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The asm keyword can be used to insert inline assembly, which is needed for a very small set of features such as fiber switching and system calls:

# x86-64 targets only
dst = 0
asm("mov $$1234, $0" : "=r"(dst))
dst # => 1234

An asm expression consists of up to 5 colon-separated sections, and components inside each section are separated by commas. For example:

  # the assembly template string, following the
  # syntax for LLVM's integrated assembler
  "nop" :
  # output operands
  "=r"(foo), "=r"(bar) :
  # input operands
  "r"(1), "r"(baz) :
  # names of clobbered registers
  "eax", "memory" :
  # optional flags, corresponding to the LLVM IR
  # sideeffect / alignstack / inteldialect / unwind attributes
  "volatile", "alignstack", "intel", "unwind"

Only the template string is mandatory, all other sections can be empty or omitted:

asm("nop" :: "b"(1), "c"(2)) # output operands are empty

For more details, refer to the LLVM documentation's section on inline assembler expressions.