Understanding __PAIR__ macro from IDA PRO Pseudo Decompiler to look better

Question

When I decompile using IDA PRO I get code which could look like both of these

Understanding the __PAIR__ macro.

Macro itself.

// The following definition is not quite correct because it always returns
// uint64. The above C++ functions are good, though.
#define __PAIR__(high, low) (((unsigned long)(high)<<sizeof(high)*8) | low)

if ( (__PAIR__(a4, a3) & 0x8000000000000000ui64) == 0i64 ) {
  //condition code.
}

I also have code that looks like this

  if ( (__PAIR__(v17, v12) & 0x8000000000000000ui64) != 0i64 ) {
    //condition code.
  }

I actually found some guy who helped me repair this bad looking code and as far I could tell by the pattern they could simply be replaced like so.

   if( (__PAIR__(a4, a3) & 0x8000000000000000ui64) == 0i64 )

becomes

   if(a4 >= 0 && a3 >= 0)

Now I found a very similar pattern that looks like above but instead of == 0i64 it has != 0i64

So I figured if my pattern is correct then the following code below

  if ( (__PAIR__(v17, v12) & 0x8000000000000000ui64) != 0i64 ) {

should look like this

  if( v17 < 0 || v12 < 0 ) {

Then again that's just a guess I wonder if that's correct.

I can't post the assembly for the != 0i64 example as I couldn't find it the assembly code is very big.. but the one with == 0i64 looks like this

.text:004049F0                 mov     edx, [esp+a3]
.text:004049F4                 test    edx, edx
.text:004049F6                 jl      short locret_404A3E
.text:004049F8                 mov     eax, [esp+a4]
.text:004049FC                 test    eax, eax
.text:004049FE                 jl      short locret_404A3E

Answer 1

 if( (__PAIR__(a4, a3) & 0x8000000000000000ui64) == 0i64 )

becomes

 if(a4 >= 0 && a3 >= 0)

No, that's not correct. The correct simplification is:

if ((int)a4 >= 0)

 if ( (__PAIR__(v17, v12) & 0x8000000000000000ui64) != 0i64 ) {

should look like this

 if( v17 < 0 || v12 < 0 ) {

Nope, it should look like this:

if ((int)v17 < 0) {

You may want to read up on how the most significant bit (MSB) is used as the sign bit in two's complement notation.