[llvm-dev] Floating point ordered and unordered comparisons

[Wang, Pengfei via llvm-dev]

Hi Lijia,

IEEE 754 standard has a strict definition on comparison predicates. In short, symbol “<=” is not the flip of “>”. There’s no such symbol in C language currently (Maybe in draft, I’m not sure), but LLVM IR does support all predicates. So the opposition of predicates ogt (“>” is C) is ule and the opposition of predicates ole (“<=” is C) is ugt.
That’s why you get opposite results from cmp1 and cmp2. The equivalent to cmp1 in IR is:
define zeroext i1 @_Z4cmp2f(float %a) {
entry:
  %cmp = fcmp ule float %a, 0.000000e+00
  %not = xor i1 %cmp, -1
  ret i1 %not
}

Thanks
Pengfei

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of ?? via llvm-dev
Sent: Monday, January 4, 2021 8:39 PM
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] Floating point ordered and unordered comparisons

Hi All,

I have some questions about Floating point ordered and unordered comparisons,
I did some searching and found link below
http://lists.llvm.org/pipermail/llvm-dev/2013-July/063571.html

But I still don't quite understand Floating point ordered and unordered
comparisons. For example, the following code
```
bool cmp1(float a) {
  if (a > 0.0)
   return true;
  else
   return false;
}

bool cmp2(float a) {
  if (!(a <= 0.0))
   return true;
  else
   return false;
}
```
The generated ir code is similar to the following:
```
define zeroext i1 @_Z4cmp1f(float %a) {
entry:
  %cmp = fcmp ogt float %a, 0.000000e+00
  ret i1 %cmp
}

define zeroext i1 @_Z4cmp2f(float %a) {
entry:
  %cmp = fcmp ugt float %a, 0.000000e+00
  ret i1 %cmp
}
```
If the parameter passed is NaN in this example, the result of cmp1 is false and
the result of cmp2 is true. In this case, the result is completely opposite.
When will we get 'fcmp ugt' and when will we get 'fcmp ugt' ? When comparing
ordered and unorder can be converted ? Such as converting an ordered less than
"setolt" into unordered greater than "setuge" operation.

Thanks in advance,
Lijia He
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