[all-commits] [llvm/llvm-project] 982527: [flang] Use saturated intrinsics for floating poin...

[Asher Mancinelli via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 982527eef0f284161dd5e09a9b4fc952b332ec92
      https://github.com/llvm/llvm-project/commit/982527eef0f284161dd5e09a9b4fc952b332ec92
  Author: Asher Mancinelli <ashermancinelli at gmail.com>
  Date:   2025-03-12 (Wed, 12 Mar 2025)

  Changed paths:
    M flang/lib/Optimizer/CodeGen/CodeGen.cpp
    M flang/test/Fir/convert-to-llvm.fir

  Log Message:
  -----------
  [flang] Use saturated intrinsics for floating point to integer conversions (#130686)

The saturated floating point conversion intrinsics match the semantics in the standard more closely than the fptosi/fptoui instructions.

Case 2 of 16.9.100 is

> INT (A [, KIND])
> If A is of type real, there are two cases: if |A| < 1, INT (A) has the
value 0; if |A| ≥ 1, INT (A) is the integer whose magnitude is the
largest integer that does not exceed the magnitude of A and whose sign
is the same as the sign of A.

Currently, converting a floating point value into an integer type too
small to hold the constant will be converted to poison in opt, leaving
us with garbage:

```
> cat t.f90
program main
  real(kind=16)   :: f
  integer(kind=4) :: i
  f=huge(f)
  i=f
  print *, i
end program main

# current upstream
> for i in `seq 10`; do; ./a.out; done
 -862156992
 -1497393344
 -739096768
 -1649494208
 1761228608
 -1959270592
 -746244288
 -1629194432
 -231217344
 382322496
```

With the saturated fptoui/fptosi intrinsics, we get the appropriate
values

```
# mine
> flang -O2 ./t.f90 && ./a.out
 2147483647

> perl -e 'printf "%d\n", (2 ** 31) - 1'
2147483647
```

One notable difference: NaNs being converted to ints will become zero, unlike current flang (and some other compilers). Newer versions of GCC have this behavior.



To unsubscribe from these emails, change your notification settings at https://github.com/llvm/llvm-project/settings/notifications