void matrix_transpose_4x4d(const double m[16], double result[16]) {
  result[0] = m[0];
  result[1] = m[4];
  result[2] = m[8];
  result[3] = m[12];
  result[4] = m[1];
  result[5] = m[5];
  result[6] = m[9];
  result[7] = m[13];
  result[8] = m[2];
  result[9] = m[6];
  result[10] = m[10];
  result[11] = m[14];
  result[12] = m[3];
  result[13] = m[7];
  result[14] = m[11];
  result[15] = m[15];
}


x86 assembly (clang, -O0):
<matrix_transpose_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  0x20(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x8(%rax)
mov    -0x8(%rbp),%rax
movsd  0x40(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x10(%rax)
mov    -0x8(%rbp),%rax
movsd  0x60(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x18(%rax)
mov    -0x8(%rbp),%rax
movsd  0x8(%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  0x48(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x30(%rax)
mov    -0x8(%rbp),%rax
movsd  0x68(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x38(%rax)
mov    -0x8(%rbp),%rax
movsd  0x10(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x40(%rax)
mov    -0x8(%rbp),%rax
movsd  0x30(%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    -0x8(%rbp),%rax
movsd  0x70(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x58(%rax)
mov    -0x8(%rbp),%rax
movsd  0x18(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x60(%rax)
mov    -0x8(%rbp),%rax
movsd  0x38(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x68(%rax)
mov    -0x8(%rbp),%rax
movsd  0x58(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x70(%rax)
mov    -0x8(%rbp),%rax
movsd  0x78(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x78(%rax)
pop    %rbp
ret

x86 assembly (clang, -O2):
<matrix_transpose_4x4d>:
movsd  (%rdi),%xmm0
movsd  %xmm0,(%rsi)
movsd  0x20(%rdi),%xmm0
movsd  %xmm0,0x8(%rsi)
movsd  0x40(%rdi),%xmm0
movsd  %xmm0,0x10(%rsi)
movsd  0x60(%rdi),%xmm0
movsd  %xmm0,0x18(%rsi)
movsd  0x8(%rdi),%xmm0
movsd  %xmm0,0x20(%rsi)
movsd  0x28(%rdi),%xmm0
movsd  %xmm0,0x28(%rsi)
movsd  0x48(%rdi),%xmm0
movsd  %xmm0,0x30(%rsi)
movsd  0x68(%rdi),%xmm0
movsd  %xmm0,0x38(%rsi)
movsd  0x10(%rdi),%xmm0
movsd  %xmm0,0x40(%rsi)
movsd  0x30(%rdi),%xmm0
movsd  %xmm0,0x48(%rsi)
movsd  0x50(%rdi),%xmm0
movsd  %xmm0,0x50(%rsi)
movsd  0x70(%rdi),%xmm0
movsd  %xmm0,0x58(%rsi)
movsd  0x18(%rdi),%xmm0
movsd  %xmm0,0x60(%rsi)
movsd  0x38(%rdi),%xmm0
movsd  %xmm0,0x68(%rsi)
movsd  0x58(%rdi),%xmm0
movsd  %xmm0,0x70(%rsi)
movsd  0x78(%rdi),%xmm0
movsd  %xmm0,0x78(%rsi)
ret

LLVM IR (-O0):
define void @matrix_transpose_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 4
%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 8
%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** %3
%21 = getelementptr inbounds double, double* %20, i64 12
%22 = load double, double* %21
%23 = load double*, double** %4
%24 = getelementptr inbounds double, double* %23, i64 3
store double %22, double* %24
%25 = load double*, double** %3
%26 = getelementptr inbounds double, double* %25, i64 1
%27 = load double, double* %26
%28 = load double*, double** %4
%29 = getelementptr inbounds double, double* %28, i64 4
store double %27, double* %29
%30 = load double*, double** %3
%31 = getelementptr inbounds double, double* %30, i64 5
%32 = load double, double* %31
%33 = load double*, double** %4
%34 = getelementptr inbounds double, double* %33, i64 5
store double %32, double* %34
%35 = load double*, double** %3
%36 = getelementptr inbounds double, double* %35, i64 9
%37 = load double, double* %36
%38 = load double*, double** %4
%39 = getelementptr inbounds double, double* %38, i64 6
store double %37, double* %39
%40 = load double*, double** %3
%41 = getelementptr inbounds double, double* %40, i64 13
%42 = load double, double* %41
%43 = load double*, double** %4
%44 = getelementptr inbounds double, double* %43, i64 7
store double %42, double* %44
%45 = load double*, double** %3
%46 = getelementptr inbounds double, double* %45, i64 2
%47 = load double, double* %46
%48 = load double*, double** %4
%49 = getelementptr inbounds double, double* %48, i64 8
store double %47, double* %49
%50 = load double*, double** %3
%51 = getelementptr inbounds double, double* %50, i64 6
%52 = load double, double* %51
%53 = load double*, double** %4
%54 = getelementptr inbounds double, double* %53, i64 9
store double %52, double* %54
%55 = load double*, double** %3
%56 = getelementptr inbounds double, double* %55, i64 10
%57 = load double, double* %56
%58 = load double*, double** %4
%59 = getelementptr inbounds double, double* %58, i64 10
store double %57, double* %59
%60 = load double*, double** %3
%61 = getelementptr inbounds double, double* %60, i64 14
%62 = load double, double* %61
%63 = load double*, double** %4
%64 = getelementptr inbounds double, double* %63, i64 11
store double %62, double* %64
%65 = load double*, double** %3
%66 = getelementptr inbounds double, double* %65, i64 3
%67 = load double, double* %66
%68 = load double*, double** %4
%69 = getelementptr inbounds double, double* %68, i64 12
store double %67, double* %69
%70 = load double*, double** %3
%71 = getelementptr inbounds double, double* %70, i64 7
%72 = load double, double* %71
%73 = load double*, double** %4
%74 = getelementptr inbounds double, double* %73, i64 13
store double %72, double* %74
%75 = load double*, double** %3
%76 = getelementptr inbounds double, double* %75, i64 11
%77 = load double, double* %76
%78 = load double*, double** %4
%79 = getelementptr inbounds double, double* %78, i64 14
store double %77, double* %79
%80 = load double*, double** %3
%81 = getelementptr inbounds double, double* %80, i64 15
%82 = load double, double* %81
%83 = load double*, double** %4
%84 = getelementptr inbounds double, double* %83, i64 15
store double %82, double* %84
ret void
}

LLVM IR (-O2):
define void @matrix_transpose_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 4
%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 8
%8 = load double, double* %7
%9 = getelementptr inbounds double, double* %1, i64 2
store double %8, double* %9
%10 = getelementptr inbounds double, double* %0, i64 12
%11 = load double, double* %10
%12 = getelementptr inbounds double, double* %1, i64 3
store double %11, double* %12
%13 = getelementptr inbounds double, double* %0, i64 1
%14 = load double, double* %13
%15 = getelementptr inbounds double, double* %1, i64 4
store double %14, double* %15
%16 = getelementptr inbounds double, double* %0, i64 5
%17 = load double, double* %16
%18 = getelementptr inbounds double, double* %1, i64 5
store double %17, double* %18
%19 = getelementptr inbounds double, double* %0, i64 9
%20 = load double, double* %19
%21 = getelementptr inbounds double, double* %1, i64 6
store double %20, double* %21
%22 = getelementptr inbounds double, double* %0, i64 13
%23 = load double, double* %22
%24 = getelementptr inbounds double, double* %1, i64 7
store double %23, double* %24
%25 = getelementptr inbounds double, double* %0, i64 2
%26 = load double, double* %25
%27 = getelementptr inbounds double, double* %1, i64 8
store double %26, double* %27
%28 = getelementptr inbounds double, double* %0, i64 6
%29 = load double, double* %28
%30 = getelementptr inbounds double, double* %1, i64 9
store double %29, double* %30
%31 = getelementptr inbounds double, double* %0, i64 10
%32 = load double, double* %31
%33 = getelementptr inbounds double, double* %1, i64 10
store double %32, double* %33
%34 = getelementptr inbounds double, double* %0, i64 14
%35 = load double, double* %34
%36 = getelementptr inbounds double, double* %1, i64 11
store double %35, double* %36
%37 = getelementptr inbounds double, double* %0, i64 3
%38 = load double, double* %37
%39 = getelementptr inbounds double, double* %1, i64 12
store double %38, double* %39
%40 = getelementptr inbounds double, double* %0, i64 7
%41 = load double, double* %40
%42 = getelementptr inbounds double, double* %1, i64 13
store double %41, double* %42
%43 = getelementptr inbounds double, double* %0, i64 11
%44 = load double, double* %43
%45 = getelementptr inbounds double, double* %1, i64 14
store double %44, double* %45
%46 = getelementptr inbounds double, double* %0, i64 15
%47 = load double, double* %46
%48 = getelementptr inbounds double, double* %1, i64 15
store double %47, double* %48
ret void
}