void BackwardElimination(double *A, int N) {
  register int i = 0;
  double *e, *tu;
  register double *t0;
  e = &A[4 * N - 6];
  t0 = &A[4 * N - 3];
  tu = &A[4 * (N - 2)];
  *e /= *t0;
  t0 -= 4;
  for (i = 1; i < (N - 1); i++) {
    t0[-3] /= t0[0] - (t0[-1]) * (t0[+1]);
    t0 -= 4;
  }
}


x86 assembly (clang, -O0):
<BackwardElimination>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    %esi,-0xc(%rbp)
movl   $0x0,-0x10(%rbp)
mov    -0x8(%rbp),%rax
mov    -0xc(%rbp),%ecx
shl    $0x2,%ecx
sub    $0x6,%ecx
movslq %ecx,%rcx
shl    $0x3,%rcx
add    %rcx,%rax
mov    %rax,-0x18(%rbp)
mov    -0x8(%rbp),%rax
mov    -0xc(%rbp),%ecx
shl    $0x2,%ecx
sub    $0x3,%ecx
movslq %ecx,%rcx
shl    $0x3,%rcx
add    %rcx,%rax
mov    %rax,-0x28(%rbp)
mov    -0x8(%rbp),%rax
mov    -0xc(%rbp),%ecx
sub    $0x2,%ecx
shl    $0x2,%ecx
movslq %ecx,%rcx
shl    $0x3,%rcx
add    %rcx,%rax
mov    %rax,-0x20(%rbp)
mov    -0x28(%rbp),%rax
movsd  (%rax),%xmm1
mov    -0x18(%rbp),%rax
movsd  (%rax),%xmm0
divsd  %xmm1,%xmm0
movsd  %xmm0,(%rax)
mov    -0x28(%rbp),%rax
add    $0xffffffffffffffe0,%rax
mov    %rax,-0x28(%rbp)
movl   $0x1,-0x10(%rbp)
mov    -0x10(%rbp),%eax
mov    -0xc(%rbp),%ecx
sub    $0x1,%ecx
cmp    %ecx,%eax
jge    1204 <BackwardElimination+0x104>
mov    -0x28(%rbp),%rax
movsd  (%rax),%xmm0
mov    -0x28(%rbp),%rax
movsd  -0x8(%rax),%xmm1
mov    -0x28(%rbp),%rax
movsd  0x8(%rax),%xmm2
movq   %xmm1,%rax
movabs $0x8000000000000000,%rcx
xor    %rcx,%rax
movq   %rax,%xmm1
mulsd  %xmm2,%xmm1
addsd  %xmm0,%xmm1
mov    -0x28(%rbp),%rax
movsd  -0x18(%rax),%xmm0
divsd  %xmm1,%xmm0
movsd  %xmm0,-0x18(%rax)
mov    -0x28(%rbp),%rax
add    $0xffffffffffffffe0,%rax
mov    %rax,-0x28(%rbp)
mov    -0x10(%rbp),%eax
add    $0x1,%eax
mov    %eax,-0x10(%rbp)
jmp    118e <BackwardElimination+0x8e>
pop    %rbp
ret

x86 assembly (clang, -O2):
<BackwardElimination>:
lea    0x0(,%rsi,4),%eax
movslq %eax,%rcx
lea    -0x3(%rcx),%rax
movsd  -0x30(%rdi,%rcx,8),%xmm0
divsd  -0x18(%rdi,%rcx,8),%xmm0
movsd  %xmm0,-0x30(%rdi,%rcx,8)
cmp    $0x3,%esi
jl     11bf <BackwardElimination+0xbf>
jne    113e <BackwardElimination+0x3e>
lea    (%rdi,%rcx,8),%rax
add    $0xffffffffffffffe8,%rax
test   $0x1,%sil
jne    119e <BackwardElimination+0x9e>
jmp    11bf <BackwardElimination+0xbf>
lea    -0x2(%rsi),%ecx
and    $0xfffffffe,%ecx
lea    (%rdi,%rax,8),%rax
add    $0xffffffffffffffa8,%rax
nopl   0x0(%rax)
movsd  0x38(%rax),%xmm0
movsd  0x30(%rax),%xmm1
mulsd  0x40(%rax),%xmm1
subsd  %xmm1,%xmm0
movsd  0x20(%rax),%xmm1
divsd  %xmm0,%xmm1
movsd  %xmm1,0x20(%rax)
movsd  (%rax),%xmm0
movsd  0x18(%rax),%xmm2
mulsd  0x10(%rax),%xmm1
subsd  %xmm1,%xmm2
divsd  %xmm2,%xmm0
movsd  %xmm0,(%rax)
add    $0xffffffffffffffc0,%rax
add    $0xfffffffe,%ecx
jne    1150 <BackwardElimination+0x50>
add    $0x58,%rax
test   $0x1,%sil
je     11bf <BackwardElimination+0xbf>
movsd  -0x20(%rax),%xmm0
movsd  -0x28(%rax),%xmm1
mulsd  -0x18(%rax),%xmm1
movsd  -0x38(%rax),%xmm2
subsd  %xmm1,%xmm0
divsd  %xmm0,%xmm2
movsd  %xmm2,-0x38(%rax)
ret

LLVM IR (-O0):
define void @BackwardElimination(double* %0, i32 %1) {
%3 = alloca double*
%4 = alloca i32
%5 = alloca i32
%6 = alloca double*
%7 = alloca double*
%8 = alloca double*
store double* %0, double** %3
store i32 %1, i32* %4
store i32 0, i32* %5
%9 = load double*, double** %3
%10 = load i32, i32* %4
%11 = mul nsw i32 4, %10
%12 = sub nsw i32 %11, 6
%13 = sext i32 %12 to i64
%14 = getelementptr inbounds double, double* %9, i64 %13
store double* %14, double** %6
%15 = load double*, double** %3
%16 = load i32, i32* %4
%17 = mul nsw i32 4, %16
%18 = sub nsw i32 %17, 3
%19 = sext i32 %18 to i64
%20 = getelementptr inbounds double, double* %15, i64 %19
store double* %20, double** %8
%21 = load double*, double** %3
%22 = load i32, i32* %4
%23 = sub nsw i32 %22, 2
%24 = mul nsw i32 4, %23
%25 = sext i32 %24 to i64
%26 = getelementptr inbounds double, double* %21, i64 %25
store double* %26, double** %7
%27 = load double*, double** %8
%28 = load double, double* %27
%29 = load double*, double** %6
%30 = load double, double* %29
%31 = fdiv double %30, %28
store double %31, double* %29
%32 = load double*, double** %8
%33 = getelementptr inbounds double, double* %32, i64 -4
store double* %33, double** %8
store i32 1, i32* %5
br label %34
34: ; preds = %57, %2
%35 = load i32, i32* %5
%36 = load i32, i32* %4
%37 = sub nsw i32 %36, 1
%38 = icmp slt i32 %35, %37
br i1 %38, label %39, label %60
39: ; preds = %34
%40 = load double*, double** %8
%41 = getelementptr inbounds double, double* %40, i64 0
%42 = load double, double* %41
%43 = load double*, double** %8
%44 = getelementptr inbounds double, double* %43, i64 -1
%45 = load double, double* %44
%46 = load double*, double** %8
%47 = getelementptr inbounds double, double* %46, i64 1
%48 = load double, double* %47
%49 = fneg double %45
%50 = call double @llvm.fmuladd.f64(double %49, double %48, double %42)
%51 = load double*, double** %8
%52 = getelementptr inbounds double, double* %51, i64 -3
%53 = load double, double* %52
%54 = fdiv double %53, %50
store double %54, double* %52
%55 = load double*, double** %8
%56 = getelementptr inbounds double, double* %55, i64 -4
store double* %56, double** %8
br label %57
57: ; preds = %39
%58 = load i32, i32* %5
%59 = add nsw i32 %58, 1
store i32 %59, i32* %5
br label %34
60: ; preds = %34
ret void
}

LLVM IR (-O2):
define void @BackwardElimination(double* nocapture %0, i32 %1) {
%3 = shl nsw i32 %1, 2
%4 = add nsw i32 %3, -6
%5 = sext i32 %4 to i64
%6 = getelementptr inbounds double, double* %0, i64 %5
%7 = add nsw i32 %3, -3
%8 = sext i32 %7 to i64
%9 = getelementptr inbounds double, double* %0, i64 %8
%10 = load double, double* %9
%11 = load double, double* %6
%12 = fdiv double %11, %10
store double %12, double* %6
%13 = icmp sgt i32 %1, 2
br i1 %13, label %14, label %60
14: ; preds = %2
%15 = and i32 %1, 1
%16 = icmp eq i32 %1, 3
br i1 %16, label %45, label %17
17: ; preds = %14
%18 = add i32 %1, -2
%19 = and i32 %18, -2
br label %20
20: ; preds = %20, %17
%21 = phi double* [ %9, %17 ], [ %34, %20 ]
%22 = phi i32 [ 0, %17 ], [ %43, %20 ]
%23 = getelementptr inbounds double, double* %21, i64 -4
%24 = load double, double* %23
%25 = getelementptr inbounds double, double* %21, i64 -5
%26 = load double, double* %25
%27 = getelementptr inbounds double, double* %21, i64 -3
%28 = load double, double* %27
%29 = fneg double %26
%30 = tail call double @llvm.fmuladd.f64(double %29, double %28, double %24)
%31 = getelementptr inbounds double, double* %21, i64 -7
%32 = load double, double* %31
%33 = fdiv double %32, %30
store double %33, double* %31
%34 = getelementptr inbounds double, double* %21, i64 -8
%35 = load double, double* %34
%36 = getelementptr inbounds double, double* %21, i64 -9
%37 = load double, double* %36
%38 = fneg double %37
%39 = tail call double @llvm.fmuladd.f64(double %38, double %33, double %35)
%40 = getelementptr inbounds double, double* %21, i64 -11
%41 = load double, double* %40
%42 = fdiv double %41, %39
store double %42, double* %40
%43 = add i32 %22, 2
%44 = icmp eq i32 %43, %19
br i1 %44, label %45, label %20
45: ; preds = %20, %14
%46 = phi double* [ %9, %14 ], [ %34, %20 ]
%47 = icmp eq i32 %15, 0
br i1 %47, label %60, label %48
48: ; preds = %45
%49 = getelementptr inbounds double, double* %46, i64 -4
%50 = load double, double* %49
%51 = getelementptr inbounds double, double* %46, i64 -5
%52 = load double, double* %51
%53 = getelementptr inbounds double, double* %46, i64 -3
%54 = load double, double* %53
%55 = fneg double %52
%56 = tail call double @llvm.fmuladd.f64(double %55, double %54, double %50)
%57 = getelementptr inbounds double, double* %46, i64 -7
%58 = load double, double* %57
%59 = fdiv double %58, %56
store double %59, double* %57
br label %60
60: ; preds = %48, %45, %2
ret void
}