c - In Microchip C18, why does the insertion of a NOP cause much larger code? -


i have code in isr. code given completeness, question commented-out __asm_ block.

without __asm_ block, compiled 82 instructions. __asm_ block, result 107 instructions long. why big difference?

here's c code:

if (pir1bits.sspif) {     spi_rec_buffer.read_cursor = 0;     spi_rec_buffer.write_cursor = 0;      latabits.lata4 ^= 1; //      _asm nop nop _endasm     latabits.lata4 ^= 1;      while (!portabits.not_ss && spi_rec_buffer.write_cursor < spi_rec_buffer.size)     {         spi_rec_buffer.data[spi_rec_buffer.write_cursor] = sspbuf;         sspbuf = spi_out_msg_buffer.data[spi_out_msg_buffer.read_cursor];         pir1bits.sspif = 0;         spi_rec_buffer.write_cursor++;         spi_out_msg_buffer.read_cursor++;         if (spi_out_msg_buffer.read_cursor == spi_out_msg_buffer.write_cursor)             latabits.lata4 = 0;         latbbits.latb1 = 1;         while (!portabits.not_ss && !pir1bits.sspif);         latbbits.latb1 = 0;     }      spi_message_locked = true;     spi_message_received = true;  }

without nops:

btfss     0x9e,0x3,0x0      if (pir1bits.sspif) bra       0x2ba                             { movlb     0xf                   spi_rec_buffer.read_cursor = 0; clrf      0x4,0x1 clrf      0x5,0x1 clrf      0x6,0x1               spi_rec_buffer.write_cursor = 0; clrf      0x7,0x1 btg       0x89,0x4,0x0          latabits.lata4 ^= 1; btg       0x89,0x4,0x0          latabits.lata4 ^= 1; movf      0x80,0x0,0x0          while (!portabits.not_ss && spi_rec_buffer.write_cursor < spi_rec_buffer.size) andlw     0x20 bnz       0x2b0 movlb     0xf movf      0x7,0x0,0x1 xorwf     0x3,0x0,0x1 btfss     0xe8,0x7,0x0 bra       0x254 rlcf      0x3,0x0,0x1 bra       0x25c movf      0x2,0x0,0x1 subwf     0x6,0x0,0x1 movf      0x3,0x0,0x1 subwfb    0x7,0x0,0x1 bc        0x2b0 bra       0x240                                 { movf      0x0,0x0,0x1               spi_rec_buffer.data[spi_rec_buffer.write_cursor] = sspbuf; addwf     0x6,0x0,0x1 movwf     0xe9,0x0 movf      0x1,0x0,0x1 addwfc    0x7,0x0,0x1 movwf     0xea,0x0 movff     0xfc9,0xfef movlb     0xf                       sspbuf = spi_out_msg_buffer.data[spi_out_msg_buffer.read_cursor]; movf      0x10,0x0,0x1 addwf     0x14,0x0,0x1 movwf     0xe9,0x0 movf      0x11,0x0,0x1 addwfc    0x15,0x0,0x1 movwf     0xea,0x0 movf      0xef,0x0,0x0 movwf     0xc9,0x0 bcf       0x9e,0x3,0x0              pir1bits.sspif = 0; movlb     0xf                       spi_rec_buffer.write_cursor++; incf      0x6,0x1,0x1 movlw     0x0 addwfc    0x7,0x1,0x1 movlb     0xf                       spi_out_msg_buffer.read_cursor++; incf      0x14,0x1,0x1 addwfc    0x15,0x1,0x1 movf      0x16,0x0,0x1              if (spi_out_msg_buffer.read_cursor == spi_out_msg_buffer.write_cursor) xorwf     0x14,0x0,0x1 bnz       0x29e movf      0x17,0x0,0x1 xorwf     0x15,0x0,0x1 bnz       0x29e bcf       0x89,0x4,0x0                  latabits.lata4 = 0; bsf       0x8a,0x1,0x0              latbbits.latb1 = 1; movf      0x80,0x0,0x0              while (!portabits.not_ss && !pir1bits.sspif); andlw     0x20 bnz       0x2ac movf      0x9e,0x0,0x0 andlw     0x8 bz        0x2a0 bcf       0x8a,0x1,0x0              latbbits.latb1 = 0;                                 } movlb     0xf                   spi_message_locked = true; movlw     0x1 movwf     0x18,0x1 movlb     0xf                   spi_message_received = true; movwf     0x19,0x1                             } movlw     0x4            } subwf     0xe1,0x0,0x0 bc        0x2c4 clrf      0xe1,0x0 movf      0xe5,0x1,0x0 movwf     0xe1,0x0 movf      0xe5,0x1,0x0 movff     0xfe7,0xfd9 movf      0xe5,0x1,0x0 movff     0xfe5,0xfea movff     0xfe5,0xfe9 movff     0xfe5,0xfda retfie    0x1

with nops:

btfss     0x9e,0x3,0x0      if (pir1bits.sspif) bra       0x30e                             { movlb     0xf                   spi_rec_buffer.read_cursor = 0; clrf      0x4,0x1 clrf      0x5,0x1 movlb     0xf                   spi_rec_buffer.write_cursor = 0; clrf      0x6,0x1 clrf      0x7,0x1 btg       0x89,0x4,0x0          latabits.lata4 ^= 1; nop                             _asm nop nop _endasm nop btg       0x89,0x4,0x0          latabits.lata4 ^= 1; movf      0x80,0x0,0x0          while (!portabits.not_ss && spi_rec_buffer.write_cursor < spi_rec_buffer.size) andlw     0x20 bnz       0x302 movlb     0xf movf      0x7,0x0,0x1 movlb     0xf xorwf     0x3,0x0,0x1 btfss     0xe8,0x7,0x0 bra       0x27e rlcf      0x3,0x0,0x1 bra       0x28c movf      0x2,0x0,0x1 movlb     0xf subwf     0x6,0x0,0x1 movlb     0xf movf      0x3,0x0,0x1 movlb     0xf subwfb    0x7,0x0,0x1 bc        0x302 bra       0x268                                 { movlb     0xf                       spi_rec_buffer.data[spi_rec_buffer.write_cursor] = sspbuf; movlb     0xf movf      0x0,0x0,0x1 movlb     0xf addwf     0x6,0x0,0x1 movwf     0xe9,0x0 movlb     0xf movlb     0xf movf      0x1,0x0,0x1 movlb     0xf addwfc    0x7,0x0,0x1 movwf     0xea,0x0 movff     0xfc9,0xfef movlb     0xf                       sspbuf = spi_out_msg_buffer.data[spi_out_msg_buffer.read_cursor]; movlb     0xf movf      0x10,0x0,0x1 movlb     0xf addwf     0x14,0x0,0x1 movwf     0xe9,0x0 movlb     0xf movlb     0xf movf      0x11,0x0,0x1 movlb     0xf addwfc    0x15,0x0,0x1 movwf     0xea,0x0 movf      0xef,0x0,0x0 movwf     0xc9,0x0 bcf       0x9e,0x3,0x0              pir1bits.sspif = 0;                           // interruptflag löschen... movlb     0xf                       spi_rec_buffer.write_cursor++; incf      0x6,0x1,0x1 movlw     0x0 addwfc    0x7,0x1,0x1 movlb     0xf                       spi_out_msg_buffer.read_cursor++; incf      0x14,0x1,0x1 movlw     0x0 addwfc    0x15,0x1,0x1 movlb     0xf                       if (spi_out_msg_buffer.read_cursor == spi_out_msg_buffer.write_cursor) movf      0x16,0x0,0x1 movlb     0xf xorwf     0x14,0x0,0x1 bnz       0x2ea movlb     0xf movf      0x17,0x0,0x1 movlb     0xf xorwf     0x15,0x0,0x1 bnz       0x2ee bcf       0x89,0x4,0x0                  latabits.lata4 = 0; bsf       0x8a,0x1,0x0              latbbits.latb1 = 1; movf      0x80,0x0,0x0              while (!portabits.not_ss && !pir1bits.sspif); andlw     0x20 bnz       0x2fe movf      0x9e,0x0,0x0 andlw     0x8 bnz       0x2fe bra       0x2f0 bcf       0x8a,0x1,0x0              latbbits.latb1 = 0;                                 } movlb     0xf                   spi_message_locked = true; movlw     0x1 movwf     0x18,0x1 movlb     0xf                   spi_message_received = true; movlw     0x1 movwf     0x19,0x1                             } movlw     0x4            } subwf     0xe1,0x0,0x0 bc        0x318 clrf      0xe1,0x0 movf      0xe5,0x1,0x0 movwf     0xe1,0x0 movf      0xe5,0x1,0x0 movff     0xfe7,0xfd9 movf      0xe5,0x1,0x0 movff     0xfe5,0xfea movff     0xfe5,0xfe9 movff     0xfe5,0xfda retfie    0x1

here's screenshot of partly diff (click enlarge): diff

so people don't have guess, here's statement the microchip c18 manual (emphasis added):

it recommended limit use of inline assembly minimum. any functions containing inline assembly not optimized compiler. write large fragments of assembly code, use mpasm assembler , link modules c modules using mplink linker.

i think common situation inline asm. gcc exception - optimize inline assembly along surrounding c code; in order correctly, gcc's inline assembly quite complex (you have let know registers , memory clobbered).


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