I'm testing some byte swap code from William Hohl's book on ARM
assembly language programming.
According to Hohl (p. 74) the byte swap should be coded as follows:
eor r1, r0, r0, ror #16
bic r1, r1, #0xff0000
mov r0, r0, ror #8
eor r0, r0, r1, lsr #8
When I convert to GCC extended ASM:
unsigned int byteswap(unsigned int value)
{
unsigned int temp;
__asm__ ("eor %1, %0, %0, ror #16 \n\t"
"bic %1, %1, #0xff0000 \n\t"
"mov %0, %0, ror #8 \n\t"
"eor %0, %0, %1, lsr #8 \n\t"
: "+r" (value), "+r" (temp));
return value;
}
There is a spurious move when a scratch register is used. Instead of 4
instructions there are 5. The mov r3, #0 was introduced with the
scratch register.
mov r3, #0
eor r3, r0, r0, ror #16
bic r3, r3, #0xff0000
mov r0, r0, ror #8
eor r0, r0, r3, lsr #8
According to Goldbot (https://godbolt.org/z/jmIya9 ), the spurious
move is present in GCC 6.4 - GCC 8.3.
I don't want to do the temp register that way (claiming it is an input
and an output that I want), but it is the only way to get a scratch
register according to GCC Extended ASM HowTo.
How do I code this up and avoid the extra move?
Jeff
