19, r19 //remove this, the register is unused.
778: 82 2b or r24, r18 // change ito "mov" since r24 is zero
77c: 08 95 ret
00000768 <genGetTickCount>:
768: 80 91 1d 01 lds r24, 0x011D //directly fill this with r25, since the
value r24 is destroyed after the move
76c: 20 91 1c 01 lds r18, 0x011C
772: 98 2f mov r25, r24 //remove this since r25 will be filled
directly
774: 88 27 eor r24, r24 //remove this, since it directly
overwritten afterwards
778: 82 2b mov r24, r18
77c: 08 95 ret
00000768 <genGetTickCount>:
768: 80 91 1d 01 lds r25, 0x011D
76c: 20 91 1c 01 lds r18, 0x011C //direcly fill this with r24 since r18 is
unsed after the move
778: 82 2b mov r24, r18 //remove this since it r24 will be filled
directly
77c: 08 95 ret
00000768 <genGetTickCount>:
768: 80 91 1d 01 lds r25, 0x011D
76c: 20 91 1c 01 lds r24, 0x011C
77c: 08 95 ret
Could such post compiler optimization steps be integrated in the compiler?
Like to hear your comments.
Ruud.
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