ADD has a smaller encoding for immediates that can be expressed as an
8-bit signed integer (in other words, between -128 and 127). MOV lacks
this compact representation.
Since addition allows us to swap the source registers, we can always get
the shortest sequence here by carefully checking if we're dealing with a
small immediate first. If we are, move the other source into the
destination and add the small immediate onto that. For large immediates
the reverse is preferrable.
Before:
41 BE 40 00 00 00 mov r14d,40h
44 03 75 A8 add r14d,dword ptr [rbp-58h]
After:
44 8B 75 A8 mov r14d,dword ptr [rbp-58h]
41 83 C6 40 add r14d,40h
Before:
44 8B 7D F8 mov r15d,dword ptr [rbp-8]
41 81 C7 00 68 00 CC add r15d,0CC006800h
After:
41 BF 00 68 00 CC mov r15d,0CC006800h
44 03 7D F8 add r15d,dword ptr [rbp-8]
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