void antisym(double *u, double *A) {
  A[0] = A[4] = A[8] = 0;
  A[1] = -u[2];
  A[2] = u[1];
  A[3] = u[2];
  A[5] = -u[0];
  A[6] = -u[1];
  A[7] = u[0];
}


x86 assembly (clang, -O0):
<antisym>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    %rsi,-0x10(%rbp)
mov    -0x10(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x40(%rax)
mov    -0x10(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,0x20(%rax)
mov    -0x10(%rbp),%rax
xorps  %xmm0,%xmm0
movsd  %xmm0,(%rax)
mov    -0x8(%rbp),%rax
movsd  0x10(%rax),%xmm0
movq   %xmm0,%rax
movabs $0x8000000000000000,%rcx
xor    %rcx,%rax
movq   %rax,%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x8(%rax)
mov    -0x8(%rbp),%rax
movsd  0x8(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x10(%rax)
mov    -0x8(%rbp),%rax
movsd  0x10(%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x18(%rax)
mov    -0x8(%rbp),%rax
movsd  (%rax),%xmm0
movq   %xmm0,%rax
movabs $0x8000000000000000,%rcx
xor    %rcx,%rax
movq   %rax,%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x28(%rax)
mov    -0x8(%rbp),%rax
movsd  0x8(%rax),%xmm0
movq   %xmm0,%rax
movabs $0x8000000000000000,%rcx
xor    %rcx,%rax
movq   %rax,%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x30(%rax)
mov    -0x8(%rbp),%rax
movsd  (%rax),%xmm0
mov    -0x10(%rbp),%rax
movsd  %xmm0,0x38(%rax)
pop    %rbp
ret

x86 assembly (clang, -O2):
<antisym>:
movq   $0x0,0x40(%rsi)
movq   $0x0,0x20(%rsi)
movq   $0x0,(%rsi)
movsd  0x10(%rdi),%xmm0
movaps 0xedd(%rip),%xmm1
xorps  %xmm1,%xmm0
movlps %xmm0,0x8(%rsi)
movsd  0x8(%rdi),%xmm0
movsd  %xmm0,0x10(%rsi)
movsd  0x10(%rdi),%xmm0
movsd  %xmm0,0x18(%rsi)
movsd  (%rdi),%xmm0
xorps  %xmm1,%xmm0
movlps %xmm0,0x28(%rsi)
movsd  0x8(%rdi),%xmm0
xorps  %xmm1,%xmm0
movlps %xmm0,0x30(%rsi)
movsd  (%rdi),%xmm0
movsd  %xmm0,0x38(%rsi)
ret

LLVM IR (-O0):
define void @antisym(double* %0, double* %1) {
%3 = alloca double*
%4 = alloca double*
store double* %0, double** %3
store double* %1, double** %4
%5 = load double*, double** %4
%6 = getelementptr inbounds double, double* %5, i64 8
store double 0.000000e+00, double* %6
%7 = load double*, double** %4
%8 = getelementptr inbounds double, double* %7, i64 4
store double 0.000000e+00, double* %8
%9 = load double*, double** %4
%10 = getelementptr inbounds double, double* %9, i64 0
store double 0.000000e+00, double* %10
%11 = load double*, double** %3
%12 = getelementptr inbounds double, double* %11, i64 2
%13 = load double, double* %12
%14 = fneg double %13
%15 = load double*, double** %4
%16 = getelementptr inbounds double, double* %15, i64 1
store double %14, double* %16
%17 = load double*, double** %3
%18 = getelementptr inbounds double, double* %17, i64 1
%19 = load double, double* %18
%20 = load double*, double** %4
%21 = getelementptr inbounds double, double* %20, i64 2
store double %19, double* %21
%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 3
store double %24, double* %26
%27 = load double*, double** %3
%28 = getelementptr inbounds double, double* %27, i64 0
%29 = load double, double* %28
%30 = fneg double %29
%31 = load double*, double** %4
%32 = getelementptr inbounds double, double* %31, i64 5
store double %30, double* %32
%33 = load double*, double** %3
%34 = getelementptr inbounds double, double* %33, i64 1
%35 = load double, double* %34
%36 = fneg double %35
%37 = load double*, double** %4
%38 = getelementptr inbounds double, double* %37, i64 6
store double %36, double* %38
%39 = load double*, double** %3
%40 = getelementptr inbounds double, double* %39, i64 0
%41 = load double, double* %40
%42 = load double*, double** %4
%43 = getelementptr inbounds double, double* %42, i64 7
store double %41, double* %43
ret void
}

LLVM IR (-O2):
define void @antisym(double* nocapture %0, double* nocapture writeonly %1) {
%3 = getelementptr inbounds double, double* %1, i64 8
store double 0.000000e+00, double* %3
%4 = getelementptr inbounds double, double* %1, i64 4
store double 0.000000e+00, double* %4
store double 0.000000e+00, double* %1
%5 = getelementptr inbounds double, double* %0, i64 2
%6 = load double, double* %5
%7 = fneg double %6
%8 = getelementptr inbounds double, double* %1, i64 1
store double %7, double* %8
%9 = getelementptr inbounds double, double* %0, i64 1
%10 = load double, double* %9
%11 = getelementptr inbounds double, double* %1, i64 2
store double %10, double* %11
%12 = load double, double* %5
%13 = getelementptr inbounds double, double* %1, i64 3
store double %12, double* %13
%14 = load double, double* %0
%15 = fneg double %14
%16 = getelementptr inbounds double, double* %1, i64 5
store double %15, double* %16
%17 = load double, double* %9
%18 = fneg double %17
%19 = getelementptr inbounds double, double* %1, i64 6
store double %18, double* %19
%20 = load double, double* %0
%21 = getelementptr inbounds double, double* %1, i64 7
store double %20, double* %21
ret void
}