void matrix_transpose_3x3d(const double m[9], double result[9]) {
  result[0] = m[0];
  result[1] = m[3];
  result[2] = m[6];
  result[3] = m[1];
  result[4] = m[4];
  result[5] = m[7];
  result[6] = m[2];
  result[7] = m[5];
  result[8] = m[8];
}


x86 assembly (clang, -O0):
<matrix_transpose_3x3d>:
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  0x18(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x8(%rax)
mov    -0x8(%rbp),%rax
movsd  0x30(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x10(%rax)
mov    -0x8(%rbp),%rax
movsd  0x8(%rax),%xmm0
mov    -0x10(%rbp),%rax
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  0x38(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x28(%rax)
mov    -0x8(%rbp),%rax
movsd  0x10(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x30(%rax)
mov    -0x8(%rbp),%rax
movsd  0x28(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x38(%rax)
mov    -0x8(%rbp),%rax
movsd  0x40(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x40(%rax)
pop    %rbp
ret

x86 assembly (clang, -O2):
<matrix_transpose_3x3d>:
movsd  (%rdi),%xmm0
movsd  %xmm0,(%rsi)
movsd  0x18(%rdi),%xmm0
movsd  %xmm0,0x8(%rsi)
movsd  0x30(%rdi),%xmm0
movsd  %xmm0,0x10(%rsi)
movsd  0x8(%rdi),%xmm0
movsd  %xmm0,0x18(%rsi)
movsd  0x20(%rdi),%xmm0
movsd  %xmm0,0x20(%rsi)
movsd  0x38(%rdi),%xmm0
movsd  %xmm0,0x28(%rsi)
movsd  0x10(%rdi),%xmm0
movsd  %xmm0,0x30(%rsi)
movsd  0x28(%rdi),%xmm0
movsd  %xmm0,0x38(%rsi)
movsd  0x40(%rdi),%xmm0
movsd  %xmm0,0x40(%rsi)
ret

LLVM IR (-O0):
define void @matrix_transpose_3x3d(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 3
%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 6
%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 1
%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 4
%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 7
%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 2
%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 5
%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 8
%47 = load double, double* %46
%48 = load double*, double** %4
%49 = getelementptr inbounds double, double* %48, i64 8
store double %47, double* %49
ret void
}

LLVM IR (-O2):
define void @matrix_transpose_3x3d(double* nocapture %0, double* nocapture writeonly %1) {
%3 = load double, double* %0
store double %3, double* %1
%4 = getelementptr inbounds double, double* %0, i64 3
%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 6
%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 1
%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 4
%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 7
%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 2
%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 5
%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 8
%26 = load double, double* %25
%27 = getelementptr inbounds double, double* %1, i64 8
store double %26, double* %27
ret void
}