Shifting a value wider than the shifter
A 32-bit shift instruction can't move bits across the boundary between the two halves of a 64-bit value, so a variable-distance 64-bit shift also defers to a runtime helper — same pattern as division: a prologue, a bl, an epilogue:
stwu r1, -16(r1)
mflr r0
mr r5, r6 # marshal the shift amount into r5
stw r0, 20(r1)
bl __shl2i # 64-bit shift-left helper
lwz r0, 20(r1)
mtlr r0
addi r1, r1, 16
blr
The left-shift helper is __shl2i. Right shifts pick the helper by signedness: __shr2u for a logical (unsigned) shift and __shr2i for an arithmetic (signed) one. As with division, the mr r5, r6 beforehand is the compiler arranging the arguments the helper expects — here, moving the shift distance into place.
So the full set of 64-bit operations that can't be done inline — and therefore announce themselves with a bl __…2i/__…2u — is divide, modulo, and shift. Everything else (add, subtract, multiply, and the bitwise ops coming up next) stays inline.
Your task
Write shl_64 to match the target.