[IR] Handle NaN in StructuralEqual and StructuralHash (#17249)

* [IR] Handle NaN in StructuralEqual and StructuralHash

Prior to this commit, `NaN` values did not have any special handling
in either `StructuralEqual` or `StructuralHash`.

`StructuralEqual` checked whether the LHS and RHS were within some
tolerance of each other.  If the LHS and RHS are both `NaN`, this
would evaluate to false.  The updated `StructuralEqual` now checks for
this case, and returns true if both sides are `NaN`.

`StructuralHash` used the bit-pattern of a floating-point number to
compute the hash.  A `NaN` value may have any non-zero value in its
mantissa, and so this could produce distinct hashes for ASTs that
differ only by the choice of non-zero value.  The updated
`StructuralHash` uses the same
`std::numeric_limits<double::quiet_NaN()` value for all `NaN` values.

With these changes, `StructuralEqual` and `StructuralHash` can now
compare two IR functions, even if they contain `NaN`.

Closes #17247

* lint fix

Author: Lunderberg

include/tvm/node/structural_equal.h

@@ -28,6 +28,7 @@
 #include <tvm/runtime/container/array.h>
 #include <tvm/runtime/data_type.h>
 
+#include <cmath>
 #include <string>
 
 namespace tvm {
@@ -38,11 +39,21 @@ namespace tvm {
 class BaseValueEqual {
  public:
   bool operator()(const double& lhs, const double& rhs) const {
-    // fuzzy float pt comparison
-    constexpr double atol = 1e-9;
-    if (lhs == rhs) return true;
-    double diff = lhs - rhs;
-    return diff > -atol && diff < atol;
+    if (std::isnan(lhs) && std::isnan(rhs)) {
+      // IEEE floats do not compare as equivalent to each other.
+      // However, for the purpose of comparing IR representation, two
+      // NaN values are equivalent.
+      return true;
+    } else if (std::isnan(lhs) || std::isnan(rhs)) {
+      return false;
+    } else if (lhs == rhs) {
+      return true;
+    } else {
+      // fuzzy float pt comparison
+      constexpr double atol = 1e-9;
+      double diff = lhs - rhs;
+      return diff > -atol && diff < atol;
+    }
   }
 
   bool operator()(const int64_t& lhs, const int64_t& rhs) const { return lhs == rhs; }

include/tvm/node/structural_hash.h

@@ -27,7 +27,9 @@
 #include <tvm/runtime/data_type.h>
 #include <tvm/runtime/ndarray.h>
 
+#include <cmath>
 #include <functional>
+#include <limits>
 #include <string>
 
 namespace tvm {
@@ -52,7 +54,16 @@ class BaseValueHash {
 
  public:
   uint64_t operator()(const float& key) const { return Reinterpret<float, uint32_t>(key); }
-  uint64_t operator()(const double& key) const { return Reinterpret<double, uint64_t>(key); }
+  uint64_t operator()(const double& key) const {
+    if (std::isnan(key)) {
+      // The IEEE format defines more than one bit-pattern that
+      // represents NaN.  For the purpose of comparing IR
+      // representations, all NaN values are considered equivalent.
+      return Reinterpret<double, uint64_t>(std::numeric_limits<double>::quiet_NaN());
+    } else {
+      return Reinterpret<double, uint64_t>(key);
+    }
+  }
   uint64_t operator()(const int64_t& key) const { return Reinterpret<int64_t, uint64_t>(key); }
   uint64_t operator()(const uint64_t& key) const { return key; }
   uint64_t operator()(const int& key) const { return Reinterpret<int, uint32_t>(key); }

tests/python/tir-base/test_tir_structural_equal_hash.py

@@ -419,5 +419,48 @@ def func(A: T.Buffer(1, "int32")):
     assert '<root>.functions[I.GlobalVar("func")].body.extent.value' in err.value.args[0]
 
 
+def test_nan_values_are_equivalent():
+    """Structural equality treats two NaN values as equivalent.
+
+    By IEEE, a check of `NaN == NaN` returns false, as does
+    `abs(NaN - NaN) < tolerance`.  However, for the purpose of
+    comparing IR representations, both NaN values are equivalent.
+
+    """
+
+    @T.prim_func(private=True)
+    def func_1():
+        return T.float32("nan")
+
+    @T.prim_func(private=True)
+    def func_2():
+        return T.float32("nan")
+
+    tvm.ir.assert_structural_equal(func_1, func_2)
+    assert tvm.ir.structural_hash(func_1) == tvm.ir.structural_hash(func_2)
+
+
+def test_all_nan_values_are_equivalent():
+    """Structural equality treats two NaN values as equivalent.
+
+    IEEE defines NaN as any value that has all exponent bits set,
+    and has a non-zero mantissa.  For the purposes of comparing IR
+    representations, all NaN values are considered equivalent.
+
+    """
+
+    # A NaN with the first payload bit set.
+    nan_all_zeros = np.int32(0x7FC00000).view("float32")
+
+    # A NaN with the last payload bit set.
+    nan_with_payload = np.int32(0x7F800001).view("float32")
+
+    float_1 = T.float32(nan_all_zeros)
+    float_2 = T.float32(nan_with_payload)
+
+    tvm.ir.assert_structural_equal(float_1, float_2)
+    assert tvm.ir.structural_hash(float_1) == tvm.ir.structural_hash(float_2)
+
+
 if __name__ == "__main__":
     tvm.testing.main()