[avr-gcc-list] Got strange compilation problem

[David Bourgeois]

Hi all,

Replacing the 2 lines
    length = (p->inIdx) - (p->outIdx);
    length %= FIFO_SIZE;
by
    length = (((p->inIdx) - (p->outIdx)) % FIFO_SIZE);

completely broke my code. I'm using avr-gcc 3.4.5 (latest winavr release)  
under avrstudio, -Os.
Here's the generated assembly part. I guess there something fundamentally  
wrong I'm doing here because it also brake without optimization (-O0) but  
with much bigger code and the failure behaves a bit different.

Thanks for any opinion about this.
David

uint8_t fifoLength(fifo_t *p)
{
        uint8_t length;

        length = (p->inIdx) - (p->outIdx);
        length %= FIFO_SIZE;
      return length;
}

@00000224: fifoLength
46:       {
+00000224:   01FC        MOVW    R30,R24          Copy register pair
48:             length = (p->inIdx) - (p->outIdx);
+00000225:   8580        LDD     R24,Z+8          Load indirect with  
displacement
+00000226:   8591        LDD     R25,Z+9          Load indirect with  
displacement
+00000227:   1B89        SUB     R24,R25          Subtract without carry
49:           length %= FIFO_SIZE;
+00000228:   7087        ANDI    R24,0x07         Logical AND with  
immediate
51:       }
+00000229:   2799        CLR     R25              Clear Register
+0000022A:   9508        RET                      Subroutine return

uint8_t fifoLength(fifo_t *p)
{
        uint8_t length;

        length = (((p->inIdx) - (p->outIdx)) % FIFO_SIZE);
    return length;
}
@00000224: fifoLength
46:       {
+00000224:   01FC        MOVW    R30,R24          Copy register pair
49:             length = (((c) % FIFO_SIZE);
+00000225:   8580        LDD     R24,Z+8          Load indirect with  
displacement
+00000226:   2F28        MOV     R18,R24          Copy register
+00000227:   2733        CLR     R19              Clear Register
+00000228:   8581        LDD     R24,Z+9          Load indirect with  
displacement
+00000229:   1B28        SUB     R18,R24          Subtract without carry
+0000022A:   0931        SBC     R19,R1           Subtract with carry
+0000022B:   01C9        MOVW    R24,R18          Copy register pair
+0000022C:   FD37        SBRC    R19,7            Skip if bit in register  
cleared
+0000022D:   9607        ADIW    R24,0x07         Add immediate to word
+0000022E:   7F88        ANDI    R24,0xF8         Logical AND with  
immediate
+0000022F:   1B28        SUB     R18,R24          Subtract without carry
+00000230:   2F82        MOV     R24,R18          Copy register
51:       }
+00000231:   2799        CLR     R25              Clear Register
+00000232:   9508        RET                      Subroutine return

Here, the problem occurs when p->inIdx) - (p->outIdx) is <0, then the  
modulus is not done, the return value is 0xF9 for example.