View assembly code for built in functions

[Marija]

Hi,

Is it possible to see generated assembly code for built-in functions? If I use just print_code with a simple program, like [].push(1), nothing is generated.

Is --print_builtin_code flag the right thing to use? For array push the output looks like:

kind = BUILTIN

name = ArrayPush

compiler = unknown

Instructions (size = 10)

0x246f7340     0  bb20c45908     mov ebx,0x859c420           ;; external reference (Builtin_ArrayPush)

0x246f7345     5  e9f682feff     jmp 0x246df640  (AdaptorWithBuiltinExitFrame)    ;; code: BUILTIN

RelocInfo (size = 3)

0x246f7341  external reference (Builtin_ArrayPush)  (0x859c420)

0x246f7346  code target (BUILTIN)  (0x246df640)

There is another code for FastArrayPush. 

Can you also help me to understand the output if the flag --print_builtin_code is the right thing to use?

Thanks!

[Rong Jie]

You need to build V8 with v8_enable_disassembler enabled (disabled by default in release build). See https://cs.chromium.org/chromium/src/v8/BUILD.gn?type=cs&q=v8_enable_disassembler

[Jakob Kummerow]

ArrayPush is implemented in C++, see src/builtins/builtins-array.cc. The code object you've disassembled is a small wrapper/trampoline stub, the first step in the C++ entry sequence.

In general, there is no single way to inspect "built-in functions", because there are several ways to implement them (a few are still in handwritten assembly; many are in C++ [exposed as "runtime functions" or "builtins"] and CodeStubAssembler; some are in JavaScript), and that implies different ways to dump their code. Many functions also have several implementations, e.g. a fast path in a generated stub (FastArrayPush is an example), with a C++ fallback for non-fast cases.

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