KnownFPClass.cpp

Go to the documentation of this file.

//===- llvm/Support/KnownFPClass.h - Stores known fplcass -------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file contains a class for representing known fpclasses used by
// computeKnownFPClass.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/Support/KnownFPClass.h"
#include "llvm/ADT/APFloat.h"
#include "llvm/Support/ErrorHandling.h"
 
using namespace llvm;
 
KnownFPClass::KnownFPClass(const APFloat &C)
    : KnownFPClasses(C.classify()), SignBit(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;
}
 
static bool inputDenormalIsIEEEOrPosZero(DenormalMode Mode) {
  return Mode.Input == DenormalMode::IEEE ||
         Mode.Input == DenormalMode::PositiveZero;
}
 
bool KnownFPClass::isKnownNeverLogicalZero(DenormalMode Mode) const {
  return isKnownNeverZero() &&
         (isKnownNeverSubnormal() || inputDenormalIsIEEE(Mode));
}
 
bool KnownFPClass::isKnownNeverLogicalNegZero(DenormalMode Mode) const {
  return isKnownNeverNegZero() &&
         (isKnownNeverNegSubnormal() || inputDenormalIsIEEEOrPosZero(Mode));
}
 
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");
}
 
void KnownFPClass::propagateDenormal(const KnownFPClass &Src,
                                     DenormalMode Mode) {
  KnownFPClasses = Src.KnownFPClasses;
  // If we aren't assuming the source can't be a zero, we don't have to check if
  // a denormal input could be flushed.
  if (!Src.isKnownNeverPosZero() && !Src.isKnownNeverNegZero())
    return;
 
  // If we know the input can't be a denormal, it can't be flushed to 0.
  if (Src.isKnownNeverSubnormal())
    return;
 
  if (!Src.isKnownNeverPosSubnormal() && Mode != DenormalMode::getIEEE())
    KnownFPClasses |= fcPosZero;
 
  if (!Src.isKnownNeverNegSubnormal() && Mode != DenormalMode::getIEEE()) {
    if (Mode != DenormalMode::getPositiveZero())
      KnownFPClasses |= fcNegZero;
 
    if (Mode.Input == DenormalMode::PositiveZero ||
        Mode.Output == DenormalMode::PositiveZero ||
        Mode.Input == DenormalMode::Dynamic ||
        Mode.Output == DenormalMode::Dynamic)
      KnownFPClasses |= fcPosZero;
  }
}
 
KnownFPClass KnownFPClass::minMaxLike(const KnownFPClass &LHS_,
                                      const KnownFPClass &RHS_, MinMaxKind Kind,
                                      DenormalMode Mode) {
  KnownFPClass KnownLHS = LHS_;
  KnownFPClass KnownRHS = RHS_;
 
  bool NeverNaN = KnownLHS.isKnownNeverNaN() || KnownRHS.isKnownNeverNaN();
  KnownFPClass Known = KnownLHS | KnownRHS;
 
  // If either operand is not NaN, the result is not NaN.
  if (NeverNaN &&
      (Kind == MinMaxKind::minnum || Kind == MinMaxKind::maxnum ||
       Kind == MinMaxKind::minimumnum || Kind == MinMaxKind::maximumnum))
    Known.knownNot(fcNan);
 
  if (Kind == MinMaxKind::maxnum || Kind == MinMaxKind::maximumnum) {
    // If at least one operand is known to be positive, the result must be
    // positive.
    if ((KnownLHS.cannotBeOrderedLessThanZero() &&
         KnownLHS.isKnownNeverNaN()) ||
        (KnownRHS.cannotBeOrderedLessThanZero() && KnownRHS.isKnownNeverNaN()))
      Known.knownNot(KnownFPClass::OrderedLessThanZeroMask);
  } else if (Kind == MinMaxKind::maximum) {
    // If at least one operand is known to be positive, the result must be
    // positive.
    if (KnownLHS.cannotBeOrderedLessThanZero() ||
        KnownRHS.cannotBeOrderedLessThanZero())
      Known.knownNot(KnownFPClass::OrderedLessThanZeroMask);
  } else if (Kind == MinMaxKind::minnum || Kind == MinMaxKind::minimumnum) {
    // If at least one operand is known to be negative, the result must be
    // negative.
    if ((KnownLHS.cannotBeOrderedGreaterThanZero() &&
         KnownLHS.isKnownNeverNaN()) ||
        (KnownRHS.cannotBeOrderedGreaterThanZero() &&
         KnownRHS.isKnownNeverNaN()))
      Known.knownNot(KnownFPClass::OrderedGreaterThanZeroMask);
  } else if (Kind == MinMaxKind::minimum) {
    // If at least one operand is known to be negative, the result must be
    // negative.
    if (KnownLHS.cannotBeOrderedGreaterThanZero() ||
        KnownRHS.cannotBeOrderedGreaterThanZero())
      Known.knownNot(KnownFPClass::OrderedGreaterThanZeroMask);
  } else
    llvm_unreachable("unhandled intrinsic");
 
  // Fixup zero handling if denormals could be returned as a zero.
  //
  // As there's no spec for denormal flushing, be conservative with the
  // treatment of denormals that could be flushed to zero. For older
  // subtargets on AMDGPU the min/max instructions would not flush the
  // output and return the original value.
  //
  if ((Known.KnownFPClasses & fcZero) != fcNone &&
      !Known.isKnownNeverSubnormal()) {
    if (Mode != DenormalMode::getIEEE())
      Known.KnownFPClasses |= fcZero;
  }
 
  if (Known.isKnownNeverNaN()) {
    if (KnownLHS.SignBit && KnownRHS.SignBit &&
        *KnownLHS.SignBit == *KnownRHS.SignBit) {
      if (*KnownLHS.SignBit)
        Known.signBitMustBeOne();
      else
        Known.signBitMustBeZero();
    } else if ((Kind == MinMaxKind::maximum || Kind == MinMaxKind::minimum ||
                Kind == MinMaxKind::maximumnum ||
                Kind == MinMaxKind::minimumnum) ||
               // FIXME: Should be using logical zero versions
               ((KnownLHS.isKnownNeverNegZero() ||
                 KnownRHS.isKnownNeverPosZero()) &&
                (KnownLHS.isKnownNeverPosZero() ||
                 KnownRHS.isKnownNeverNegZero()))) {
      // Don't take sign bit from NaN operands.
      if (!KnownLHS.isKnownNeverNaN())
        KnownLHS.SignBit = std::nullopt;
      if (!KnownRHS.isKnownNeverNaN())
        KnownRHS.SignBit = std::nullopt;
      if ((Kind == MinMaxKind::maximum || Kind == MinMaxKind::maximumnum ||
           Kind == MinMaxKind::maxnum) &&
          (KnownLHS.SignBit == false || KnownRHS.SignBit == false))
        Known.signBitMustBeZero();
      else if ((Kind == MinMaxKind::minimum || Kind == MinMaxKind::minimumnum ||
                Kind == MinMaxKind::minnum) &&
               (KnownLHS.SignBit == true || KnownRHS.SignBit == true))
        Known.signBitMustBeOne();
    }
  }
 
  return Known;
}
 
KnownFPClass KnownFPClass::canonicalize(const KnownFPClass &KnownSrc,
                                        DenormalMode DenormMode) {
  KnownFPClass Known;
 
  // This is essentially a stronger form of
  // propagateCanonicalizingSrc. Other "canonicalizing" operations don't
  // actually have an IR canonicalization guarantee.
 
  // Canonicalize may flush denormals to zero, so we have to consider the
  // denormal mode to preserve known-not-0 knowledge.
  Known.KnownFPClasses = KnownSrc.KnownFPClasses | fcZero | fcQNan;
 
  // Stronger version of propagateNaN
  // Canonicalize is guaranteed to quiet signaling nans.
  if (KnownSrc.isKnownNeverNaN())
    Known.knownNot(fcNan);
  else
    Known.knownNot(fcSNan);
 
  // FIXME: Missing check of IEEE like types.
 
  // If the parent function flushes denormals, the canonical output cannot be a
  // denormal.
  if (DenormMode == DenormalMode::getIEEE()) {
    if (KnownSrc.isKnownNever(fcPosZero))
      Known.knownNot(fcPosZero);
    if (KnownSrc.isKnownNever(fcNegZero))
      Known.knownNot(fcNegZero);
    return Known;
  }
 
  if (DenormMode.inputsAreZero() || DenormMode.outputsAreZero())
    Known.knownNot(fcSubnormal);
 
  if (DenormMode == DenormalMode::getPreserveSign()) {
    if (KnownSrc.isKnownNever(fcPosZero | fcPosSubnormal))
      Known.knownNot(fcPosZero);
    if (KnownSrc.isKnownNever(fcNegZero | fcNegSubnormal))
      Known.knownNot(fcNegZero);
    return Known;
  }
 
  if (DenormMode.Input == DenormalMode::PositiveZero ||
      (DenormMode.Output == DenormalMode::PositiveZero &&
       DenormMode.Input == DenormalMode::IEEE))
    Known.knownNot(fcNegZero);
 
  return Known;
}
 
KnownFPClass KnownFPClass::fmul(const KnownFPClass &KnownLHS,
                                const KnownFPClass &KnownRHS,
                                DenormalMode Mode) {
  KnownFPClass Known;
 
  // xor sign bit.
  if ((KnownLHS.isKnownNever(fcNegative) &&
       KnownRHS.isKnownNever(fcNegative)) ||
      (KnownLHS.isKnownNever(fcPositive) && KnownRHS.isKnownNever(fcPositive)))
    Known.knownNot(fcNegative);
 
  if ((KnownLHS.isKnownNever(fcPositive) &&
       KnownRHS.isKnownNever(fcNegative)) ||
      (KnownLHS.isKnownNever(fcNegative) && KnownRHS.isKnownNever(fcPositive)))
    Known.knownNot(fcPositive);
 
  // inf * anything => inf or nan
  if (KnownLHS.isKnownAlways(fcInf | fcNan) ||
      KnownRHS.isKnownAlways(fcInf | fcNan))
    Known.knownNot(fcNormal | fcSubnormal | fcZero);
 
  // 0 * anything => 0 or nan
  if (KnownRHS.isKnownAlways(fcZero | fcNan) ||
      KnownLHS.isKnownAlways(fcZero | fcNan))
    Known.knownNot(fcNormal | fcSubnormal | fcInf);
 
  // +/-0 * +/-inf = nan
  if ((KnownLHS.isKnownAlways(fcZero | fcNan) &&
       KnownRHS.isKnownAlways(fcInf | fcNan)) ||
      (KnownLHS.isKnownAlways(fcInf | fcNan) &&
       KnownRHS.isKnownAlways(fcZero | fcNan)))
    Known.knownNot(~fcNan);
 
  if (!KnownLHS.isKnownNeverNaN() || !KnownRHS.isKnownNeverNaN())
    return Known;
 
  if (KnownLHS.SignBit && KnownRHS.SignBit) {
    if (*KnownLHS.SignBit == *KnownRHS.SignBit)
      Known.signBitMustBeZero();
    else
      Known.signBitMustBeOne();
  }
 
  // If 0 * +/-inf produces NaN.
  if ((KnownRHS.isKnownNeverInfinity() ||
       KnownLHS.isKnownNeverLogicalZero(Mode)) &&
      (KnownLHS.isKnownNeverInfinity() ||
       KnownRHS.isKnownNeverLogicalZero(Mode)))
    Known.knownNot(fcNan);
 
  return Known;
}
 
KnownFPClass KnownFPClass::exp(const KnownFPClass &KnownSrc) {
  KnownFPClass Known;
  Known.knownNot(fcNegative);
 
  Known.propagateNaN(KnownSrc);
 
  if (KnownSrc.cannotBeOrderedLessThanZero()) {
    // If the source is positive this cannot underflow.
    Known.knownNot(fcPosZero);
 
    // Cannot introduce denormal values.
    Known.knownNot(fcPosSubnormal);
  }
 
  // If the source is negative, this cannot overflow to infinity.
  if (KnownSrc.cannotBeOrderedGreaterThanZero())
    Known.knownNot(fcPosInf);
 
  return Known;
}
 
void KnownFPClass::propagateCanonicalizingSrc(const KnownFPClass &Src,
                                              DenormalMode Mode) {
  propagateDenormal(Src, Mode);
  propagateNaN(Src, /*PreserveSign=*/true);
}
 
KnownFPClass KnownFPClass::log(const KnownFPClass &KnownSrc,
                               DenormalMode Mode) {
  KnownFPClass Known;
  Known.knownNot(fcNegZero);
 
  if (KnownSrc.isKnownNeverPosInfinity())
    Known.knownNot(fcPosInf);
 
  if (KnownSrc.isKnownNeverNaN() && KnownSrc.cannotBeOrderedLessThanZero())
    Known.knownNot(fcNan);
 
  if (KnownSrc.isKnownNeverLogicalZero(Mode))
    Known.knownNot(fcNegInf);
 
  return Known;
}