[llvm] [KnownFPClass] Add output/input denormal helpers (PR #225575)
Eric Ross via llvm-commits
llvm-commits at lists.llvm.org
Tue Sep 22 19:07:20 PDT 2026
https://github.com/ZERICO2005 created https://github.com/llvm/llvm-project/pull/225575
Added `applyInputDenormalMode(KnownSrc, Mode)` and `applyOutputDenormalMode(KnownSrc, Mode)`. These helpers allow us to correctly handle the output/input denormal mode correctly every single time, while also being much easier to use.
Example usage:
```c++
KnownFPClass func(const KnownFPClass& KnownSrc_, DenormalMode Mode) {
KnownFPClass KnownSrc = applyInputDenormalMode(KnownSrc_, Mode);
KnownFPClass Known;
// We can treat everything here as if it were IEEE.
return applyOutputDenormalMode(Known, Mode);
}
```
>From 5456311191c076c15d79b259ed829e63657a9da5 Mon Sep 17 00:00:00 2001
From: zerico <zerico2005 at gmail.com>
Date: Tue, 22 Sep 2026 19:53:28 -0600
Subject: [PATCH] [KnownFPClass] Add output/input denormal helpers
---
llvm/include/llvm/Support/KnownFPClass.h | 10 +++
llvm/lib/Support/KnownFPClass.cpp | 110 +++++++++++++----------
2 files changed, 71 insertions(+), 49 deletions(-)
diff --git a/llvm/include/llvm/Support/KnownFPClass.h b/llvm/include/llvm/Support/KnownFPClass.h
index 22097a70a361be..33a115b679ad0d 100644
--- a/llvm/include/llvm/Support/KnownFPClass.h
+++ b/llvm/include/llvm/Support/KnownFPClass.h
@@ -160,6 +160,16 @@ struct KnownFPClass {
return isKnownNever(fcPositive) && isKnownNeverLogicalNegZero(Mode);
}
+ /// Return the known classes of \p Src after passing through an input that
+ /// may flush subnormals to zero according to \p Mode.
+ LLVM_ABI static KnownFPClass applyInputDenormalMode(const KnownFPClass &Src,
+ DenormalMode Mode);
+
+ /// Return the known classes of \p Src after producing an output that may
+ /// flush subnormals to zero according to \p Mode.
+ LLVM_ABI static KnownFPClass applyOutputDenormalMode(const KnownFPClass &Src,
+ DenormalMode Mode);
+
KnownFPClass intersectWith(const KnownFPClass &RHS) const {
return KnownFPClass(getKnownFPClasses() | RHS.getKnownFPClasses(),
getSignBit() == RHS.getSignBit() ? getSignBit()
diff --git a/llvm/lib/Support/KnownFPClass.cpp b/llvm/lib/Support/KnownFPClass.cpp
index 48867ce3a6a6fb..a34fa8d6bc7f2e 100644
--- a/llvm/lib/Support/KnownFPClass.cpp
+++ b/llvm/lib/Support/KnownFPClass.cpp
@@ -23,48 +23,70 @@ KnownFPClass::KnownFPClass(const APFloat &C)
setSignBit(C.isNegative());
}
-/// Return true if it's possible to assume IEEE treatment of input denormals in
-/// \p F for \p Val.
-static bool inputDenormalIsIEEE(DenormalMode Mode) {
- return Mode.Input == DenormalMode::IEEE;
+KnownFPClass KnownFPClass::applyInputDenormalMode(const KnownFPClass &KnownSrc,
+ DenormalMode Mode) {
+ KnownFPClass Known = KnownSrc;
+ switch (Mode.Input) {
+ case DenormalMode::IEEE:
+ return Known;
+ case DenormalMode::PreserveSign:
+ if (KnownSrc.getKnownFPClasses() & fcPosSubnormal)
+ Known.setKnownFPClasses(Known.getKnownFPClasses() | fcPosZero);
+ if (KnownSrc.getKnownFPClasses() & fcNegSubnormal)
+ Known.setKnownFPClasses(Known.getKnownFPClasses() | fcNegZero);
+ return Known;
+ case DenormalMode::PositiveZero:
+ if (KnownSrc.getKnownFPClasses() & fcSubnormal)
+ Known.setKnownFPClasses(Known.getKnownFPClasses() | fcPosZero);
+ return Known;
+ default:
+ if (KnownSrc.getKnownFPClasses() & fcSubnormal)
+ Known.setKnownFPClasses(Known.getKnownFPClasses() | fcPosZero);
+ if (KnownSrc.getKnownFPClasses() & fcNegSubnormal)
+ Known.setKnownFPClasses(Known.getKnownFPClasses() | fcNegZero);
+ return Known;
+ }
+
+ llvm_unreachable("covered switch over denormal mode");
}
-static bool inputDenormalIsIEEEOrPosZero(DenormalMode Mode) {
- return Mode.Input == DenormalMode::IEEE ||
- Mode.Input == DenormalMode::PositiveZero;
+KnownFPClass KnownFPClass::applyOutputDenormalMode(const KnownFPClass &KnownSrc,
+ DenormalMode Mode) {
+ KnownFPClass Known = KnownSrc;
+ switch (Mode.Output) {
+ case DenormalMode::IEEE:
+ return Known;
+ case DenormalMode::PreserveSign:
+ if (KnownSrc.getKnownFPClasses() & fcPosSubnormal)
+ Known.setKnownFPClasses(Known.getKnownFPClasses() | fcPosZero);
+ if (KnownSrc.getKnownFPClasses() & fcNegSubnormal)
+ Known.setKnownFPClasses(Known.getKnownFPClasses() | fcNegZero);
+ return Known;
+ case DenormalMode::PositiveZero:
+ if (KnownSrc.getKnownFPClasses() & fcSubnormal)
+ Known.setKnownFPClasses(Known.getKnownFPClasses() | fcPosZero);
+ return Known;
+ default:
+ if (KnownSrc.getKnownFPClasses() & fcSubnormal)
+ Known.setKnownFPClasses(Known.getKnownFPClasses() | fcPosZero);
+ if (KnownSrc.getKnownFPClasses() & fcNegSubnormal)
+ Known.setKnownFPClasses(Known.getKnownFPClasses() | fcNegZero);
+ return Known;
+ }
+
+ llvm_unreachable("covered switch over denormal mode");
}
bool KnownFPClass::isKnownNeverLogicalZero(DenormalMode Mode) const {
- return isKnownNeverZero() &&
- (isKnownNeverSubnormal() || inputDenormalIsIEEE(Mode));
+ return applyInputDenormalMode(*this, Mode).isKnownNeverZero();
}
bool KnownFPClass::isKnownNeverLogicalNegZero(DenormalMode Mode) const {
- return isKnownNeverNegZero() &&
- (isKnownNeverNegSubnormal() || inputDenormalIsIEEEOrPosZero(Mode));
+ return applyInputDenormalMode(*this, Mode).isKnownNeverNegZero();
}
bool KnownFPClass::isKnownNeverLogicalPosZero(DenormalMode Mode) const {
- if (!isKnownNeverPosZero())
- return false;
-
- // If we know there are no denormals, nothing can be flushed to zero.
- if (isKnownNeverSubnormal())
- return true;
-
- switch (Mode.Input) {
- case DenormalMode::IEEE:
- return true;
- case DenormalMode::PreserveSign:
- // Negative subnormal won't flush to +0
- return isKnownNeverPosSubnormal();
- case DenormalMode::PositiveZero:
- default:
- // Both positive and negative subnormal could flush to +0
- return false;
- }
-
- llvm_unreachable("covered switch over denormal mode");
+ return applyInputDenormalMode(*this, Mode).isKnownNeverPosZero();
}
void KnownFPClass::propagateDenormal(const KnownFPClass &Src,
@@ -831,10 +853,12 @@ KnownFPClass KnownFPClass::atan(const KnownFPClass &KnownSrc) {
return Known;
}
-KnownFPClass KnownFPClass::atan2(const KnownFPClass &KnownY,
- const KnownFPClass &KnownX,
+KnownFPClass KnownFPClass::atan2(const KnownFPClass &KnownY_,
+ const KnownFPClass &KnownX_,
DenormalMode Mode) {
KnownFPClass Known;
+ KnownFPClass KnownY = applyInputDenormalMode(KnownY_, Mode);
+ KnownFPClass KnownX = applyInputDenormalMode(KnownX_, Mode);
// Even though these deductions are correct, we are ignoring the following
// potentially erroneous cases:
@@ -846,14 +870,8 @@ KnownFPClass KnownFPClass::atan2(const KnownFPClass &KnownY,
Known.propagateNonNaN(KnownY, KnownX);
- // Negative subnormals could be treated like positive zero.
- const bool XCannotHavePositiveInput = KnownX.isKnownNever(fcPositive) &&
- KnownX.isKnownNeverLogicalPosZero(Mode);
- const bool YCannotHavePositiveInput = KnownY.isKnownNever(fcPositive) &&
- KnownY.isKnownNeverLogicalPosZero(Mode);
-
// If x <= -0.0, then |atan2(y, x)| >= pi/2
- if (XCannotHavePositiveInput)
+ if (KnownX.isKnownNever(fcPositive))
Known.knownNot(fcZero | fcSubnormal);
// If y >= +0.0, then atan2(y, x) >= +0.0
@@ -861,16 +879,10 @@ KnownFPClass KnownFPClass::atan2(const KnownFPClass &KnownY,
Known.knownNot(fcNegative);
// If y <= -0.0, then atan2(y, x) <= -0.0
- // We do this deduction last in case we were able to rule out a negative
- // subnormal result earlier.
- if (YCannotHavePositiveInput) {
- Known.knownNot(fcPosSubnormal | fcPosNormal | fcPosInf);
- // Negative subnormal results can flush to +0.0.
- if (Known.isKnownNever(fcNegSubnormal) || !Mode.outputsMayBePositiveZero())
- Known.knownNot(fcPosZero);
- }
+ if (KnownY.isKnownNever(fcPositive))
+ Known.knownNot(fcPositive);
- return Known;
+ return applyOutputDenormalMode(Known, Mode);
}
KnownFPClass KnownFPClass::fpext(const KnownFPClass &KnownSrc,
More information about the llvm-commits
mailing list