Add CPU Cast op support for Float8E8M0

docs/OperatorKernels.md

@@ -69,8 +69,8 @@ The **OpSet Version** column uses the following notation:
 |BitwiseOr|*in* A:**T**<br> *in* B:**T**<br> *out* C:**T**|18+|**T** = tensor(int16), tensor(int32), tensor(int64), tensor(int8), tensor(uint16), tensor(uint32), tensor(uint64), tensor(uint8)|
 |BitwiseXor|*in* A:**T**<br> *in* B:**T**<br> *out* C:**T**|18+|**T** = tensor(int16), tensor(int32), tensor(int64), tensor(int8), tensor(uint16), tensor(uint32), tensor(uint64), tensor(uint8)|
 |BlackmanWindow|*in* size:**T1**<br> *out* output:**T2**|17+|**T1** = tensor(int32), tensor(int64)<br/> **T2** = tensor(double), tensor(float), tensor(int16), tensor(int32), tensor(int64), tensor(int8), tensor(uint16), tensor(uint32), tensor(uint64), tensor(uint8)|
-|Cast|*in* input:**T1**<br> *out* output:**T2**|25+|**T1** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)<br/> **T2** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)|
-|||24|**T1** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)<br/> **T2** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)|
+|Cast|*in* input:**T1**<br> *out* output:**T2**|25+|**T1** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(float8e8m0), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)<br/> **T2** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(float8e8m0), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)|
+|||24|**T1** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(float8e8m0), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)<br/> **T2** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(float8e8m0), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)|
 |||23|**T1** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)<br/> **T2** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)|
 |||[21, 22]|**T1** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)<br/> **T2** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)|
 |||[19, 20]|**T1** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)<br/> **T2** = tensor(bfloat16), tensor(bool), tensor(double), tensor(float), tensor(float16), tensor(float8e4m3fn), tensor(float8e4m3fnuz), tensor(float8e5m2), tensor(float8e5m2fnuz), tensor(int16), tensor(int2), tensor(int32), tensor(int4), tensor(int64), tensor(int8), tensor(string), tensor(uint16), tensor(uint2), tensor(uint32), tensor(uint4), tensor(uint64), tensor(uint8)|

include/onnxruntime/core/common/float8.h

@@ -701,7 +701,17 @@ struct Float8E8M0 {
   static constexpr ORT_HOST_DEVICE FromBitsT FromBits() { return FromBitsT(); }
   constexpr ORT_HOST_DEVICE Float8E8M0(unsigned char bits, FromBitsT) : val(bits) {}
 
-  inline explicit ORT_HOST_DEVICE Float8E8M0(float v, bool saturate = true) {
+  /// Rounding modes for Float8E8M0 conversion from float.
+  /// These correspond to the ONNX Cast op's round_mode attribute for float8e8m0.
+  /// See: https://github.com/onnx/onnx/blob/main/onnx/numpy_helper.py (to_float8e8m0)
+  enum class RoundMode : uint8_t {
+    Up,       // Ceiling: always round up to next power of 2 when not exact (default).
+    Down,     // Floor: always truncate to lower power of 2.
+    Nearest,  // Round to nearest power of 2; ties round to higher power (round-half-up).
+  };
+
+  inline explicit ORT_HOST_DEVICE Float8E8M0(float v, bool saturate = true,
+                                             RoundMode round_mode = RoundMode::Up) {
     uint32_t b;
     std::memcpy(&b, &v, sizeof(b));
 
@@ -756,34 +766,74 @@ struct Float8E8M0 {
       return;
     }
 
-    // Denormalized float32: value = 2^(-126) * (mantissa / 2^23)
-    // The largest subnormal is ~2^(-126) * (1 - 2^-23), which should round to 2^(-126) = val 1.
-    // The midpoint between 2^(-127) and 2^(-126) is 1.5 * 2^(-127).
-    // Subnormals with value >= midpoint round up to 2^(-126) (val=1), others to 2^(-127) (val=0).
-    // Midpoint in subnormal mantissa: 0x00600000 (mantissa >= 0.75 * 2^23 means value >= 1.5 * 2^-127).
+    // Denormalized float32: value = 2^(-126) * (mantissa / 2^23), range (0, 2^(-126)).
+    // E8M0 can represent 2^(-127) (val=0) and 2^(-126) (val=1). For nearest rounding,
+    // the midpoint is 1.5 * 2^(-127), which is mantissa 0x600000. Ties round up.
     if (exponent == 0) {
-      if (saturate) {
-        if (mantissa >= 0x00600000) {
-          val = 0x01;  // Round up to 2^(-126)
-        } else {
-          val = 0x00;  // Round down to 2^(-127)
-        }
+      // Subnormals with mantissa < 0x400000 have value < E8M0_MIN (2^-127) and
+      // cannot be represented. Without saturation they map to NaN.
+      // Subnormals with mantissa >= 0x400000 have value >= E8M0_MIN, so they
+      // round to val=0 or val=1, both valid E8M0 values.
+      if (!saturate && mantissa < 0x00400000) {
+        val = 0xFF;  // NaN: x < E8M0_MIN is not representable without saturation
+        return;
+      }
+      bool round_up;
+      switch (round_mode) {
+        case RoundMode::Up:
+          // Ceiling: round up only when value > 2^(-127). Denorm mantissa == 0x400000
+          // is exactly 2^(-127) (val=0), so it must NOT round up.
+          round_up = (mantissa > 0x00400000);
+          break;
+        case RoundMode::Down:
+          // Floor: always keep val=0 (2^(-127)), never increment.
+          round_up = false;
+          break;
+        case RoundMode::Nearest:
+        default:
+          round_up = (mantissa >= 0x00600000);
+          break;
+      }
+      if (round_up) {
+        val = 0x01;  // 2^(-126)
       } else {
-        val = 0xFF;  // NaN (subnormals are below E8M0 min for saturate=false)
+        val = 0x00;  // 2^(-127)
       }
       return;
     }
 
-    // Normal float32: value is 2^(exponent - 127) * (1 + mantissa/2^23)
-    // We need to round to the nearest power of 2.
-    // Round half up: round to next power of 2 when mantissa >= 0.5
-    // (i.e., when the float value is >= 1.5 * nearest lower power of 2)
-    // This aligns with the OCP Microscaling Formats (MX) spec for E8M0 scaling factors.
-    if (mantissa >= 0x00400000) {  // >= 0.5
+    // Normal float32: value is 2^(exponent - 127) * (1 + mantissa/2^23).
+    // Values with exponent=254 and mantissa>0 are in (2^127, 2^128). Since 2^128
+    // is not representable in E8M0 (val 255 = NaN), without saturation these
+    // values cannot be rounded to any valid E8M0 value and must become NaN.
+    if (!saturate && exponent == 0xFE && mantissa != 0) {
+      val = 0xFF;  // NaN: x > E8M0_MAX is not representable without saturation
+      return;
+    }
+    // Round to the nearest power of 2 using the ONNX semantics:
+    //   Up (ceiling):  round up when the float is not exactly a power of 2 (mantissa > 0).
+    //   Down (floor):  never round up; always keep the lower exponent.
+    //   Nearest:       G bit (bit 22) determines direction -- round up when mantissa >= 0x400000.
+    //                  For normal floats lsb of exponent is always considered 1, so ties
+    //                  round to the higher power of 2 (round-half-up).
+    bool round_up;
+    switch (round_mode) {
+      case RoundMode::Up:
+        round_up = (mantissa > 0);
+        break;
+      case RoundMode::Down:
+        round_up = false;
+        break;
+      case RoundMode::Nearest:
+      default:
+        round_up = (mantissa >= 0x00400000);
+        break;
+    }
+    if (round_up) {
       exponent += 1;
     }
 
-    // After rounding, exponent may overflow
+    // After rounding, exponent may overflow.
     if (exponent > 0xFE) {
       if (saturate) {
         val = 0xFE;  // Largest finite: 2^127