Remove the constraint that min / max should stride zero

Since we apply nudging for the zero point to make sure the nudged zerop points
can be in the range of [qmin, qmax], the constraint that rmin / rmax should
stride zero isn't necessary.

This also matches the documentation of tensorflow's FakeQuantWithMinMaxArgs op,
where min and max don't need to stride zero:
https://www.tensorflow.org/api_docs/python/tf/quantization/fake_quant_with_min_max_args

PiperOrigin-RevId: 268296285

1 files changed, 13 insertions, 12 deletions

diff --git a/mlir/lib/Dialect/QuantOps/Utils/FakeQuantSupport.cpp b/mlir/lib/Dialect/QuantOps/Utils/FakeQuantSupport.cpp
index 5d4561be81b..2e1bd958b79 100644
--- a/mlir/lib/Dialect/QuantOps/Utils/FakeQuantSupport.cpp
+++ b/mlir/lib/Dialect/QuantOps/Utils/FakeQuantSupport.cpp
@@ -54,8 +54,17 @@ bool getDefaultStorageParams(unsigned numBits, bool narrowRange, bool isSigned,
   return false;
 }
 
-void getScaleAndZeroPoint(int64_t qmin, int64_t qmax, double rmin, double rmax,
-                          double &scale, int64_t &nudgedZeroPoint) {
+// This is a specific implementation of nudging:
+// If 0.0 < rmin < rmax or rmin < rmax < 0.0, the range will be shifted
+// to include 0.0, but the range width size (rmax-rmin) isn't changed. The zero
+// point is derived from the shifted range, and the scale isn't changed. As
+// a consequence some values, which are supposeed in the original [rmin, rmax]
+// range will be outside the shifted range and be clamped during quantization.
+// TODO(fengliuai): we should nudge the scale as well, but that requires the
+// fake quant op used in the training to use the nudged scale as well.
+void getNudgedScaleAndZeroPoint(int64_t qmin, int64_t qmax, double rmin,
+                                double rmax, double &scale,
+                                int64_t &nudgedZeroPoint) {
   // Determine the scale.
   const double qminDouble = qmin;
   const double qmaxDouble = qmax;
@@ -100,14 +109,6 @@ UniformQuantizedType fakeQuantAttrsToType(Location loc, unsigned numBits,
                                           double rmin, double rmax,
                                           bool narrowRange, Type expressedType,
                                           bool isSigned) {
-  // Range must straddle zero.
-  // TODO(b/140641593): remove this constraint.
-  if (rmin > 0.0 || rmax < 0.0) {
-    return (emitError(loc, "FakeQuant range must straddle zero: [")
-                << rmin << "," << rmax << "]",
-            nullptr);
-  }
-
   MLIRContext *ctx = expressedType.getContext();
   unsigned flags = isSigned ? QuantizationFlags::Signed : 0;
   Type storageType;
@@ -129,7 +130,7 @@ UniformQuantizedType fakeQuantAttrsToType(Location loc, unsigned numBits,
 
   double scale;
   int64_t nudgedZeroPoint;
-  getScaleAndZeroPoint(qmin, qmax, rmin, rmax, scale, nudgedZeroPoint);
+  getNudgedScaleAndZeroPoint(qmin, qmax, rmin, rmax, scale, nudgedZeroPoint);
 
   return UniformQuantizedType::getChecked(flags, storageType, expressedType,
                                           scale, nudgedZeroPoint, qmin, qmax,
@@ -172,7 +173,7 @@ fakeQuantAttrsToType(Location loc, unsigned numBits, int32_t quantizedDimension,
 
     double scale;
     int64_t nudgedZeroPoint;
-    getScaleAndZeroPoint(qmin, qmax, rmin, rmax, scale, nudgedZeroPoint);
+    getNudgedScaleAndZeroPoint(qmin, qmax, rmin, rmax, scale, nudgedZeroPoint);
     scales.push_back(scale);
     zeroPoints.push_back(nudgedZeroPoint);
   }