[libc-commits] [libc] c0064f7 - [libc][math] Fix missing underflow exception in DyadicFloat::generic_as (#186734)

[via libc-commits]

Author: Anonmiraj
Date: 2026-03-18T10:02:36-04:00
New Revision: c0064f744c01bef79babbf5853288a11cd7a3c9f

URL: https://github.com/llvm/llvm-project/commit/c0064f744c01bef79babbf5853288a11cd7a3c9f
DIFF: https://github.com/llvm/llvm-project/commit/c0064f744c01bef79babbf5853288a11cd7a3c9f.diff

LOG: [libc][math] Fix missing underflow exception in DyadicFloat::generic_as (#186734)

The `generic_as` function in `dyadic_float.h` had a missing `underflow =
true` at the exact boundary where `unbiased_exp == -EXP_BIAS -
FRACTION_LEN`.
At this boundary, the mantissa MSB maps exactly to the round bit, so
out_mantissa is 0 and the result can only be 0 or min_subnormal. The
value is at most min_subnormal / 2, so it is always tiny and always
inexact `underflow` must be signaled. The < case and the general
subnormal range both already set underflow = true this boundary case was
the only gap.

this specifically fix this error in the erfcf16 function 
```
Extracted erfcf16.cpp.o from archive for linking
Running exhaustive check in --rndn mode...
Missing underflow exception for x=0x1.eacp+1 (y=0x1p-24)
```

this fix may also apply to other bfloat16 missing exceptions
(@Sukumarsawant)

part of: #186483 

CC: @lntue

Added: 
    

Modified: 
    libc/src/__support/FPUtil/dyadic_float.h

Removed: 
    


################################################################################
diff  --git a/libc/src/__support/FPUtil/dyadic_float.h b/libc/src/__support/FPUtil/dyadic_float.h
index cc0710fbf7b02..8ce041247716b 100644
--- a/libc/src/__support/FPUtil/dyadic_float.h
+++ b/libc/src/__support/FPUtil/dyadic_float.h
@@ -219,6 +219,9 @@ template <size_t Bits> struct DyadicFloat {
       underflow = true;
     } else if (unbiased_exp == -FPBits::EXP_BIAS - FPBits::FRACTION_LEN) {
       round = true;
+      // underflow is detected pre-rounding FE_UNDERFLOW may be raised
+      // even if rounding produces a non-underflow result
+      underflow = true;
       MantissaType sticky_mask = (MantissaType(1) << (Bits - 1)) - 1;
       sticky = (mantissa & sticky_mask) != 0;
     } else {