[Bug inline-asm/37895] New: AVR inline assembly clobbers input value

[Rudolf dot Leitgeb at gmx dot at]

I carefully followed the instructions how to write proper inline assembly for
avr-gcc, however, the resulting code didn't work. I tracked down the problem
and came to the conclusion that gcc assigned wrong registers to the output
variable and therefore corrupted my input data.

Here's sample code, I tried to write a fast routine for multiplying a 16-bit
number with an 8 bit number, the result would be written to a 32 bit number:


static inline int32_t mul_16_8(uint16_t left, uint8_t right) {
        int32_t result;
        asm volatile(
                "clr %C0;" "\n\t"
                "clr %D0;" "\n\t"
                "mul %A1, %2;" "\n\t"
                "movw %A0, r0;" "\n\t"
                "mul %B1, %2;" "\n\t"
                "add %B0, r0;" "\n\t"
                "adc %C0, r1;" "\n\t"
                "clr __zero_reg__" "\n\t"
                :"=&r" (result)
                :"r" (left),"r" (right)
                :"r0", "r1"
        );
        return result;
}

If I invoke this function from regular C code, such as

int32_t res = mul_16_8(0x1234, 0x56);

I get incorrect results. I believe this is the case because the "left" variable
is assigned to the same registers as "result", and "left" is overwritten during
the multiplication. According to several web pages the  "=&r" (result)
directive should prevent exactly this ....

Since the bug reporting guide explicitly discourages from posting assembly code
generated by the compiler, I refrain from posting it here, although it might
help clarify this issue. I will post it upon request, though :)


Some extra information:

I don't use plain vanilla avr gcc, but the win avr version: WinAVR 20071221,
which is 4.2.2 plus some AVR specific patches (some bug fixes, extra device
support). Please let me know, if gcc-bugzilla is the wrong address for these
bugs!


-- 
           Summary: AVR inline assembly clobbers input value
           Product: gcc
           Version: 4.2.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: inline-asm
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: Rudolf dot Leitgeb at gmx dot at
 GCC build triplet: i686-unknown-linux
  GCC host triplet: i686-unknown-linux
GCC target triplet: avr


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37895