[llvm] Revert "LangRef: Clarify llvm.minnum and llvm.maxnum about sNaN and signed zero (#112852)" (PR #138451)
Nikita Popov via llvm-commits
llvm-commits at lists.llvm.org
Sun May 4 04:32:51 PDT 2025
https://github.com/nikic created https://github.com/llvm/llvm-project/pull/138451
This reverts commit 363b05944f9212511ee6811d0eb1af841c177226.
I'm not a big fan of the change itself (because it is incoherent with LLVM's general sNaN semantics), but even if we want to do it, it needs to be phased in a lot more carefully than what has actually happened. You can't just change the semantics in LangRef and then completely ignore the consequences of this change in semantics.
Revert to the status quo for now. If we actually want to do this, we first need to ensure that the new minimumnum intrinsics have optimization and codegen parity with the old minnum, and ensure produces are migrated to them in advance, and then we need to land the LangRef changes as part of a patch stack that adjusts optimizations to respect the new semantics.
Fixes https://github.com/llvm/llvm-project/issues/138303.
>From 9211a4b3644b435aa09020ef3d3e1722178b355e Mon Sep 17 00:00:00 2001
From: Nikita Popov <nikita.ppv at gmail.com>
Date: Sun, 4 May 2025 13:27:22 +0200
Subject: [PATCH] Revert "LangRef: Clarify llvm.minnum and llvm.maxnum about
sNaN and signed zero (#112852)"
This reverts commit 363b05944f9212511ee6811d0eb1af841c177226.
I don't agree with the change itself, but even if we want to do it,
it needs to be phased in a lot more carefully than it was. You can't
just change the semantics in LangRef without actually dealing with
the consequences of the change.
Revert to the status quo for now.
Fixes https://github.com/llvm/llvm-project/issues/138303.
---
llvm/docs/LangRef.rst | 103 ++++++++++++-------------
llvm/include/llvm/CodeGen/ISDOpcodes.h | 20 ++---
2 files changed, 54 insertions(+), 69 deletions(-)
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index c8cc38c23cff3..2dd297a795c26 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -16936,7 +16936,7 @@ 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)
@@ -16980,30 +16980,21 @@ 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 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) returns either operand.
-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 similiar 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 implemention.
+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:
@@ -17040,30 +17031,20 @@ 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 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) returns either -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 similiar 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 implemention.
+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:
@@ -19942,8 +19923,12 @@ 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:
""""""""""
@@ -19971,8 +19956,12 @@ 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:
""""""""""
@@ -22262,7 +22251,7 @@ 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:
@@ -22311,7 +22300,7 @@ 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:
@@ -23610,7 +23599,10 @@ 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.
@@ -23677,7 +23669,10 @@ 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.
@@ -28349,7 +28344,7 @@ 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
@@ -28381,7 +28376,7 @@ 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
diff --git a/llvm/include/llvm/CodeGen/ISDOpcodes.h b/llvm/include/llvm/CodeGen/ISDOpcodes.h
index 80ef32aff62ae..9bd1ebb2abd55 100644
--- a/llvm/include/llvm/CodeGen/ISDOpcodes.h
+++ b/llvm/include/llvm/CodeGen/ISDOpcodes.h
@@ -1024,20 +1024,13 @@ 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,
@@ -1051,9 +1044,6 @@ 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,
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