user: Please show me the C implementation that matches the assembly code listed below:

```asm
<bitrevR2>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  mov    %rdi,%rbx
  mov    %rdi,-0x48(%rsp)
  mov    %rsi,%rdi
  mov    %rdx,%r11
  mov    %rdx,-0x20(%rsp)
  lea    0x3(%rsi),%rax
  mov    %rax,%r8
  shr    $0x3f,%r8
  add    %rax,%r8
  sar    %r8
  mov    $0x1,%edx
  mov    %rdx,%rsi
  mov    %edi,%ecx
  shl    %cl,%rsi
  mov    %rdx,%rax
  mov    %r8d,%ecx
  shl    %cl,%rax
  mov    %rsi,%rcx
  sub    %rax,%rcx
  mov    %rcx,-0x28(%rsp)
  mov    %rdi,%rax
  shr    $0x3f,%rax
  add    %rdi,%rax
  sar    %rax
  lea    -0x1(%rax),%ecx
  shl    %cl,%rdx
  lea    -0x1(%rdx),%r14
  mov    %r14,-0x40(%rsp)
  lea    0x1(%rdi),%r9
  mov    %r9,%rcx
  shr    $0x3f,%rcx
  add    %r9,%rcx
  sar    %rcx
  add    $0x1,%rcx
  mov    %rcx,-0x38(%rsp)
  add    $0x1,%rsi
  mov    $0x4,%r9d
  mov    %r8d,%ecx
  shl    %cl,%r9
  mov    %r9,-0x60(%rsp)
  mov    %rbx,%rcx
  add    %r9,%rbx
  mov    %rbx,-0x8(%rsp)
  cmp    %rbx,%rcx
  jae    L4
  lea    (%r11,%r14,2),%rbx
  mov    %rbx,-0x30(%rsp)
  mov    $0x4,%r9d
  mov    %r9,%rbx
  mov    %edi,%ecx
  shl    %cl,%rbx
  mov    %rbx,%rdi
  shl    $0x2,%rsi
  lea    0x4(%rsi),%r8
  lea    0x8(%rsi),%r13
  add    $0x1,%eax
  mov    %eax,%ecx
  shl    %cl,%r9
  mov    %r9,-0x10(%rsp)
  lea    -0x2(%rdx),%rax
  mov    %rax,-0x18(%rsp)
L10:
  mov    -0x40(%rsp),%r12
L16:
  mov    %r12,-0x58(%rsp)
  mov    -0x20(%rsp),%r10
  movswl (%r10,%r12,2),%eax
  shl    $0x2,%eax
  cltq
  add    -0x28(%rsp),%rax
  mov    -0x48(%rsp),%rbx
  lea    (%rbx,%rax,4),%rbp
  mov    %r12,%rax
  movzbl -0x38(%rsp),%ecx
  shl    %cl,%rax
  lea    (%rbx,%rax,4),%r14
  mov    -0x30(%rsp),%rax
  movswl (%rax),%eax
  shl    $0x2,%eax
  cltq
  lea    (%r14,%rax,4),%rdx
  movss  0x0(%rbp),%xmm1
  movss  0x4(%rbp),%xmm0
  movss  0x0(%rbp,%rdi,1),%xmm9
  movss  0x0(%rbp,%rsi,1),%xmm8
  mov    -0x40(%rsp),%rax
  cmp    %r12,%rax
  jle    L48
  mov    %rax,%rcx
  mov    %rbp,%rax
  movss  %xmm0,-0x64(%rsp)
  mov    %rbp,-0x50(%rsp)
  mov    %r10,%rbp
L31:
  movss  0x8(%rax),%xmm7
  movss  0xc(%rax),%xmm6
  movss  (%rax,%r8,1),%xmm15
  movss  (%rax,%r13,1),%xmm14
  movss  (%rdx),%xmm5
  movss  0x4(%rdx),%xmm4
  lea    (%rdx,%rdi,1),%rbx
  movss  (%rbx),%xmm13
  lea    (%rdx,%rsi,1),%r11
  movss  (%r11),%xmm12
  movss  0x8(%rdx),%xmm3
  movss  0xc(%rdx),%xmm2
  lea    (%rdx,%r8,1),%r10
  movss  (%r10),%xmm11
  lea    (%rdx,%r13,1),%r9
  movss  (%r9),%xmm10
  movaps %xmm1,%xmm0
  addss  %xmm9,%xmm0
  movss  %xmm0,(%rdx)
  movss  -0x64(%rsp),%xmm0
  addss  %xmm8,%xmm0
  movss  %xmm0,0x4(%rdx)
  subss  %xmm9,%xmm1
  movss  %xmm1,0x8(%rdx)
  movss  -0x64(%rsp),%xmm0
  subss  %xmm8,%xmm0
  movss  %xmm0,0xc(%rdx)
  movaps %xmm7,%xmm0
  addss  %xmm15,%xmm0
  movss  %xmm0,(%rbx)
  movaps %xmm6,%xmm0
  addss  %xmm14,%xmm0
  movss  %xmm0,(%r11)
  subss  %xmm15,%xmm7
  movss  %xmm7,(%r10)
  subss  %xmm14,%xmm6
  movss  %xmm6,(%r9)
  movaps %xmm5,%xmm0
  addss  %xmm13,%xmm0
  movss  %xmm0,(%rax)
  movaps %xmm4,%xmm0
  addss  %xmm12,%xmm0
  movss  %xmm0,0x4(%rax)
  subss  %xmm13,%xmm5
  movss  %xmm5,0x8(%rax)
  subss  %xmm12,%xmm4
  movss  %xmm4,0xc(%rax)
  movaps %xmm3,%xmm0
  addss  %xmm11,%xmm0
  movss  %xmm0,(%rax,%rdi,1)
  movaps %xmm2,%xmm0
  addss  %xmm10,%xmm0
  movss  %xmm0,(%rax,%rsi,1)
  subss  %xmm11,%xmm3
  movss  %xmm3,(%rax,%r8,1)
  subss  %xmm10,%xmm2
  movss  %xmm2,(%rax,%r13,1)
  sub    -0x60(%rsp),%rax
  movss  (%rax),%xmm1
  movss  0x4(%rax),%xmm7
  movss  %xmm7,-0x64(%rsp)
  movss  (%rax,%rdi,1),%xmm9
  movss  (%rax,%rsi,1),%xmm8
  sub    $0x1,%rcx
  movswl 0x0(%rbp,%rcx,2),%edx
  shl    $0x2,%edx
  movslq %edx,%rdx
  lea    (%r14,%rdx,4),%rdx
  cmp    %r12,%rcx
  jg     L31
  movaps %xmm7,%xmm0
  mov    -0x50(%rsp),%rbp
  mov    -0x18(%rsp),%rbx
  mov    %rbx,%rax
  sub    -0x58(%rsp),%rax
  cmp    %r12,%rbx
  mov    $0x0,%ecx
  cmovl  %rcx,%rax
  mov    -0x60(%rsp),%rcx
  imul   %rcx,%rax
  sub    %rcx,%rbp
  sub    %rax,%rbp
L48:
  movss  0x8(%rbp),%xmm3
  movss  0xc(%rbp),%xmm2
  lea    0x0(%rbp,%r8,1),%rdx
  movss  (%rdx),%xmm5
  lea    0x0(%rbp,%r13,1),%rax
  movss  (%rax),%xmm4
  movaps %xmm1,%xmm6
  addss  %xmm9,%xmm6
  movss  %xmm6,0x0(%rbp)
  movaps %xmm0,%xmm6
  addss  %xmm8,%xmm6
  movss  %xmm6,0x4(%rbp)
  subss  %xmm9,%xmm1
  movss  %xmm1,0x8(%rbp)
  subss  %xmm8,%xmm0
  movss  %xmm0,0xc(%rbp)
  movaps %xmm3,%xmm0
  addss  %xmm5,%xmm0
  movss  %xmm0,0x0(%rbp,%rdi,1)
  movaps %xmm2,%xmm0
  addss  %xmm4,%xmm0
  movss  %xmm0,0x0(%rbp,%rsi,1)
  subss  %xmm5,%xmm3
  movss  %xmm3,(%rdx)
  subss  %xmm4,%xmm2
  movss  %xmm2,(%rax)
  sub    $0x1,%r12
  jns    L16
  mov    -0x10(%rsp),%rcx
  add    %rcx,-0x48(%rsp)
  mov    -0x48(%rsp),%rax
  cmp    %rax,-0x8(%rsp)
  ja     L10
L4:
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
```

assistant: ```c
void bitrevR2(float *ioptr, long M, short *BRLow) {
  float f0r;
  float f0i;
  float f1r;
  float f1i;
  float f2r;
  float f2i;
  float f3r;
  float f3i;
  float f4r;
  float f4i;
  float f5r;
  float f5i;
  float f6r;
  float f6i;
  float f7r;
  float f7i;
  float t0r;
  float t0i;
  float t1r;
  float t1i;
  float *p0r;
  float *p1r;
  float *IOP;
  float *iolimit;
  long Colstart;
  long iCol;
  unsigned long posA;
  unsigned long posAi;
  unsigned long posB;
  unsigned long posBi;
  const unsigned long Nrems2 = ((unsigned long)1 << ((M + 3) / 2));
  const unsigned long Nroot_1_ColInc = ((unsigned long)1 << (M)) - Nrems2;
  const unsigned long Nroot_1 = ((unsigned long)1 << (M / 2 - 1)) - 1;
  const unsigned long ColstartShift = (M + 1) / 2 + 1;
  posA = ((unsigned long)1 << (M));
  posAi = posA + 1;
  posB = posA + 2;
  posBi = posB + 1;
  iolimit = ioptr + Nrems2;
  for (; ioptr < iolimit; ioptr += ((unsigned long)1 << (M / 2 + 1))) {
    for (Colstart = Nroot_1; Colstart >= 0; Colstart--) {
      iCol = Nroot_1;
      p0r = ioptr + Nroot_1_ColInc + BRLow[Colstart] * 4;
      IOP = ioptr + (Colstart << ColstartShift);
      p1r = IOP + BRLow[iCol] * 4;
      f0r = *(p0r);
      f0i = *(p0r + 1);
      f1r = *(p0r + posA);
      f1i = *(p0r + posAi);
      for (; iCol > Colstart;) {
        f2r = *(p0r + 2);
        f2i = *(p0r + (2 + 1));
        f3r = *(p0r + posB);
        f3i = *(p0r + posBi);
        f4r = *(p1r);
        f4i = *(p1r + 1);
        f5r = *(p1r + posA);
        f5i = *(p1r + posAi);
        f6r = *(p1r + 2);
        f6i = *(p1r + (2 + 1));
        f7r = *(p1r + posB);
        f7i = *(p1r + posBi);
        t0r = f0r + f1r;
        t0i = f0i + f1i;
        f1r = f0r - f1r;
        f1i = f0i - f1i;
        t1r = f2r + f3r;
        t1i = f2i + f3i;
        f3r = f2r - f3r;
        f3i = f2i - f3i;
        f0r = f4r + f5r;
        f0i = f4i + f5i;
        f5r = f4r - f5r;
        f5i = f4i - f5i;
        f2r = f6r + f7r;
        f2i = f6i + f7i;
        f7r = f6r - f7r;
        f7i = f6i - f7i;
        *(p1r) = t0r;
        *(p1r + 1) = t0i;
        *(p1r + 2) = f1r;
        *(p1r + (2 + 1)) = f1i;
        *(p1r + posA) = t1r;
        *(p1r + posAi) = t1i;
        *(p1r + posB) = f3r;
        *(p1r + posBi) = f3i;
        *(p0r) = f0r;
        *(p0r + 1) = f0i;
        *(p0r + 2) = f5r;
        *(p0r + (2 + 1)) = f5i;
        *(p0r + posA) = f2r;
        *(p0r + posAi) = f2i;
        *(p0r + posB) = f7r;
        *(p0r + posBi) = f7i;
        p0r -= Nrems2;
        f0r = *(p0r);
        f0i = *(p0r + 1);
        f1r = *(p0r + posA);
        f1i = *(p0r + posAi);
        iCol -= 1;
        p1r = IOP + BRLow[iCol] * 4;
      };
      f2r = *(p0r + 2);
      f2i = *(p0r + (2 + 1));
      f3r = *(p0r + posB);
      f3i = *(p0r + posBi);
      t0r = f0r + f1r;
      t0i = f0i + f1i;
      f1r = f0r - f1r;
      f1i = f0i - f1i;
      t1r = f2r + f3r;
      t1i = f2i + f3i;
      f3r = f2r - f3r;
      f3i = f2i - f3i;
      *(p0r) = t0r;
      *(p0r + 1) = t0i;
      *(p0r + 2) = f1r;
      *(p0r + (2 + 1)) = f1i;
      *(p0r + posA) = t1r;
      *(p0r + posAi) = t1i;
      *(p0r + posB) = f3r;
      *(p0r + posBi) = f3i;
    };
  };
}
```