Mastodawn

Day 22 of Advent of Compiler Optimisations!

Comparing a string_view against "ABCDEFG" should call memcmp, right? Watch what Clang actually generates — no function call at all, just a handful of inline instructions using some rather cunning tricks. How does it compare 7 bytes so efficiently when they don't fit in a single register?

#AoCO2025

Clever memory tricks — Matt Godbolt’s blog

We learn that compilers have tricks to access memory efficiently

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Riley S. Faelan Dec 22

@mattgodbolt WTF. I'm really disappointed that Clang is not smart enough to do

t1:
  cmp rdi, 1                ; is length 1?
  jne .LBB0_1               ; if not 1, goto "return false"
  cmp byte ptr [rsi], 65    ; is the byte 65 ('A')?
.LBB0_1:
  sete al                   ; set result to 0 or 1 accordingly
  ret                       ; return

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matt godbolt Dec 22

@riley these kinds of things are always worth asking the clang Devs and/or filing bugs to report :). Even if we don't know how to improve the compiler ourselves, a respectful issue request can help them help us!

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Riley S. Faelan

@mattgodbolt Yeah, that would seem like good idea, but I'm afraid I'm probably juggling too many rabbit-holes for my dwindling spoon collection already.