double v_dot_p6(const double x[6], const double y[6]) {
  double res = 0.0;
  int i;
  for (i = 0; i < 6; i++)
    res += x[i] * y[i];
  return res;
}


x86 assembly (clang, -O0):
<v_dot_p6>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    %rsi,-0x10(%rbp)
xorps  %xmm0,%xmm0
movsd  %xmm0,-0x18(%rbp)
movl   $0x0,-0x1c(%rbp)
cmpl   $0x6,-0x1c(%rbp)
jge    115f <v_dot_p6+0x5f>
mov    -0x8(%rbp),%rax
movslq -0x1c(%rbp),%rcx
movsd  (%rax,%rcx,8),%xmm0
mov    -0x10(%rbp),%rax
movslq -0x1c(%rbp),%rcx
movsd  (%rax,%rcx,8),%xmm2
movsd  -0x18(%rbp),%xmm1
mulsd  %xmm2,%xmm0
addsd  %xmm1,%xmm0
movsd  %xmm0,-0x18(%rbp)
mov    -0x1c(%rbp),%eax
add    $0x1,%eax
mov    %eax,-0x1c(%rbp)
jmp    111b <v_dot_p6+0x1b>
movsd  -0x18(%rbp),%xmm0
pop    %rbp
ret

x86 assembly (clang, -O2):
<v_dot_p6>:
movsd  (%rdi),%xmm0
movsd  0x8(%rdi),%xmm1
mulsd  (%rsi),%xmm0
xorpd  %xmm2,%xmm2
addsd  %xmm0,%xmm2
mulsd  0x8(%rsi),%xmm1
addsd  %xmm2,%xmm1
movsd  0x10(%rdi),%xmm0
mulsd  0x10(%rsi),%xmm0
addsd  %xmm1,%xmm0
movsd  0x18(%rdi),%xmm1
mulsd  0x18(%rsi),%xmm1
addsd  %xmm0,%xmm1
movsd  0x20(%rdi),%xmm2
mulsd  0x20(%rsi),%xmm2
addsd  %xmm1,%xmm2
movsd  0x28(%rdi),%xmm0
mulsd  0x28(%rsi),%xmm0
addsd  %xmm2,%xmm0
ret

LLVM IR (-O0):
define double @v_dot_p6(double* %0, double* %1) {
%3 = alloca double*
%4 = alloca double*
%5 = alloca double
%6 = alloca i32
store double* %0, double** %3
store double* %1, double** %4
store double 0.000000e+00, double* %5
store i32 0, i32* %6
br label %7
7: ; preds = %23, %2
%8 = load i32, i32* %6
%9 = icmp slt i32 %8, 6
br i1 %9, label %10, label %26
10: ; preds = %7
%11 = load double*, double** %3
%12 = load i32, i32* %6
%13 = sext i32 %12 to i64
%14 = getelementptr inbounds double, double* %11, i64 %13
%15 = load double, double* %14
%16 = load double*, double** %4
%17 = load i32, i32* %6
%18 = sext i32 %17 to i64
%19 = getelementptr inbounds double, double* %16, i64 %18
%20 = load double, double* %19
%21 = load double, double* %5
%22 = call double @llvm.fmuladd.f64(double %15, double %20, double %21)
store double %22, double* %5
br label %23
23: ; preds = %10
%24 = load i32, i32* %6
%25 = add nsw i32 %24, 1
store i32 %25, i32* %6
br label %7
26: ; preds = %7
%27 = load double, double* %5
ret double %27
}

LLVM IR (-O2):
define double @v_dot_p6(double* nocapture %0, double* nocapture %1) {
%3 = load double, double* %0
%4 = load double, double* %1
%5 = tail call double @llvm.fmuladd.f64(double %3, double %4, double 0.000000e+00)
%6 = getelementptr inbounds double, double* %0, i64 1
%7 = load double, double* %6
%8 = getelementptr inbounds double, double* %1, i64 1
%9 = load double, double* %8
%10 = tail call double @llvm.fmuladd.f64(double %7, double %9, double %5)
%11 = getelementptr inbounds double, double* %0, i64 2
%12 = load double, double* %11
%13 = getelementptr inbounds double, double* %1, i64 2
%14 = load double, double* %13
%15 = tail call double @llvm.fmuladd.f64(double %12, double %14, double %10)
%16 = getelementptr inbounds double, double* %0, i64 3
%17 = load double, double* %16
%18 = getelementptr inbounds double, double* %1, i64 3
%19 = load double, double* %18
%20 = tail call double @llvm.fmuladd.f64(double %17, double %19, double %15)
%21 = getelementptr inbounds double, double* %0, i64 4
%22 = load double, double* %21
%23 = getelementptr inbounds double, double* %1, i64 4
%24 = load double, double* %23
%25 = tail call double @llvm.fmuladd.f64(double %22, double %24, double %20)
%26 = getelementptr inbounds double, double* %0, i64 5
%27 = load double, double* %26
%28 = getelementptr inbounds double, double* %1, i64 5
%29 = load double, double* %28
%30 = tail call double @llvm.fmuladd.f64(double %27, double %29, double %25)
ret double %30
}