float *mat3_assign_mat4(float *result, float *m) {
  result[0] = m[0];
  result[1] = m[1];
  result[2] = m[2];
  result[3] = m[4];
  result[4] = m[5];
  result[5] = m[6];
  result[6] = m[8];
  result[7] = m[9];
  result[8] = m[10];
  return result;
}


x86 assembly (clang, -O0):
<mat3_assign_mat4>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    %rsi,-0x10(%rbp)
mov    -0x10(%rbp),%rax
movss  (%rax),%xmm0
mov    -0x8(%rbp),%rax
movss  %xmm0,(%rax)
mov    -0x10(%rbp),%rax
movss  0x4(%rax),%xmm0
mov    -0x8(%rbp),%rax
movss  %xmm0,0x4(%rax)
mov    -0x10(%rbp),%rax
movss  0x8(%rax),%xmm0
mov    -0x8(%rbp),%rax
movss  %xmm0,0x8(%rax)
mov    -0x10(%rbp),%rax
movss  0x10(%rax),%xmm0
mov    -0x8(%rbp),%rax
movss  %xmm0,0xc(%rax)
mov    -0x10(%rbp),%rax
movss  0x14(%rax),%xmm0
mov    -0x8(%rbp),%rax
movss  %xmm0,0x10(%rax)
mov    -0x10(%rbp),%rax
movss  0x18(%rax),%xmm0
mov    -0x8(%rbp),%rax
movss  %xmm0,0x14(%rax)
mov    -0x10(%rbp),%rax
movss  0x20(%rax),%xmm0
mov    -0x8(%rbp),%rax
movss  %xmm0,0x18(%rax)
mov    -0x10(%rbp),%rax
movss  0x24(%rax),%xmm0
mov    -0x8(%rbp),%rax
movss  %xmm0,0x1c(%rax)
mov    -0x10(%rbp),%rax
movss  0x28(%rax),%xmm0
mov    -0x8(%rbp),%rax
movss  %xmm0,0x20(%rax)
mov    -0x8(%rbp),%rax
pop    %rbp
ret

x86 assembly (clang, -O2):
<mat3_assign_mat4>:
mov    %rdi,%rax
movss  (%rsi),%xmm0
movss  %xmm0,(%rdi)
movss  0x4(%rsi),%xmm0
movss  %xmm0,0x4(%rdi)
movss  0x8(%rsi),%xmm0
movss  %xmm0,0x8(%rdi)
movss  0x10(%rsi),%xmm0
movss  %xmm0,0xc(%rdi)
movss  0x14(%rsi),%xmm0
movss  %xmm0,0x10(%rdi)
movss  0x18(%rsi),%xmm0
movss  %xmm0,0x14(%rdi)
movss  0x20(%rsi),%xmm0
movss  %xmm0,0x18(%rdi)
movss  0x24(%rsi),%xmm0
movss  %xmm0,0x1c(%rdi)
movss  0x28(%rsi),%xmm0
movss  %xmm0,0x20(%rdi)
ret

LLVM IR (-O0):
define float* @mat3_assign_mat4(float* %0, float* %1) {
%3 = alloca float*
%4 = alloca float*
store float* %0, float** %3
store float* %1, float** %4
%5 = load float*, float** %4
%6 = getelementptr inbounds float, float* %5, i64 0
%7 = load float, float* %6
%8 = load float*, float** %3
%9 = getelementptr inbounds float, float* %8, i64 0
store float %7, float* %9
%10 = load float*, float** %4
%11 = getelementptr inbounds float, float* %10, i64 1
%12 = load float, float* %11
%13 = load float*, float** %3
%14 = getelementptr inbounds float, float* %13, i64 1
store float %12, float* %14
%15 = load float*, float** %4
%16 = getelementptr inbounds float, float* %15, i64 2
%17 = load float, float* %16
%18 = load float*, float** %3
%19 = getelementptr inbounds float, float* %18, i64 2
store float %17, float* %19
%20 = load float*, float** %4
%21 = getelementptr inbounds float, float* %20, i64 4
%22 = load float, float* %21
%23 = load float*, float** %3
%24 = getelementptr inbounds float, float* %23, i64 3
store float %22, float* %24
%25 = load float*, float** %4
%26 = getelementptr inbounds float, float* %25, i64 5
%27 = load float, float* %26
%28 = load float*, float** %3
%29 = getelementptr inbounds float, float* %28, i64 4
store float %27, float* %29
%30 = load float*, float** %4
%31 = getelementptr inbounds float, float* %30, i64 6
%32 = load float, float* %31
%33 = load float*, float** %3
%34 = getelementptr inbounds float, float* %33, i64 5
store float %32, float* %34
%35 = load float*, float** %4
%36 = getelementptr inbounds float, float* %35, i64 8
%37 = load float, float* %36
%38 = load float*, float** %3
%39 = getelementptr inbounds float, float* %38, i64 6
store float %37, float* %39
%40 = load float*, float** %4
%41 = getelementptr inbounds float, float* %40, i64 9
%42 = load float, float* %41
%43 = load float*, float** %3
%44 = getelementptr inbounds float, float* %43, i64 7
store float %42, float* %44
%45 = load float*, float** %4
%46 = getelementptr inbounds float, float* %45, i64 10
%47 = load float, float* %46
%48 = load float*, float** %3
%49 = getelementptr inbounds float, float* %48, i64 8
store float %47, float* %49
%50 = load float*, float** %3
ret float* %50
}

LLVM IR (-O2):
define float* @mat3_assign_mat4(float* returned writeonly %0, float* nocapture %1) {
%3 = load float, float* %1
store float %3, float* %0
%4 = getelementptr inbounds float, float* %1, i64 1
%5 = load float, float* %4
%6 = getelementptr inbounds float, float* %0, i64 1
store float %5, float* %6
%7 = getelementptr inbounds float, float* %1, i64 2
%8 = load float, float* %7
%9 = getelementptr inbounds float, float* %0, i64 2
store float %8, float* %9
%10 = getelementptr inbounds float, float* %1, i64 4
%11 = load float, float* %10
%12 = getelementptr inbounds float, float* %0, i64 3
store float %11, float* %12
%13 = getelementptr inbounds float, float* %1, i64 5
%14 = load float, float* %13
%15 = getelementptr inbounds float, float* %0, i64 4
store float %14, float* %15
%16 = getelementptr inbounds float, float* %1, i64 6
%17 = load float, float* %16
%18 = getelementptr inbounds float, float* %0, i64 5
store float %17, float* %18
%19 = getelementptr inbounds float, float* %1, i64 8
%20 = load float, float* %19
%21 = getelementptr inbounds float, float* %0, i64 6
store float %20, float* %21
%22 = getelementptr inbounds float, float* %1, i64 9
%23 = load float, float* %22
%24 = getelementptr inbounds float, float* %0, i64 7
store float %23, float* %24
%25 = getelementptr inbounds float, float* %1, i64 10
%26 = load float, float* %25
%27 = getelementptr inbounds float, float* %0, i64 8
store float %26, float* %27
ret float* %0
}