Don't know assembler, but a whale ago I come across this blog
Arduino – AVR GCC multiplication | Mekonikuv blog and here macro implementation:
#define MultiU16X16to32(longRes, intIn1, intIn2) \
asm volatile ( \
"clr r26 \n\t" \
//<< Removed for this post, look on a blog full version
: \
"=&r" (longRes) \
: \
"a" (intIn1), \<< This is you looking for?
"a" (intIn2) \
: \
"r26" \
)