[mlir][tosa] Add check for invalid tosa.rescale parameters

[tom-arm]

• mlir::tosa::computeMultiplierAndShift returns a boolean, depending on validity of the shift value

[github-actions]

Thank you for submitting a Pull Request (PR) to the LLVM Project!

This PR will be automatically labeled and the relevant teams will be
notified.

If you wish to, you can add reviewers by using the "Reviewers" section on this page.

If this is not working for you, it is probably because you do not have write
permissions for the repository. In which case you can instead tag reviewers by
name in a comment by using @ followed by their GitHub username.

If you have received no comments on your PR for a week, you can request a review
by "ping"ing the PR by adding a comment “Ping”. The common courtesy "ping" rate
is once a week. Please remember that you are asking for valuable time from other developers.

If you have further questions, they may be answered by the LLVM GitHub User Guide.

You can also ask questions in a comment on this PR, on the LLVM Discord or on the forums.

[llvmbot]

@llvm/pr-subscribers-mlir

@llvm/pr-subscribers-mlir-tosa

Author: Tom Allsop (tom-arm)

Changes

• mlir::tosa::computeMultiplierAndShift returns a boolean, depending on validity of the shift value

---

Full diff: https://github.com/llvm/llvm-project/pull/96480.diff

2 Files Affected:

• (modified) mlir/include/mlir/Dialect/Tosa/Utils/QuantUtils.h (+1-1)
• (modified) mlir/lib/Dialect/Tosa/Utils/QuantUtils.cpp (+16-3)

diff --git a/mlir/include/mlir/Dialect/Tosa/Utils/QuantUtils.h b/mlir/include/mlir/Dialect/Tosa/Utils/QuantUtils.h
index 298c97015fe2e..7fba62e9d002b 100644
--- a/mlir/include/mlir/Dialect/Tosa/Utils/QuantUtils.h
+++ b/mlir/include/mlir/Dialect/Tosa/Utils/QuantUtils.h
@@ -28,7 +28,7 @@ namespace tosa {
 
 /// From a scale value, computes multiplier and shift values
 /// for 16 or 32-bit scale widths.
-void computeMultiplierAndShift(double scale, int32_t &multiplier,
+bool computeMultiplierAndShift(double scale, int32_t &multiplier,
                                int32_t &shift, int32_t scaleWidth);
 
 //// Builds ConvOpQuantizationAttr from input and weight.
diff --git a/mlir/lib/Dialect/Tosa/Utils/QuantUtils.cpp b/mlir/lib/Dialect/Tosa/Utils/QuantUtils.cpp
index 5c546f59cde41..c318fe3240520 100644
--- a/mlir/lib/Dialect/Tosa/Utils/QuantUtils.cpp
+++ b/mlir/lib/Dialect/Tosa/Utils/QuantUtils.cpp
@@ -92,18 +92,31 @@ static void computeMultiplierAndShiftTosaScale32(double scale,
 }
 
 /// Generates a quantized multiplier/shift from double.
-void mlir::tosa::computeMultiplierAndShift(double scale, int32_t &multiplier,
+bool mlir::tosa::computeMultiplierAndShift(double scale, int32_t &multiplier,
                                            int32_t &shift, int32_t scaleWidth) {
 
   switch (scaleWidth) {
   case 16:
     computeMultiplierAndShiftTosaScale16(scale, multiplier, shift);
-    return;
+
+    // In some cases computeMultiplierAndShiftTosaScale16 can return
+    // a value less then 2, which is not valid in the TOSA spec.
+    if (shift < 2)
+      return false;
+    else
+      return true;
   case 32:
     computeMultiplierAndShiftTosaScale32(scale, multiplier, shift);
-    return;
+
+    // In some cases computeMultiplierAndShiftTosaScale32 can return
+    // a value less then 2, which is not valid in the TOSA spec.
+    if (shift < 2)
+      return false;
+    else
+      return true;
   default:
     assert(0 && "Unsupported Tosa quantized_scale regime specified!");
+    return false;
   }
 }
 

[GeorgeARM]

Minor comment

[GeorgeARM]

Similar code in the two cases. Maybe worth pulling it into a lambda? e.g. isShiftWithinRange and check that is between 2 and 62?