Add buildRescale helper function with scale width and double rounding option (#567)

Summary:
These scaling options can be used by lowering of convolution.

Pull Request resolved: #567

Reviewed By: larryliu0820

Differential Revision: D49897715

Pulled By: digantdesai

fbshipit-source-id: f833aab567bdc4d227231f5845c4f0ff42bf2be1

Author: tatwaichong

backends/arm/tosa_quant_utils.py

@@ -32,30 +32,42 @@ def isQuantArg(arg):
     return consumer_node.target == q_op
 
 
+# Check if scale32 mode is used for given output element type
+def isScale32(type):
+    return type == ts.DType.INT8
+
+
 # TOSA uses the RESCALE operation to scale between values with differing precision.
 # The RESCALE operator is defined using an integer multiply, add, and shift.
 # This utility function is for calculating the multier and shift given a scale.
 # Ref: https://www.mlplatform.org/tosa/tosa_spec.html#_precision_scaling
-def computeMultiplierAndShift(scale):
+def computeMultiplierAndShift(scale, scaleWidth=32):
+    if scaleWidth == 16:
+        offset = 15
+    elif scaleWidth == 32:
+        offset = 31
+    else:
+        raise AssertionError("unsupported scale width")
+
     assert isinstance(scale, float)
 
     mantissa, exponent = math.frexp(scale)
     shift = exponent
 
-    const_two_to_31 = 1 << 31
-    shifted_mantissa = round(mantissa * const_two_to_31)
+    const_2_power_15_or_31 = 1 << offset
+    shifted_mantissa = round(mantissa * const_2_power_15_or_31)
 
-    assert shifted_mantissa <= const_two_to_31
+    assert shifted_mantissa <= const_2_power_15_or_31
 
-    if shifted_mantissa == const_two_to_31:
+    if shifted_mantissa == const_2_power_15_or_31:
         shifted_mantissa = shifted_mantissa / 2
         shift += 1
 
-    # TOSA expects right shift to be positive, and embed (1 << 31) into right shift bits.
-    shift = 31 - shift
+    # TOSA expects right shift to be positive, and embed (1 << offset) into right shift bits.
+    shift = offset - shift
 
     # INT32_MAX, 2^31 - 1
-    assert shifted_mantissa <= (const_two_to_31 - 1)
+    assert shifted_mantissa <= (const_2_power_15_or_31 - 1)
 
     multiplier = shifted_mantissa
 
@@ -65,8 +77,41 @@ def computeMultiplierAndShift(scale):
     return multiplier, shift
 
 
+def buildRescale(
+    tosa_fb,
+    scale,
+    input_node,
+    output_type,
+    output_shape,
+    input_zp,
+    output_zp,
+    is_double_round,
+):
+    is_scale32 = isScale32(output_type)
+    scale_width = 32 if is_scale32 else 16
+    multiplier, shift = computeMultiplierAndShift(scale, scale_width)
+
+    attr_rescale = ts.TosaSerializerAttribute()
+    attr_rescale.RescaleAttribute(
+        input_zp=input_zp,
+        output_zp=output_zp,
+        multiplier=[multiplier],
+        shift=[shift],
+        scale32=is_scale32,
+        double_round=is_double_round,
+        per_channel=False,
+    )
+
+    rescale_out = tosa_fb.addIntermediate(output_shape, output_type)
+    tosa_fb.addOperator(
+        TosaOp.Op().RESCALE, [input_node.name], [rescale_out.name], attr_rescale
+    )
+
+    return rescale_out
+
+
 def buildRescaleToInt32(
-    tosa_fb, input, input_zp, rescale_scale
+    tosa_fb, input, input_zp, rescale_scale, is_scale32=True, is_double_round=True
 ) -> TosaSerializerTensor:
     multiplier, shift = computeMultiplierAndShift(rescale_scale)
     attr_rescale = ts.TosaSerializerAttribute()
@@ -75,8 +120,8 @@ def buildRescaleToInt32(
         output_zp=0,
         multiplier=[multiplier],
         shift=[shift],
-        scale32=True,
-        double_round=True,
+        scale32=is_scale32,
+        double_round=is_double_round,
         per_channel=False,
     )
     input_A_rescaled_to_int32 = tosa_fb.addIntermediate(input.shape, ts.DType.INT32)
@@ -91,7 +136,13 @@ def buildRescaleToInt32(
 
 
 def buildRescaleFromInt32(
-    tosa_fb, input_name, output_name, output_zp, rescale_scale
+    tosa_fb,
+    input_name,
+    output_name,
+    output_zp,
+    rescale_scale,
+    is_scale32=True,
+    is_double_round=True,
 ) -> TosaSerializerTensor:
     multiplier, shift = computeMultiplierAndShift(rescale_scale)
     attr_rescale_output = ts.TosaSerializerAttribute()
@@ -100,8 +151,8 @@ def buildRescaleFromInt32(
         output_zp=output_zp,
         multiplier=[multiplier],
         shift=[shift],
-        scale32=True,
-        double_round=True,
+        scale32=is_scale32,
+        double_round=is_double_round,
         per_channel=False,
     )