double dot(double *a, double *b) {
  double ss = 0;
  ss += a[0] * b[0];
  ss += a[1] * b[1];
  ss += a[2] * b[2];
  return ss;
}


x86 assembly (clang, -O0):
<dot>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    %rsi,-0x10(%rbp)
xorps  %xmm0,%xmm0
movsd  %xmm0,-0x18(%rbp)
mov    -0x8(%rbp),%rax
movsd  (%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  (%rax),%xmm2
movsd  -0x18(%rbp),%xmm1
mulsd  %xmm2,%xmm0
addsd  %xmm1,%xmm0
movsd  %xmm0,-0x18(%rbp)
mov    -0x8(%rbp),%rax
movsd  0x8(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  0x8(%rax),%xmm2
movsd  -0x18(%rbp),%xmm1
mulsd  %xmm2,%xmm0
addsd  %xmm1,%xmm0
movsd  %xmm0,-0x18(%rbp)
mov    -0x8(%rbp),%rax
movsd  0x10(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  0x10(%rax),%xmm2
movsd  -0x18(%rbp),%xmm1
mulsd  %xmm2,%xmm0
addsd  %xmm1,%xmm0
movsd  %xmm0,-0x18(%rbp)
movsd  -0x18(%rbp),%xmm0
pop    %rbp
ret

x86 assembly (clang, -O2):
<dot>:
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
ret

LLVM IR (-O0):
define double @dot(double* %0, double* %1) {
%3 = alloca double*
%4 = alloca double*
%5 = alloca double
store double* %0, double** %3
store double* %1, double** %4
store double 0.000000e+00, double* %5
%6 = load double*, double** %3
%7 = getelementptr inbounds double, double* %6, i64 0
%8 = load double, double* %7
%9 = load double*, double** %4
%10 = getelementptr inbounds double, double* %9, i64 0
%11 = load double, double* %10
%12 = load double, double* %5
%13 = call double @llvm.fmuladd.f64(double %8, double %11, double %12)
store double %13, double* %5
%14 = load double*, double** %3
%15 = getelementptr inbounds double, double* %14, i64 1
%16 = load double, double* %15
%17 = load double*, double** %4
%18 = getelementptr inbounds double, double* %17, i64 1
%19 = load double, double* %18
%20 = load double, double* %5
%21 = call double @llvm.fmuladd.f64(double %16, double %19, double %20)
store double %21, double* %5
%22 = load double*, double** %3
%23 = getelementptr inbounds double, double* %22, i64 2
%24 = load double, double* %23
%25 = load double*, double** %4
%26 = getelementptr inbounds double, double* %25, i64 2
%27 = load double, double* %26
%28 = load double, double* %5
%29 = call double @llvm.fmuladd.f64(double %24, double %27, double %28)
store double %29, double* %5
%30 = load double, double* %5
ret double %30
}

LLVM IR (-O2):
define double @dot(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)
ret double %15
}