You can use all preprocessor features, such as comments and macros, in the assembler. In multi-line macros, you must end each assembly statement with a semicolon (;) because the (\) operator removes newlines.
For example:
#define remainder(x,y,z) \
sdiv z,x,y; \
mul z,z,y; \
sub z,z,x
asm void func(void)
{
remainder(r3,r4,r5)
bx lr
}