asm¶
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:
asm(
# 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")
asm("nop" :: "b"(1), "c"(2)) # output operands are empty
For more details, refer to the LLVM documentation's section on inline assembler expressions.