double vectmult(double *xy1, double *xy2, double *xy3) {
  return (xy2[0] - xy1[0]) * (xy3[1] - xy1[1]) -
         (xy3[0] - xy1[0]) * (xy2[1] - xy1[1]);
}


x86 assembly (clang, -O0):
<vectmult>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    %rsi,-0x10(%rbp)
mov    %rdx,-0x18(%rbp)
mov    -0x10(%rbp),%rax
movsd  (%rax),%xmm0
mov    -0x8(%rbp),%rax
subsd  (%rax),%xmm0
mov    -0x18(%rbp),%rax
movsd  0x8(%rax),%xmm2
mov    -0x8(%rbp),%rax
subsd  0x8(%rax),%xmm2
mov    -0x18(%rbp),%rax
movsd  (%rax),%xmm1
mov    -0x8(%rbp),%rax
subsd  (%rax),%xmm1
mov    -0x10(%rbp),%rax
movsd  0x8(%rax),%xmm3
mov    -0x8(%rbp),%rax
subsd  0x8(%rax),%xmm3
mulsd  %xmm3,%xmm1
movq   %xmm1,%rax
movabs $0x8000000000000000,%rcx
xor    %rcx,%rax
movq   %rax,%xmm1
mulsd  %xmm2,%xmm0
addsd  %xmm1,%xmm0
pop    %rbp
ret

x86 assembly (clang, -O2):
<vectmult>:
movsd  (%rdi),%xmm1
movsd  0x8(%rdi),%xmm2
movsd  (%rsi),%xmm3
movsd  0x8(%rsi),%xmm4
subsd  %xmm1,%xmm3
movsd  (%rdx),%xmm5
movsd  0x8(%rdx),%xmm0
subsd  %xmm2,%xmm0
mulsd  %xmm3,%xmm0
subsd  %xmm1,%xmm5
subsd  %xmm2,%xmm4
mulsd  %xmm5,%xmm4
subsd  %xmm4,%xmm0
ret

LLVM IR (-O0):
define double @vectmult(double* %0, double* %1, double* %2) {
%4 = alloca double*
%5 = alloca double*
%6 = alloca double*
store double* %0, double** %4
store double* %1, double** %5
store double* %2, double** %6
%7 = load double*, double** %5
%8 = getelementptr inbounds double, double* %7, i64 0
%9 = load double, double* %8
%10 = load double*, double** %4
%11 = getelementptr inbounds double, double* %10, i64 0
%12 = load double, double* %11
%13 = fsub double %9, %12
%14 = load double*, double** %6
%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 = fsub double %16, %19
%21 = load double*, double** %6
%22 = getelementptr inbounds double, double* %21, i64 0
%23 = load double, double* %22
%24 = load double*, double** %4
%25 = getelementptr inbounds double, double* %24, i64 0
%26 = load double, double* %25
%27 = fsub double %23, %26
%28 = load double*, double** %5
%29 = getelementptr inbounds double, double* %28, i64 1
%30 = load double, double* %29
%31 = load double*, double** %4
%32 = getelementptr inbounds double, double* %31, i64 1
%33 = load double, double* %32
%34 = fsub double %30, %33
%35 = fmul double %27, %34
%36 = fneg double %35
%37 = call double @llvm.fmuladd.f64(double %13, double %20, double %36)
ret double %37
}

LLVM IR (-O2):
define double @vectmult(double* nocapture %0, double* nocapture %1, double* nocapture %2) {
%4 = load double, double* %1
%5 = load double, double* %0
%6 = fsub double %4, %5
%7 = getelementptr inbounds double, double* %2, i64 1
%8 = load double, double* %7
%9 = getelementptr inbounds double, double* %0, i64 1
%10 = load double, double* %9
%11 = fsub double %8, %10
%12 = load double, double* %2
%13 = fsub double %12, %5
%14 = getelementptr inbounds double, double* %1, i64 1
%15 = load double, double* %14
%16 = fsub double %15, %10
%17 = fneg double %13
%18 = fmul double %16, %17
%19 = tail call double @llvm.fmuladd.f64(double %6, double %11, double %18)
ret double %19
}