void clear_matrix(double *matrix) {
  matrix[0] = 1;
  matrix[1] = 0;
  matrix[2] = 0;
  matrix[3] = 0;
  matrix[4] = 0;
  matrix[5] = 1;
  matrix[6] = 0;
  matrix[7] = 0;
  matrix[8] = 0;
  matrix[9] = 0;
  matrix[10] = 1;
  matrix[11] = 0;
  matrix[12] = 0;
  matrix[13] = 0;
  matrix[14] = 0;
  matrix[15] = 1;
}


x86 assembly (clang, -O0):
<clear_matrix>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    -0x8(%rbp),%rax
movsd  0xeec(%rip),%xmm0
movsd  %xmm0,(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x8(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x10(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x18(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x20(%rax)
mov    -0x8(%rbp),%rax
movsd  0xeac(%rip),%xmm0
movsd  %xmm0,0x28(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x30(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x38(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x40(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x48(%rax)
mov    -0x8(%rbp),%rax
movsd  0xe6b(%rip),%xmm0
movsd  %xmm0,0x50(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x58(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x60(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x68(%rax)
mov    -0x8(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x70(%rax)
mov    -0x8(%rbp),%rax
movsd  0xe2a(%rip),%xmm0
movsd  %xmm0,0x78(%rax)
pop    %rbp
ret

x86 assembly (clang, -O2):
<clear_matrix>:
movabs $0x3ff0000000000000,%rax
mov    %rax,(%rdi)
xorps  %xmm0,%xmm0
movups %xmm0,0x8(%rdi)
movups %xmm0,0x18(%rdi)
mov    %rax,0x28(%rdi)
movups %xmm0,0x30(%rdi)
movups %xmm0,0x40(%rdi)
mov    %rax,0x50(%rdi)
movups %xmm0,0x58(%rdi)
movups %xmm0,0x68(%rdi)
mov    %rax,0x78(%rdi)
ret

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

LLVM IR (-O2):
define void @clear_matrix(double* nocapture writeonly %0) {
store double 1.000000e+00, double* %0
%2 = getelementptr inbounds double, double* %0, i64 1
%3 = getelementptr inbounds double, double* %0, i64 5
%4 = bitcast double* %2 to i8*
store double 1.000000e+00, double* %3
%5 = getelementptr inbounds double, double* %0, i64 6
%6 = getelementptr inbounds double, double* %0, i64 10
%7 = bitcast double* %5 to i8*
store double 1.000000e+00, double* %6
%8 = getelementptr inbounds double, double* %0, i64 11
%9 = getelementptr inbounds double, double* %0, i64 15
%10 = bitcast double* %8 to i8*
store double 1.000000e+00, double* %9
ret void
}