[Mlir-commits] [mlir] Introduce `arith.scaling_extf` and `arith.scaling_truncf` (PR #141965)

[Umang Yadav]

================
@@ -409,6 +421,125 @@ struct F8E8M0TruncFOpConverter : public OpRewritePattern<arith::TruncFOp> {
   }
 };
 
+struct ScalingExtFOpConverter : public OpRewritePattern<arith::ScalingExtFOp> {
+  using OpRewritePattern::OpRewritePattern;
+  LogicalResult matchAndRewrite(arith::ScalingExtFOp op,
+                                PatternRewriter &rewriter) const final {
+    ImplicitLocOpBuilder b(op.getLoc(), rewriter);
+    Value inputOperand = op.getIn();
+    Value scaleOperand = op.getScale();
+    Type scaleETy = getElementTypeOrSelf(scaleOperand);
+    // allow implicit exponent extraction from 16/32 bits floats
+    if (scaleETy.getIntOrFloatBitWidth() >= 16) {
+      scaleETy = b.getF8E8M0Type();
+      scaleOperand = b.create<arith::TruncFOp>(scaleETy, scaleOperand);
+    }
+    if (!llvm::isa<Float8E8M0FNUType>(scaleETy)) {
+      return rewriter.notifyMatchFailure(
+          op, "scaling_extf is using scales of type which can not be converted "
+              "to f8E8M0FNU");
+    }
+    Type resultTy = op.getType();
+    // extf on scale will essentially create floating point number
+    // of type resulTy that is 2^scale and will also propagate NaNs
+    Value scaleExt = b.create<arith::ExtFOp>(resultTy, scaleOperand);
+    Value inputExt = b.create<arith::ExtFOp>(resultTy, inputOperand);
+    Value result = b.create<arith::MulFOp>(inputExt, scaleExt);
+    rewriter.replaceOp(op, result);
+    return success();
+  }
+};
+
+struct ScalingTruncFOpConverter
+    : public OpRewritePattern<arith::ScalingTruncFOp> {
+  using OpRewritePattern::OpRewritePattern;
+  LogicalResult matchAndRewrite(arith::ScalingTruncFOp op,
+                                PatternRewriter &rewriter) const final {
+    ImplicitLocOpBuilder b(op.getLoc(), rewriter);
+    Value inputOperand = op.getIn();
+    Value scaleOperand = op.getScale();
+    Type scaleTy = scaleOperand.getType();
+    Type scaleETy = getElementTypeOrSelf(scaleOperand);
+    // allow implicit exponent extraction from 16/32 bits floats
+    if (scaleETy.getIntOrFloatBitWidth() >= 16) {
+      scaleETy = b.getF8E8M0Type();
+      scaleOperand = b.create<arith::TruncFOp>(scaleETy, scaleOperand);
+      scaleTy = scaleOperand.getType();
+    }
+    if (!llvm::isa<Float8E8M0FNUType>(scaleETy)) {
+      return rewriter.notifyMatchFailure(
+          op, "scaling_truncf is using scales type which can not be converted "
+              "to f8E8M0FNU");
+    }
+
+    Type resultTy = op.getType();
+    Type resultETy = getElementTypeOrSelf(op.getOut());
+
+    Type inputTy = inputOperand.getType();
+    Type inputETy = getElementTypeOrSelf(inputOperand);
+
+    Type i8Ty = cloneToShapedType(resultTy, b.getI8Type());
+    Type i32Ty = cloneToShapedType(resultTy, b.getI32Type());
+    Type f32Ty = cloneToShapedType(resultTy, b.getF32Type());
+
+    if (inputETy.getIntOrFloatBitWidth() < 32) {
+      inputOperand = b.create<arith::ExtFOp>(f32Ty, inputOperand);
+    } else if (inputETy.getIntOrFloatBitWidth() > 32) {
+      inputOperand = b.create<arith::TruncFOp>(f32Ty, inputOperand);
+    }
+    inputTy = inputOperand.getType();
+    inputETy = getElementTypeOrSelf(inputOperand);
+
+    // normalize scale by exponent of the max normal value (emax) in result type
+    // as per the OCP MXFP spec
+    // https://github.com/microsoft/microxcaling/blob/7bc41952de394f5cc5e782baf132e7c7542eb4e4/mx/mx_ops.py#L277
+    // here this normalization is carried in f32. Therefore instead of
+    // subtraction it does the DivFOp
+    const llvm::fltSemantics &resultFltSemantics =
+        llvm::cast<FloatType>(resultETy).getFloatSemantics();
+    int maxExponent = APFloat::semanticsMaxExponent(resultFltSemantics);
+    Value cEmax = createConst(op->getLoc(), i32Ty, maxExponent, rewriter);
+    Value c1 = createConst(op->getLoc(), i32Ty, 1, rewriter);
+    Value cPow2 = b.create<arith::ShLIOp>(c1, cEmax);
+    Value cPow2F32 = b.create<arith::SIToFPOp>(f32Ty, cPow2);
+    Value scaleF32 = b.create<arith::ExtFOp>(f32Ty, scaleOperand);
+    // note that spec also does the clamping but it should only be done for
+    // underflows because diving by 2^emax will only make it smaller.
----------------
umangyadav wrote:

```suggestion
    // underflows because dividing by 2^emax will only make it smaller.
```

https://github.com/llvm/llvm-project/pull/141965