react-native/React/Profiler/RCTProfileTrampoline-x86_64.S

#include "RCTDefines.h"

#if RCT_DEV && defined(__x86_64__)

  .globl _RCTProfileTrampoline
_RCTProfileTrampoline:

  /**
   * Saves all the state so we can restore it before calling the functions being
   * profiled. Registers have the same value at the point of the function call,
   * the only thing we can change is the return value, so we return to
   * `RCTProfileTrampoline` rather than to its caller.
   *
   * Save all the parameters registers (%rdi, %rsi, %rdx, %rcx, %r8, %r9), they
   * have the 6 first arguments of the function call, and %rax which in special
   * cases might be a pointer used for struct returns.
   *
   * We have to save %r12 since its value should be preserved across function
   * calls and we'll use it to keep the stack pointer
   */
  pushq %rdi
  pushq %rsi
  pushq %rdx
  pushq %rcx
  pushq %r8
  pushq %r9
  pushq %rax
  pushq %r12

  /**
   * Store the stack pointer in the callee saved register %r12 and align the
   * stack - it has to 16-byte aligned at the point of the function call
   */
  movq %rsp, %r12
  andq $-0x10, %rsp

  /**
   * void RCTProfileGetImplementation(id object, SEL selector)
   *
   * This is a C function defined in `RCTProfile.m`, the object and the selector
   * already have to be on %rdi and %rsi respectively, as in any ObjC call.
   */
  callq _RCTProfileGetImplementation

  // Restore/unalign the stack pointer, so we can access the registers we stored
  movq %r12, %rsp

  /**
   * pop %r12 before pushing %rax, which contains the address of the actual
   * function we have to call, than we keep %r12 at the bottom of the stack to
   * reference the stack pointer
   */
  popq %r12
  pushq %rax
  pushq %r12

  // align stack
  movq %rsp, %r12
  andq $-0x10, %rsp

  /**
   * Allocate memory to save parent before start profiling: the address is put
   * at the bottom of the stack at the function call, so ret can actually return
   * to the caller. In this case it has the address of RCTProfileTrampoline's
   * caller where we'll have to return to after we're finished.
   *
   * We can't store it on the stack or in any register, since we have to be in
   * the exact same state we were at the moment we were called, so the solution
   * is to allocate a tiny bit of memory to save this address
   */

  // allocate 16 bytes
  movq $0x10, %rdi
  callq _malloc

  // store the initial value of calle saved registers %r13 and %r14
  movq %r13, 0x0(%rax)
  movq %r14, 0x8(%rax)

  // mov the pointers we need to the callee saved registers
  movq 0x48(%rsp), %r13 // caller of RCTProfileTrampoline
  movq %rax, %r14 // allocated memory's address

  /**
   * Move self and cmd back to the registers and call start profile: it uses
   * the object and the selector to label the call in the profile.
   */
  movq 0x40(%r12), %rdi // object
  movq 0x38(%r12), %rsi // selector

  // void RCTProfileTrampolineStart(id, SEL) in RCTProfile.m
  callq _RCTProfileTrampolineStart

  // unalign the stack and restore %r12
  movq %r12, %rsp
  popq %r12

  // Restore registers for actual function call
  popq %r11
  popq %rax
  popq %r9
  popq %r8
  popq %rcx
  popq %rdx
  popq %rsi
  popq %rdi

  /**
   * delete parent caller (saved in %r13) `call` will add the new address so
   * we return to RCTProfileTrampoline rather than to its caller
   */
  addq $0x8, %rsp

  // call the actual function and save the return value
  callq *%r11
  pushq %rax

  // align stack
  pushq %r12
  movq %rsp, %r12
  andq $-0x10, %rsp

  // void RCTProfileTrampolineEnd(void) in RCTProfile.m - just ends this profile
  callq _RCTProfileTrampolineEnd

  // unalign stack and restore %r12
  movq %r12, %rsp
  popq %r12

  // save the return of the actual function call
  popq %rax

  /**
   * Restore the initial value of the callee saved registers, saved in the
   * memory allocated.
   */
  movq %r13, %rcx
  movq %r14, %rdx
  movq 0x0(%r14), %r13
  movq 0x8(%r14), %r14

  /**
   * Save caller address and actual function return (previously in the allocated
   * memory) and align the stack
   */
  pushq %rcx
  pushq %rax
  pushq %r12
  movq %rsp, %r12
  andq $-0x10, %rsp

  // Free the memory allocated to stash callee saved registers
  movq %rdx, %rdi
  callq _free

  // unalign  stack and restore %r12
  movq %r12, %rsp
  popq %r12

  /**
   * pop the caller address to %rcx and the actual function return value to
   * %rax, so it's the return value of RCTProfileTrampoline
   */
  popq %rax
  popq %rcx

  // jump to caller
  jmpq *%rcx

#endif