user: What would the C source look like for the assembly code provided here?

```asm
<bitrevR2>:
  endbr64
  push   %r15
  mov    %rdx,%r15
  lea    0x3(%rsi),%rdx
  mov    %esi,%ecx
  push   %r14
  mov    %rdx,%rax
  mov    %rsi,%r8
  mov    $0x4,%r10d
  push   %r13
  shr    $0x3f,%rax
  mov    %r10,%r11
  push   %r12
  add    %rdx,%rax
  mov    $0x1,%edx
  push   %rbp
  sar    %rax
  mov    %rdx,%rsi
  push   %rbx
  mov    %rdi,%rbx
  mov    %rdi,-0x50(%rsp)
  mov    %rdx,%rdi
  shl    %cl,%rdi
  mov    %eax,%ecx
  shl    %cl,%rsi
  mov    %rsi,%rcx
  mov    %rdi,%rsi
  add    $0x1,%rdi
  sub    %rcx,%rsi
  mov    %rsi,-0x40(%rsp)
  mov    %r8,%rsi
  shr    $0x3f,%rsi
  add    %r8,%rsi
  sar    %rsi
  lea    -0x1(%rsi),%ecx
  shl    %cl,%rdx
  lea    0x1(%r8),%rcx
  mov    %rcx,%r9
  lea    -0x1(%rdx),%r14
  shr    $0x3f,%r9
  mov    %r14,-0x58(%rsp)
  add    %rcx,%r9
  mov    %eax,%ecx
  shl    %cl,%r11
  sar    %r9
  lea    (%rbx,%r11,1),%rcx
  add    $0x1,%r9
  mov    %rcx,-0x10(%rsp)
  cmp    %rcx,%rbx
  jae    L15
  movswl (%r15,%r14,2),%ecx
  mov    %r10,%r14
  shl    $0x2,%rdi
  mov    %r11,%r13
  mov    %r9d,-0x34(%rsp)
  neg    %r13
  mov    %r15,%rbx
  lea    0x4(%rdi),%r12
  shl    $0x2,%ecx
  lea    0x8(%rdi),%r15
  movslq %ecx,%rcx
  shl    $0x2,%rcx
  mov    %rcx,-0x48(%rsp)
  mov    %r8d,%ecx
  shl    %cl,%r14
  lea    0x1(%rsi),%ecx
  lea    -0x2(%rdx),%rsi
  mov    %rsi,-0x30(%rsp)
  mov    %r14,%rsi
  shl    %cl,%r10
  mov    %r14,%r8
  sub    %r11,%rsi
  mov    %r10,-0x18(%rsp)
  mov    %rsi,-0x28(%rsp)
  mov    %rdi,%rsi
  sub    %r11,%rsi
  mov    %rsi,-0x20(%rsp)
  mov    %r13,%rsi
  nopl   0x0(%rax)
L43:
  movq   $0xffffffffffffffff,-0x60(%rsp)
  mov    -0x58(%rsp),%rbp
  xchg   %ax,%ax
L22:
  movswl (%rbx,%rbp,2),%eax
  mov    -0x50(%rsp),%rdx
  movzbl -0x34(%rsp),%ecx
  shl    $0x2,%eax
  cltq
  add    -0x40(%rsp),%rax
  lea    (%rdx,%rax,4),%r13
  mov    %rbp,%rax
  shl    %cl,%rax
  lea    0x0(%r13,%r8,1),%rcx
  movss  0x0(%r13),%xmm0
  movss  0x4(%r13),%xmm1
  lea    (%rdx,%rax,4),%r14
  mov    -0x48(%rsp),%rax
  movss  (%rcx),%xmm9
  lea    (%r14,%rax,1),%rdx
  lea    0x0(%r13,%rdi,1),%rax
  movss  (%rax),%xmm8
  cmp    -0x58(%rsp),%rbp
  jge    L58
  mov    -0x58(%rsp),%rcx
  mov    %r14,%r9
  mov    %r13,%rax
  movss  %xmm8,-0x64(%rsp)
  mov    %r13,%r14
  nop
L9:
  movaps %xmm9,%xmm8
  movss  0x8(%rax),%xmm7
  movss  0xc(%rax),%xmm6
  lea    (%rdx,%r8,1),%r13
  lea    (%rdx,%rdi,1),%r11
  lea    (%rdx,%r15,1),%r10
  movss  (%rdx),%xmm5
  sub    $0x1,%rcx
  addss  %xmm0,%xmm8
  subss  %xmm9,%xmm0
  movss  (%rax,%r12,1),%xmm15
  movss  (%rax,%r15,1),%xmm14
  movss  0x4(%rdx),%xmm4
  movss  0x0(%r13),%xmm13
  movss  (%r11),%xmm12
  movss  0x8(%rdx),%xmm3
  movss  %xmm8,-0x38(%rsp)
  movss  0xc(%rdx),%xmm2
  movss  (%rdx,%r12,1),%xmm11
  movss  -0x64(%rsp),%xmm8
  movss  (%r10),%xmm10
  addss  %xmm1,%xmm8
  subss  -0x64(%rsp),%xmm1
  unpcklps %xmm1,%xmm0
  movss  -0x38(%rsp),%xmm1
  unpcklps %xmm8,%xmm1
  movlhps %xmm0,%xmm1
  movaps %xmm7,%xmm0
  subss  %xmm15,%xmm7
  addss  %xmm15,%xmm0
  movups %xmm1,(%rdx)
  movss  %xmm0,0x0(%r13)
  movaps %xmm6,%xmm0
  subss  %xmm14,%xmm6
  addss  %xmm14,%xmm0
  unpcklps %xmm7,%xmm0
  movlps %xmm0,(%r11)
  movaps %xmm5,%xmm0
  subss  %xmm13,%xmm5
  addss  %xmm13,%xmm0
  movss  %xmm6,(%r10)
  movss  %xmm5,0x8(%rax)
  movss  %xmm0,(%rax)
  movaps %xmm4,%xmm0
  subss  %xmm12,%xmm4
  addss  %xmm12,%xmm0
  movss  %xmm4,0xc(%rax)
  movss  %xmm0,0x4(%rax)
  movaps %xmm3,%xmm0
  subss  %xmm11,%xmm3
  addss  %xmm11,%xmm0
  movss  %xmm0,(%rax,%r8,1)
  movaps %xmm2,%xmm0
  subss  %xmm10,%xmm2
  addss  %xmm10,%xmm0
  unpcklps %xmm3,%xmm0
  movlps %xmm0,(%rax,%rdi,1)
  movss  %xmm2,(%rax,%r15,1)
  add    %rsi,%rax
  movss  (%rax,%rdi,1),%xmm6
  movss  (%rax),%xmm0
  movss  0x4(%rax),%xmm1
  movss  (%rax,%r8,1),%xmm9
  movss  %xmm6,-0x64(%rsp)
  movswl (%rbx,%rcx,2),%edx
  shl    $0x2,%edx
  movslq %edx,%rdx
  lea    (%r9,%rdx,4),%rdx
  cmp    %rbp,%rcx
  jg     L9
  mov    -0x30(%rsp),%rdx
  mov    %r14,%r13
  mov    -0x60(%rsp),%r14
  xor    %eax,%eax
  mov    -0x28(%rsp),%rcx
  movaps %xmm6,%xmm8
  cmp    %rdx,%rbp
  lea    0x1(%r14),%r9
  cmovle %r14,%rax
  imul   %rsi,%rax
  add    %rax,%rcx
  add    -0x20(%rsp),%rax
  add    %r13,%rcx
  add    %r13,%rax
  cmp    %rdx,%rbp
  mov    $0x1,%edx
  cmovle %r9,%rdx
  imul   %rsi,%rdx
  add    %rdx,%r13
L4:
  movaps %xmm0,%xmm3
  movaps %xmm8,%xmm5
  subss  %xmm9,%xmm0
  sub    $0x1,%rbp
  addss  %xmm1,%xmm5
  addss  %xmm9,%xmm3
  movq   0x8(%r13),%xmm2
  mov    %r9,-0x60(%rsp)
  subss  %xmm8,%xmm1
  lea    0x0(%r13,%r12,1),%rdx
  movq   (%rdx),%xmm4
  movaps %xmm2,%xmm7
  shufps $0xe5,%xmm7,%xmm7
  unpcklps %xmm1,%xmm0
  movaps %xmm3,%xmm1
  movaps %xmm4,%xmm6
  unpcklps %xmm5,%xmm1
  shufps $0xe5,%xmm6,%xmm6
  movlhps %xmm0,%xmm1
  movaps %xmm4,%xmm0
  addss  %xmm2,%xmm0
  movups %xmm1,0x0(%r13)
  subps  %xmm4,%xmm2
  movss  %xmm0,(%rcx)
  movaps %xmm6,%xmm0
  addss  %xmm7,%xmm0
  movss  %xmm0,(%rax)
  movlps %xmm2,(%rdx)
  cmp    $0xffffffffffffffff,%rbp
  jne    L22
  mov    -0x18(%rsp),%rcx
  add    %rcx,-0x50(%rsp)
  mov    -0x50(%rsp),%rax
  cmp    %rax,-0x10(%rsp)
  ja     L43
L15:
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  nopw   0x0(%rax,%rax,1)
L58:
  mov    -0x60(%rsp),%rdx
  lea    0x1(%rdx),%r9
  jmp    L4
```

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;
    };
  };
}
```