void matrix_rigid_body_transform_get_rotation_matrix_4x4d(const double m[16],
                                                          double rot[16]) {
  rot[0] = m[0];
  rot[1] = m[1];
  rot[2] = m[2];
  rot[3] = 0;
  rot[4] = m[4];
  rot[5] = m[5];
  rot[6] = m[6];
  rot[7] = 0;
  rot[8] = m[8];
  rot[9] = m[9];
  rot[10] = m[10];
  rot[11] = 0;
  rot[12] = rot[13] = rot[14] = 0;
  rot[15] = 1;
}


x86 assembly (clang, -O0):
<matrix_rigid_body_transform_get_rotation_matrix_4x4d>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    %rsi,-0x10(%rbp)
mov    -0x8(%rbp),%rax
movsd  (%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,(%rax)
mov    -0x8(%rbp),%rax
movsd  0x8(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x8(%rax)
mov    -0x8(%rbp),%rax
movsd  0x10(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x10(%rax)
mov    -0x10(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x18(%rax)
mov    -0x8(%rbp),%rax
movsd  0x20(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x20(%rax)
mov    -0x8(%rbp),%rax
movsd  0x28(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x28(%rax)
mov    -0x8(%rbp),%rax
movsd  0x30(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x30(%rax)
mov    -0x10(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x38(%rax)
mov    -0x8(%rbp),%rax
movsd  0x40(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x40(%rax)
mov    -0x8(%rbp),%rax
movsd  0x48(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x48(%rax)
mov    -0x8(%rbp),%rax
movsd  0x50(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x50(%rax)
mov    -0x10(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x58(%rax)
mov    -0x10(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x70(%rax)
mov    -0x10(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x68(%rax)
mov    -0x10(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x60(%rax)
mov    -0x10(%rbp),%rax
movsd  0xe00(%rip),%xmm0
movsd  %xmm0,0x78(%rax)
pop    %rbp
ret

x86 assembly (clang, -O2):
<matrix_rigid_body_transform_get_rotation_matrix_4x4d>:
movsd  (%rdi),%xmm0
movsd  %xmm0,(%rsi)
movsd  0x8(%rdi),%xmm0
movsd  %xmm0,0x8(%rsi)
movsd  0x10(%rdi),%xmm0
movsd  %xmm0,0x10(%rsi)
movq   $0x0,0x18(%rsi)
movsd  0x20(%rdi),%xmm0
movsd  %xmm0,0x20(%rsi)
movsd  0x28(%rdi),%xmm0
movsd  %xmm0,0x28(%rsi)
movsd  0x30(%rdi),%xmm0
movsd  %xmm0,0x30(%rsi)
movq   $0x0,0x38(%rsi)
movsd  0x40(%rdi),%xmm0
movsd  %xmm0,0x40(%rsi)
movsd  0x48(%rdi),%xmm0
movsd  %xmm0,0x48(%rsi)
movsd  0x50(%rdi),%xmm0
movsd  %xmm0,0x50(%rsi)
xorps  %xmm0,%xmm0
movups %xmm0,0x58(%rsi)
movups %xmm0,0x68(%rsi)
movabs $0x3ff0000000000000,%rax
mov    %rax,0x78(%rsi)
ret

LLVM IR (-O0):
define void @matrix_rigid_body_transform_get_rotation_matrix_4x4d(double* %0, double* %1) {
%3 = alloca double*
%4 = alloca double*
store double* %0, double** %3
store double* %1, double** %4
%5 = load double*, double** %3
%6 = getelementptr inbounds double, double* %5, i64 0
%7 = load double, double* %6
%8 = load double*, double** %4
%9 = getelementptr inbounds double, double* %8, i64 0
store double %7, double* %9
%10 = load double*, double** %3
%11 = getelementptr inbounds double, double* %10, i64 1
%12 = load double, double* %11
%13 = load double*, double** %4
%14 = getelementptr inbounds double, double* %13, i64 1
store double %12, double* %14
%15 = load double*, double** %3
%16 = getelementptr inbounds double, double* %15, i64 2
%17 = load double, double* %16
%18 = load double*, double** %4
%19 = getelementptr inbounds double, double* %18, i64 2
store double %17, double* %19
%20 = load double*, double** %4
%21 = getelementptr inbounds double, double* %20, i64 3
store double 0.000000e+00, double* %21
%22 = load double*, double** %3
%23 = getelementptr inbounds double, double* %22, i64 4
%24 = load double, double* %23
%25 = load double*, double** %4
%26 = getelementptr inbounds double, double* %25, i64 4
store double %24, double* %26
%27 = load double*, double** %3
%28 = getelementptr inbounds double, double* %27, i64 5
%29 = load double, double* %28
%30 = load double*, double** %4
%31 = getelementptr inbounds double, double* %30, i64 5
store double %29, double* %31
%32 = load double*, double** %3
%33 = getelementptr inbounds double, double* %32, i64 6
%34 = load double, double* %33
%35 = load double*, double** %4
%36 = getelementptr inbounds double, double* %35, i64 6
store double %34, double* %36
%37 = load double*, double** %4
%38 = getelementptr inbounds double, double* %37, i64 7
store double 0.000000e+00, double* %38
%39 = load double*, double** %3
%40 = getelementptr inbounds double, double* %39, i64 8
%41 = load double, double* %40
%42 = load double*, double** %4
%43 = getelementptr inbounds double, double* %42, i64 8
store double %41, double* %43
%44 = load double*, double** %3
%45 = getelementptr inbounds double, double* %44, i64 9
%46 = load double, double* %45
%47 = load double*, double** %4
%48 = getelementptr inbounds double, double* %47, i64 9
store double %46, double* %48
%49 = load double*, double** %3
%50 = getelementptr inbounds double, double* %49, i64 10
%51 = load double, double* %50
%52 = load double*, double** %4
%53 = getelementptr inbounds double, double* %52, i64 10
store double %51, double* %53
%54 = load double*, double** %4
%55 = getelementptr inbounds double, double* %54, i64 11
store double 0.000000e+00, double* %55
%56 = load double*, double** %4
%57 = getelementptr inbounds double, double* %56, i64 14
store double 0.000000e+00, double* %57
%58 = load double*, double** %4
%59 = getelementptr inbounds double, double* %58, i64 13
store double 0.000000e+00, double* %59
%60 = load double*, double** %4
%61 = getelementptr inbounds double, double* %60, i64 12
store double 0.000000e+00, double* %61
%62 = load double*, double** %4
%63 = getelementptr inbounds double, double* %62, i64 15
store double 1.000000e+00, double* %63
ret void
}

LLVM IR (-O2):
define void @matrix_rigid_body_transform_get_rotation_matrix_4x4d(double* nocapture %0, double* nocapture writeonly %1) {
%3 = load double, double* %0
store double %3, double* %1
%4 = getelementptr inbounds double, double* %0, i64 1
%5 = load double, double* %4
%6 = getelementptr inbounds double, double* %1, i64 1
store double %5, double* %6
%7 = getelementptr inbounds double, double* %0, i64 2
%8 = load double, double* %7
%9 = getelementptr inbounds double, double* %1, i64 2
store double %8, double* %9
%10 = getelementptr inbounds double, double* %1, i64 3
store double 0.000000e+00, double* %10
%11 = getelementptr inbounds double, double* %0, i64 4
%12 = load double, double* %11
%13 = getelementptr inbounds double, double* %1, i64 4
store double %12, double* %13
%14 = getelementptr inbounds double, double* %0, i64 5
%15 = load double, double* %14
%16 = getelementptr inbounds double, double* %1, i64 5
store double %15, double* %16
%17 = getelementptr inbounds double, double* %0, i64 6
%18 = load double, double* %17
%19 = getelementptr inbounds double, double* %1, i64 6
store double %18, double* %19
%20 = getelementptr inbounds double, double* %1, i64 7
store double 0.000000e+00, double* %20
%21 = getelementptr inbounds double, double* %0, i64 8
%22 = load double, double* %21
%23 = getelementptr inbounds double, double* %1, i64 8
store double %22, double* %23
%24 = getelementptr inbounds double, double* %0, i64 9
%25 = load double, double* %24
%26 = getelementptr inbounds double, double* %1, i64 9
store double %25, double* %26
%27 = getelementptr inbounds double, double* %0, i64 10
%28 = load double, double* %27
%29 = getelementptr inbounds double, double* %1, i64 10
store double %28, double* %29
%30 = getelementptr inbounds double, double* %1, i64 11
%31 = getelementptr inbounds double, double* %1, i64 15
%32 = bitcast double* %30 to i8*
store double 1.000000e+00, double* %31
ret void
}