double orient2dfast(double *pa, double *pb, double *pc) {
  double acx, bcx, acy, bcy;
  acx = pa[0] - pc[0];
  bcx = pb[0] - pc[0];
  acy = pa[1] - pc[1];
  bcy = pb[1] - pc[1];
  return acx * bcy - acy * bcx;
}


x86 assembly (clang, -O0):
<orient2dfast>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    %rsi,-0x10(%rbp)
mov    %rdx,-0x18(%rbp)
mov    -0x8(%rbp),%rax
movsd  (%rax),%xmm0
mov    -0x18(%rbp),%rax
subsd  (%rax),%xmm0
movsd  %xmm0,-0x20(%rbp)
mov    -0x10(%rbp),%rax
movsd  (%rax),%xmm0
mov    -0x18(%rbp),%rax
subsd  (%rax),%xmm0
movsd  %xmm0,-0x28(%rbp)
mov    -0x8(%rbp),%rax
movsd  0x8(%rax),%xmm0
mov    -0x18(%rbp),%rax
subsd  0x8(%rax),%xmm0
movsd  %xmm0,-0x30(%rbp)
mov    -0x10(%rbp),%rax
movsd  0x8(%rax),%xmm0
mov    -0x18(%rbp),%rax
subsd  0x8(%rax),%xmm0
movsd  %xmm0,-0x38(%rbp)
movsd  -0x20(%rbp),%xmm0
movsd  -0x38(%rbp),%xmm2
movsd  -0x30(%rbp),%xmm1
mulsd  -0x28(%rbp),%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):
<orient2dfast>:
movsd  (%rdi),%xmm1
movsd  0x8(%rdi),%xmm2
movsd  (%rdx),%xmm3
movsd  0x8(%rdx),%xmm4
subsd  %xmm3,%xmm1
movsd  (%rsi),%xmm5
movsd  0x8(%rsi),%xmm0
subsd  %xmm3,%xmm5
subsd  %xmm4,%xmm2
mulsd  %xmm5,%xmm2
subsd  %xmm4,%xmm0
mulsd  %xmm1,%xmm0
subsd  %xmm2,%xmm0
ret

LLVM IR (-O0):
define double @orient2dfast(double* %0, double* %1, double* %2) {
%4 = alloca double*
%5 = alloca double*
%6 = alloca double*
%7 = alloca double
%8 = alloca double
%9 = alloca double
%10 = alloca double
store double* %0, double** %4
store double* %1, double** %5
store double* %2, double** %6
%11 = load double*, double** %4
%12 = getelementptr inbounds double, double* %11, i64 0
%13 = load double, double* %12
%14 = load double*, double** %6
%15 = getelementptr inbounds double, double* %14, i64 0
%16 = load double, double* %15
%17 = fsub double %13, %16
store double %17, double* %7
%18 = load double*, double** %5
%19 = getelementptr inbounds double, double* %18, i64 0
%20 = load double, double* %19
%21 = load double*, double** %6
%22 = getelementptr inbounds double, double* %21, i64 0
%23 = load double, double* %22
%24 = fsub double %20, %23
store double %24, double* %8
%25 = load double*, double** %4
%26 = getelementptr inbounds double, double* %25, i64 1
%27 = load double, double* %26
%28 = load double*, double** %6
%29 = getelementptr inbounds double, double* %28, i64 1
%30 = load double, double* %29
%31 = fsub double %27, %30
store double %31, double* %9
%32 = load double*, double** %5
%33 = getelementptr inbounds double, double* %32, i64 1
%34 = load double, double* %33
%35 = load double*, double** %6
%36 = getelementptr inbounds double, double* %35, i64 1
%37 = load double, double* %36
%38 = fsub double %34, %37
store double %38, double* %10
%39 = load double, double* %7
%40 = load double, double* %10
%41 = load double, double* %9
%42 = load double, double* %8
%43 = fmul double %41, %42
%44 = fneg double %43
%45 = call double @llvm.fmuladd.f64(double %39, double %40, double %44)
ret double %45
}

LLVM IR (-O2):
define double @orient2dfast(double* nocapture %0, double* nocapture %1, double* nocapture %2) {
%4 = load double, double* %0
%5 = load double, double* %2
%6 = fsub double %4, %5
%7 = load double, double* %1
%8 = fsub double %7, %5
%9 = getelementptr inbounds double, double* %0, i64 1
%10 = load double, double* %9
%11 = getelementptr inbounds double, double* %2, i64 1
%12 = load double, double* %11
%13 = fsub double %10, %12
%14 = getelementptr inbounds double, double* %1, i64 1
%15 = load double, double* %14
%16 = fsub double %15, %12
%17 = fneg double %13
%18 = fmul double %8, %17
%19 = tail call double @llvm.fmuladd.f64(double %6, double %16, double %18)
ret double %19
}