CodeGen: Strengthen definition of F{MIN|MAX}NUM_IEEE nodes (#85195)

Previously these were declared as having the 2008 behavior, with
underspecified signed zero handling. Currently, AMDGPU, PPC and
LoongArch mark these as legal. The AMDGPU and PPC instructions respect
the signed zero behavior. The LoongArch documentation doesn't state, but
I'm assuming it also does.

Author: arsenm

llvm/docs/GlobalISel/GenericOpcode.rst

@@ -521,16 +521,32 @@ The return value of (FMAXNUM 0.0, -0.0) could be either 0.0 or -0.0.
 G_FMINNUM_IEEE
 ^^^^^^^^^^^^^^
 
-Perform floating-point minimum on two values, following the IEEE-754 2008
-definition. This differs from FMINNUM in the handling of signaling NaNs. If one
-input is a signaling NaN, returns a quiet NaN.
+Perform floating-point minimum on two values, following IEEE-754
+definitions. This differs from FMINNUM in the handling of signaling
+NaNs.
+
+If one input is a signaling NaN, returns a quiet NaN. This matches
+IEEE-754 2008's minnum/maxnum for signaling NaNs (which differs from
+2019).
+
+These treat -0 as ordered less than +0, matching the behavior of
+IEEE-754 2019's minimumNumber/maximumNumber (which was unspecified in
+2008).
 
 G_FMAXNUM_IEEE
 ^^^^^^^^^^^^^^
 
-Perform floating-point maximum on two values, following the IEEE-754 2008
-definition. This differs from FMAXNUM in the handling of signaling NaNs. If one
-input is a signaling NaN, returns a quiet NaN.
+Perform floating-point maximum on two values, following IEEE-754
+definitions. This differs from FMAXNUM in the handling of signaling
+NaNs.
+
+If one input is a signaling NaN, returns a quiet NaN. This matches
+IEEE-754 2008's minnum/maxnum for signaling NaNs (which differs from
+2019).
+
+These treat -0 as ordered less than +0, matching the behavior of
+IEEE-754 2019's minimumNumber/maximumNumber (which was unspecified in
+2008).
 
 G_FMINIMUM
 ^^^^^^^^^^

llvm/include/llvm/CodeGen/ISDOpcodes.h

@@ -971,10 +971,16 @@ enum NodeType {
   FMINNUM,
   FMAXNUM,
 
-  /// FMINNUM_IEEE/FMAXNUM_IEEE - Perform floating-point minimum or maximum on
-  /// two values, following the IEEE-754 2008 definition. This differs from
-  /// FMINNUM/FMAXNUM in the handling of signaling NaNs. If one input is a
-  /// signaling NaN, returns a quiet NaN.
+  /// FMINNUM_IEEE/FMAXNUM_IEEE - Perform floating-point minimumNumber or
+  /// maximumNumber on two values, following IEEE-754 definitions. This differs
+  /// from FMINNUM/FMAXNUM in the handling of signaling NaNs, and signed zero.
+  ///
+  /// 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).
+  ///
+  /// These treat -0 as ordered less than +0, matching the behavior of IEEE-754
+  /// 2019's minimumNumber/maximumNumber.
   FMINNUM_IEEE,
   FMAXNUM_IEEE,
 

llvm/include/llvm/Target/GenericOpcodes.td

@@ -795,10 +795,17 @@ def G_FMAXNUM : GenericInstruction {
   let isCommutable = true;
 }
 
-// FMINNUM_IEEE/FMAXNUM_IEEE - Perform floating-point minimum or maximum on
-// two values, following the IEEE-754 2008 definition. This differs from
-// FMINNUM/FMAXNUM in the handling of signaling NaNs. If one input is a
-// signaling NaN, returns a quiet NaN.
+// FMINNUM_IEEE/FMAXNUM_IEEE - Perform floating-point minimumNumber or
+// maximumNumber on two values, following IEEE-754 definitions. This
+// differs from FMINNUM/FMAXNUM in the handling of signaling NaNs, and
+// signed zero.
+//
+// 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).
+//
+// These treat -0 as ordered less than +0, matching the behavior of
+// IEEE-754 2019's minimumNumber/maximumNumber.
 def G_FMINNUM_IEEE : GenericInstruction {
   let OutOperandList = (outs type0:$dst);
   let InOperandList = (ins type0:$src1, type0:$src2);