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allenhuffman
Joined: 17 Jun 2019
Posts: 552
Location: Des Moines, Iowa, USA
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| Code generation: "var1=0; var2=0;" versus "va |
 Posted: Fri Aug 30, 2019 10:11 am |
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Occasionally I go down a rabbit hole looking at what a compiler does. I have written articles about Arduino code generation on topics of "if/then" versus "switch/case" and what is better/faster/smaller.
Here's a fun one. I don't code this way, but I've seen people batch-set variables like this:
| Code: | | value1 = value2 = value3 = value4 = value5 = value6 = 0; |
Since I am currently looking at CCS code generation, I wondered how it handled it.
| Code: | .............................. value1 = value2 = value3 = value4 = value5 = value6 = 0;
00228 810070 MOV 200E,W0 : W0 = [200E]
0022A B3C000 MOV.B #0,W0L : W0L = 0
0022C 890070 MOV W0,200E : [200E] = W0
0022E 810070 MOV 200E,W0 : W0 = [200E]
00230 F8200C PUSH 200C : PUSH [200C] to TOS
00232 9FFFF0 MOV.B W0L,[W15-#1] : [W15+-1] = W0L
00234 F9200C POP 200C : POP TOS to [200C]
00236 F8200C PUSH 200C : PUSH [200C] to TOS
00238 78004F MOV [--W15],W0 : POP TOS to [--W15]
0023A DE0048 LSR W0,#8,W0 : W0 = W0 >> 8
0023C F8200C PUSH 200C : PUSH [200C] to TOS
0023E 9FFFE0 MOV.B W0L,[W15-#2] : [W15+-2] = W0L
00240 F9200C POP 200C : POP TOS to [200C]
00242 810060 MOV 200C,W0 : W0 = [200C]
00244 F8200A PUSH 200A : PUSH [200A] to TOS
00246 9FFFF0 MOV.B W0L,[W15-#1] : [W15+-1] = W0L
00248 F9200A POP 200A : POP TOS to [200A]
0024A F8200A PUSH 200A : PUSH [200A] to TOS
0024C 78004F MOV [--W15],W0 : POP TOS to [--W15]
0024E DE0048 LSR W0,#8,W0 : W0 = W0 >> 8
00250 F82000 PUSH 2000 : PUSH [2000] to TOS
00252 9FFFF0 MOV.B W0L,[W15-#1] : [W15+-1] = W0L
00254 F92000 POP 2000 : POP TOS to [2000]
00256 F82000 PUSH 2000 : PUSH [2000] to TOS
00258 78004F MOV [--W15],W0 : POP TOS to [--W15]
0025A DE0048 LSR W0,#8,W0 : W0 = W0 >> 8
0025C F82000 PUSH 2000 : PUSH [2000] to TOS
0025E 9FFFE0 MOV.B W0L,[W15-#2] : [W15+-2] = W0L
00260 F92000 POP 2000 : POP TOS to [2000] |
29 lines of assembly.
Versus the way I normally do it:
| Code: | value1 = 0;
value2 = 0;
value3 = 0;
value4 = 0;
value5 = 0;
value6 = 0; |
...which generates:
| Code: | .............................. value1 = 0;
00262 810000 MOV 2000,W0 : W0 = [2000]
00264 B3C000 MOV.B #0,W0L : W0L = 0
00266 890000 MOV W0,2000 : [2000] = W0
.............................. value2 = 0;
00268 810000 MOV 2000,W0 : W0 = [2000]
0026A EF6001 CLR.B 1 : W0H = 0
0026C 890000 MOV W0,2000 : [2000] = W0
.............................. value3 = 0;
0026E 810050 MOV 200A,W0 : W0 = [200A]
00270 EF6001 CLR.B 1 : W0H = 0
00272 890050 MOV W0,200A : [200A] = W0
.............................. value4 = 0;
00274 810060 MOV 200C,W0 : W0 = [200C]
00276 B3C000 MOV.B #0,W0L : W0L = 0
00278 890060 MOV W0,200C : [200C] = W0
.............................. value5 = 0;
0027A 810060 MOV 200C,W0 : W0 = [200C]
0027C EF6001 CLR.B 1 : W0H = 0
0027E 890060 MOV W0,200C : [200C] = W0
.............................. value6 = 0;
00280 810070 MOV 200E,W0 : W0 = [200E]
00282 B3C000 MOV.B #0,W0L : W0L = 0
00284 890070 MOV W0,200E : [200E] = W0 |
19 lines of assembly.
An aggressive optimizer might generate the same end-result, but you can't rely on that.
Just something fun I wondered about.
_________________
Allen C. Huffman, Sub-Etha Software (est. 1990) http://www.subethasoftware.com
Embedded C, Arduino, MSP430, ESP8266/32, BASIC Stamp and PIC24 programmer.
http://www.whywouldyouwanttodothat.com ? |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Aug 30, 2019 12:01 pm |
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18F works better:
| Code: |
00038: CLRF value6
0003A: MOVFF value6,value5
0003E: MOVFF value5,value4
00042: MOVFF value4,value3
00046: MOVFF value3,value2
0004A: MOVFF value2,value1
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Test program:
| Code: |
#include <18F46K22.h>
#fuses INTRC_IO, NOWDT, BROWNOUT, PUT, NOPBADEN
#use delay(clock=4M)
#use i2c(Slave, I2C1)
//======================================
void main()
{
int8 value1, value2, value3, value4, value5, value6;
value1 = value2 = value3 = value4 = value5 = value6 = 0;
while(TRUE);
} |
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temtronic
Joined: 01 Jul 2010
Posts: 9225
Location: Greensville,Ontario
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| Posted: Fri Aug 30, 2019 1:06 pm |
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It probably all hinges on the 'compiler code cutter'. If he needs 'generic - one code for all' or can he TRULY optimize the code.
Have to remember not ONE guy cut the compiler, so there's probably a few 'rules and regulations' about style and 'how-to-code'. And of course, different PICs may alter the design process...as not 'one shoe fits all'.
There's also the 'time vs size' challenge'. Some code for speed, some space, some mix it up.
Microchip used to have paperback books (yeah, I'm that old...) that had LOTS of neat 'tricks and tips' all Assembler / machine code, but interesting to see how easy it was to maximize the overall performance of a PIC.
Jay |
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