Revert "LangRef: Clarify llvm.minnum and llvm.maxnum about sNaN and signed zero (#112852)"

llvm/docs/LangRef.rst

@@ -17257,8 +17257,9 @@ LLVM Implementation:
 """"""""""""""""""""
 
 LLVM implements all ISO C flavors as listed in this table, except in the
-default floating-point environment exceptions are ignored. The constrained
-versions of the intrinsics respect the exception behavior.
+default floating-point environment exceptions are ignored and return value
+is non-deterministic if one or both inputs are sNaN. The constrained
+versions of the intrinsics respect the exception behavior and sNaN.
 
 .. list-table::
    :header-rows: 1
@@ -17290,7 +17291,7 @@ versions of the intrinsics respect the exception behavior.
      - qNaN, invalid exception
 
    * - ``+0.0 vs -0.0``
-     - +0.0(max)/-0.0(min)
+     - either one
      - +0.0(max)/-0.0(min)
      - +0.0(max)/-0.0(min)
 
@@ -17334,30 +17335,22 @@ type.
 
 Semantics:
 """"""""""
-Follows the semantics of minNum in IEEE-754-2008, except that -0.0 < +0.0 for the purposes
-of this intrinsic. As for signaling NaNs, per the minNum semantics, if either operand is sNaN,
-the result is qNaN. This matches the recommended behavior for the libm
-function ``fmin``, although not all implementations have implemented these recommended behaviors.
-
-If either operand is a qNaN, returns the other non-NaN operand. Returns NaN only if both operands are
-NaN or if either operand is sNaN. Note that arithmetic on an sNaN doesn't consistently produce a qNaN,
-so arithmetic feeding into a minnum can produce inconsistent results. For example,
-``minnum(fadd(sNaN, -0.0), 1.0)`` can produce qNaN or 1.0 depending on whether ``fadd`` is folded.
 
-IEEE-754-2008 defines minNum, and it was removed in IEEE-754-2019. As the replacement, IEEE-754-2019
-defines :ref:`minimumNumber <i_minimumnum>`.
+Follows the IEEE-754-2008 semantics for minNum, except for handling of
+signaling NaNs. This match's the behavior of libm's fmin.
 
-If the intrinsic is marked with the nsz attribute, then the effect is as in the definition in C
-and IEEE-754-2008: the result of ``minnum(-0.0, +0.0)`` may be either -0.0 or +0.0.
+If either operand is a NaN, returns the other non-NaN operand. Returns
+NaN only if both operands are NaN. If the operands compare equal,
+returns either one of the operands. For example, this means that
+fmin(+0.0, -0.0) non-deterministically returns either operand (-0.0
+or 0.0).
 
-Some architectures, such as ARMv8 (FMINNM), LoongArch (fmin), MIPSr6 (min.fmt), PowerPC/VSX (xsmindp),
-have instructions that match these semantics exactly; thus it is quite simple for these architectures.
-Some architectures have similar ones while they are not exact equivalent. Such as x86 implements ``MINPS``,
-which implements the semantics of C code ``a<b?a:b``: NUM vs qNaN always return qNaN. ``MINPS`` can be used
-if ``nsz`` and ``nnan`` are given.
-
-For existing libc implementations, the behaviors of fmin may be quite different on sNaN and signed zero behaviors,
-even in the same release of a single libm implementation.
+Unlike the IEEE-754-2008 behavior, this does not distinguish between
+signaling and quiet NaN inputs. If a target's implementation follows
+the standard and returns a quiet NaN if either input is a signaling
+NaN, the intrinsic lowering is responsible for quieting the inputs to
+correctly return the non-NaN input (e.g. by using the equivalent of
+``llvm.canonicalize``).
 
 .. _i_maxnum:
 
@@ -17394,30 +17387,21 @@ type.
 
 Semantics:
 """"""""""
-Follows the semantics of maxNum in IEEE-754-2008, except that -0.0 < +0.0 for the purposes
-of this intrinsic. As for signaling NaNs, per the maxNum semantics, if either operand is sNaN,
-the result is qNaN. This matches the recommended behavior for the libm
-function ``fmax``, although not all implementations have implemented these recommended behaviors.
-
-If either operand is a qNaN, returns the other non-NaN operand. Returns NaN only if both operands are
-NaN or if either operand is sNaN. Note that arithmetic on an sNaN doesn't consistently produce a qNaN,
-so arithmetic feeding into a maxnum can produce inconsistent results. For example,
-``maxnum(fadd(sNaN, -0.0), 1.0)`` can produce qNaN or 1.0 depending on whether ``fadd`` is folded.
+Follows the IEEE-754-2008 semantics for maxNum except for the handling of
+signaling NaNs. This matches the behavior of libm's fmax.
 
-IEEE-754-2008 defines maxNum, and it was removed in IEEE-754-2019. As the replacement, IEEE-754-2019
-defines :ref:`maximumNumber <i_maximumnum>`.
+If either operand is a NaN, returns the other non-NaN operand. Returns
+NaN only if both operands are NaN. If the operands compare equal,
+returns either one of the operands. For example, this means that
+fmax(+0.0, -0.0) non-deterministically returns either operand (-0.0
+or 0.0).
 
-If the intrinsic is marked with the nsz attribute, then the effect is as in the definition in C
-and IEEE-754-2008: the result of maxnum(-0.0, +0.0) may be either -0.0 or +0.0.
-
-Some architectures, such as ARMv8 (FMAXNM), LoongArch (fmax), MIPSr6 (max.fmt), PowerPC/VSX (xsmaxdp),
-have instructions that match these semantics exactly; thus it is quite simple for these architectures.
-Some architectures have similar ones while they are not exact equivalent. Such as x86 implements ``MAXPS``,
-which implements the semantics of C code ``a>b?a:b``: NUM vs qNaN always return qNaN. ``MAXPS`` can be used
-if ``nsz`` and ``nnan`` are given.
-
-For existing libc implementations, the behaviors of fmin may be quite different on sNaN and signed zero behaviors,
-even in the same release of a single libm implementation.
+Unlike the IEEE-754-2008 behavior, this does not distinguish between
+signaling and quiet NaN inputs. If a target's implementation follows
+the standard and returns a quiet NaN if either input is a signaling
+NaN, the intrinsic lowering is responsible for quieting the inputs to
+correctly return the non-NaN input (e.g. by using the equivalent of
+``llvm.canonicalize``).
 
 .. _i_minimum:
 
@@ -20301,8 +20285,12 @@ The '``llvm.vector.reduce.fmax.*``' intrinsics do a floating-point
 matches the element-type of the vector input.
 
 This instruction has the same comparison semantics as the '``llvm.maxnum.*``'
-intrinsic.  If the intrinsic call has the ``nnan`` fast-math flag, then the
-operation can assume that NaNs are not present in the input vector.
+intrinsic. That is, the result will always be a number unless all elements of
+the vector are NaN. For a vector with maximum element magnitude 0.0 and
+containing both +0.0 and -0.0 elements, the sign of the result is unspecified.
+
+If the intrinsic call has the ``nnan`` fast-math flag, then the operation can
+assume that NaNs are not present in the input vector.
 
 Arguments:
 """"""""""
@@ -20330,8 +20318,12 @@ The '``llvm.vector.reduce.fmin.*``' intrinsics do a floating-point
 matches the element-type of the vector input.
 
 This instruction has the same comparison semantics as the '``llvm.minnum.*``'
-intrinsic. If the intrinsic call has the ``nnan`` fast-math flag, then the
-operation can assume that NaNs are not present in the input vector.
+intrinsic. That is, the result will always be a number unless all elements of
+the vector are NaN. For a vector with minimum element magnitude 0.0 and
+containing both +0.0 and -0.0 elements, the sign of the result is unspecified.
+
+If the intrinsic call has the ``nnan`` fast-math flag, then the operation can
+assume that NaNs are not present in the input vector.
 
 Arguments:
 """"""""""
@@ -22712,7 +22704,7 @@ This is an overloaded intrinsic.
 Overview:
 """""""""
 
-Predicated floating-point IEEE-754-2008 minNum of two vectors of floating-point values.
+Predicated floating-point IEEE-754 minNum of two vectors of floating-point values.
 
 
 Arguments:
@@ -22761,7 +22753,7 @@ This is an overloaded intrinsic.
 Overview:
 """""""""
 
-Predicated floating-point IEEE-754-2008 maxNum of two vectors of floating-point values.
+Predicated floating-point IEEE-754 maxNum of two vectors of floating-point values.
 
 
 Arguments:
@@ -24060,7 +24052,10 @@ result type. If only ``nnan`` is set then the neutral value is ``-Infinity``.
 
 This instruction has the same comparison semantics as the
 :ref:`llvm.vector.reduce.fmax <int_vector_reduce_fmax>` intrinsic (and thus the
-'``llvm.maxnum.*``' intrinsic).
+'``llvm.maxnum.*``' intrinsic). That is, the result will always be a number
+unless all elements of the vector and the starting value are ``NaN``. For a
+vector with maximum element magnitude ``0.0`` and containing both ``+0.0`` and
+``-0.0`` elements, the sign of the result is unspecified.
 
 To ignore the start value, the neutral value can be used.
 
@@ -24127,7 +24122,10 @@ result type. If only ``nnan`` is set then the neutral value is ``+Infinity``.
 
 This instruction has the same comparison semantics as the
 :ref:`llvm.vector.reduce.fmin <int_vector_reduce_fmin>` intrinsic (and thus the
-'``llvm.minnum.*``' intrinsic).
+'``llvm.minnum.*``' intrinsic). That is, the result will always be a number
+unless all elements of the vector and the starting value are ``NaN``. For a
+vector with maximum element magnitude ``0.0`` and containing both ``+0.0`` and
+``-0.0`` elements, the sign of the result is unspecified.
 
 To ignore the start value, the neutral value can be used.
 
@@ -28999,7 +28997,7 @@ The third argument specifies the exception behavior as described above.
 Semantics:
 """"""""""
 
-This function follows the IEEE-754-2008 semantics for maxNum.
+This function follows the IEEE-754 semantics for maxNum.
 
 
 '``llvm.experimental.constrained.minnum``' Intrinsic
@@ -29031,7 +29029,7 @@ The third argument specifies the exception behavior as described above.
 Semantics:
 """"""""""
 
-This function follows the IEEE-754-2008 semantics for minNum.
+This function follows the IEEE-754 semantics for minNum.
 
 
 '``llvm.experimental.constrained.maximum``' Intrinsic

llvm/include/llvm/CodeGen/ISDOpcodes.h

@@ -1048,20 +1048,13 @@ enum NodeType {
   LRINT,
   LLRINT,
 
-  /// FMINNUM/FMAXNUM - Perform floating-point minimum maximum on two values,
-  /// following IEEE-754 definitions except for signed zero behavior.
+  /// FMINNUM/FMAXNUM - Perform floating-point minimum or maximum on two
+  /// values.
   ///
-  /// If one input is a signaling NaN, returns a quiet NaN. This matches
-  /// IEEE-754 2008's minNum/maxNum behavior for signaling NaNs (which differs
-  /// from 2019).
+  /// In the case where a single input is a NaN (either signaling or quiet),
+  /// the non-NaN input is returned.
   ///
-  /// These treat -0 as ordered less than +0, matching the behavior of IEEE-754
-  /// 2019's minimumNumber/maximumNumber.
-  ///
-  /// Note that that arithmetic on an sNaN doesn't consistently produce a qNaN,
-  /// so arithmetic feeding into a minnum/maxnum can produce inconsistent
-  /// results. FMAXIMUN/FMINIMUM or FMAXIMUMNUM/FMINIMUMNUM may be better choice
-  /// for non-distinction of sNaN/qNaN handling.
+  /// The return value of (FMINNUM 0.0, -0.0) could be either 0.0 or -0.0.
   FMINNUM,
   FMAXNUM,
 
@@ -1075,9 +1068,6 @@ enum NodeType {
   ///
   /// These treat -0 as ordered less than +0, matching the behavior of IEEE-754
   /// 2019's minimumNumber/maximumNumber.
-  ///
-  /// Deprecated, and will be removed soon, as FMINNUM/FMAXNUM have the same
-  /// semantics now.
   FMINNUM_IEEE,
   FMAXNUM_IEEE,