void m_ident(float *M) {
  M[0] = 1.f;
  M[4] = 0.f;
  M[8] = 0.f;
  M[12] = 0.f;
  M[1] = 0.f;
  M[5] = 1.f;
  M[9] = 0.f;
  M[13] = 0.f;
  M[2] = 0.f;
  M[6] = 0.f;
  M[10] = 1.f;
  M[14] = 0.f;
  M[3] = 0.f;
  M[7] = 0.f;
  M[11] = 0.f;
  M[15] = 1.f;
}


x86 assembly (clang, -O0):
<m_ident>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    -0x8(%rbp),%rax
movss  0xeec(%rip),%xmm0
movss  %xmm0,(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0x10(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0x20(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0x30(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0x4(%rax)
mov    -0x8(%rbp),%rax
movss  0xeac(%rip),%xmm0
movss  %xmm0,0x14(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0x24(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0x34(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0x8(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0x18(%rax)
mov    -0x8(%rbp),%rax
movss  0xe6b(%rip),%xmm0
movss  %xmm0,0x28(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0x38(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0xc(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0x1c(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movss  %xmm0,0x2c(%rax)
mov    -0x8(%rbp),%rax
movss  0xe2a(%rip),%xmm0
movss  %xmm0,0x3c(%rax)
pop    %rbp
ret

x86 assembly (clang, -O2):
<m_ident>:
movss  0xef8(%rip),%xmm0
movups %xmm0,(%rdi)
movaps 0xefe(%rip),%xmm0
movups %xmm0,0x10(%rdi)
movaps 0xf03(%rip),%xmm0
movups %xmm0,0x20(%rdi)
movaps 0xf08(%rip),%xmm0
movups %xmm0,0x30(%rdi)
ret

LLVM IR (-O0):
define void @m_ident(float* %0) {
%2 = alloca float*
store float* %0, float** %2
%3 = load float*, float** %2
%4 = getelementptr inbounds float, float* %3, i64 0
store float 1.000000e+00, float* %4
%5 = load float*, float** %2
%6 = getelementptr inbounds float, float* %5, i64 4
store float 0.000000e+00, float* %6
%7 = load float*, float** %2
%8 = getelementptr inbounds float, float* %7, i64 8
store float 0.000000e+00, float* %8
%9 = load float*, float** %2
%10 = getelementptr inbounds float, float* %9, i64 12
store float 0.000000e+00, float* %10
%11 = load float*, float** %2
%12 = getelementptr inbounds float, float* %11, i64 1
store float 0.000000e+00, float* %12
%13 = load float*, float** %2
%14 = getelementptr inbounds float, float* %13, i64 5
store float 1.000000e+00, float* %14
%15 = load float*, float** %2
%16 = getelementptr inbounds float, float* %15, i64 9
store float 0.000000e+00, float* %16
%17 = load float*, float** %2
%18 = getelementptr inbounds float, float* %17, i64 13
store float 0.000000e+00, float* %18
%19 = load float*, float** %2
%20 = getelementptr inbounds float, float* %19, i64 2
store float 0.000000e+00, float* %20
%21 = load float*, float** %2
%22 = getelementptr inbounds float, float* %21, i64 6
store float 0.000000e+00, float* %22
%23 = load float*, float** %2
%24 = getelementptr inbounds float, float* %23, i64 10
store float 1.000000e+00, float* %24
%25 = load float*, float** %2
%26 = getelementptr inbounds float, float* %25, i64 14
store float 0.000000e+00, float* %26
%27 = load float*, float** %2
%28 = getelementptr inbounds float, float* %27, i64 3
store float 0.000000e+00, float* %28
%29 = load float*, float** %2
%30 = getelementptr inbounds float, float* %29, i64 7
store float 0.000000e+00, float* %30
%31 = load float*, float** %2
%32 = getelementptr inbounds float, float* %31, i64 11
store float 0.000000e+00, float* %32
%33 = load float*, float** %2
%34 = getelementptr inbounds float, float* %33, i64 15
store float 1.000000e+00, float* %34
ret void
}

LLVM IR (-O2):
define void @m_ident(float* nocapture writeonly %0) {
%2 = getelementptr inbounds float, float* %0, i64 4
%3 = getelementptr inbounds float, float* %0, i64 8
%4 = getelementptr inbounds float, float* %0, i64 12
%5 = bitcast float* %0 to <4 x float>*
store <4 x float> <float 1.000000e+00, float 0.000000e+00, float 0.000000e+00, float 0.000000e+00>, <4 x float>* %5
%6 = bitcast float* %2 to <4 x float>*
store <4 x float> <float 0.000000e+00, float 1.000000e+00, float 0.000000e+00, float 0.000000e+00>, <4 x float>* %6
%7 = bitcast float* %3 to <4 x float>*
store <4 x float> <float 0.000000e+00, float 0.000000e+00, float 1.000000e+00, float 0.000000e+00>, <4 x float>* %7
%8 = bitcast float* %4 to <4 x float>*
store <4 x float> <float 0.000000e+00, float 0.000000e+00, float 0.000000e+00, float 1.000000e+00>, <4 x float>* %8
ret void
}