[jmalicki]

What is a single example where the optimal encoding depends on context? (I am assuming you're just doing an assembler where registers have already been chosen, vs. a compiler that can choose sse vs. scalar and do register allocation etc.)?

[chris_swenson]

“mov rcx, 0”. At least one assembler (the Go assembler) would at one point blindly (and arguably, incorrectly) rewrite this to “xor rcx, rcx”, which is smaller but modifies flags, which “mov” does not. I believe Go fixed this later, possibly by looking at surrounding instructions to see if the flags were being used, for instance by an “adc” later, to know if the assembler needs to pick the larger “mov” encoding.

Whether that logic should belong in a compiler or an assembler is a separate issue, but it definitely was in the assembler there.

[jmalicki]

Ok fair, I saw that as out of scope for an assembler - since that is a different instruction not just how to encode.

[nicebyte]

jumps is another one. jmp can have many encodings depending on where the target offset you're jumping to is. but often times, the offset is not yet known when you first encounter the jump insn and have to assemble it.