[Mlir-commits] [mlir] [MLIR][Math] Add erfc to math dialect (PR #126439)

[Benoit Jacob]

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@@ -1118,6 +1122,101 @@ ErfPolynomialApproximation::matchAndRewrite(math::ErfOp op,
   return success();
 }
 
+// Approximates erfc(x) with p((x - 2) / (x + 2)), where p is a 9 degree
+// polynomial.This approximation is based on the following stackoverflow post:
+// https://stackoverflow.com/questions/35966695/vectorizable-implementation-of-complementary-error-function-erfcf
+LogicalResult
+ErfcPolynomialApproximation::matchAndRewrite(math::ErfcOp op,
+                                             PatternRewriter &rewriter) const {
+  Value x = op.getOperand();
+  Type et = getElementTypeOrSelf(x);
+
+  if (!et.isF32())
+    return rewriter.notifyMatchFailure(op, "only f32 type is supported.");
+  std::optional<VectorShape> shape = vectorShape(x);
+
+  ImplicitLocOpBuilder builder(op->getLoc(), rewriter);
+  auto bcast = [&](Value value) -> Value {
+    return broadcast(builder, value, shape);
+  };
+
+  Value trueValue = bcast(boolCst(builder, true));
+  Value zero = bcast(floatCst(builder, 0.0f, et));
+  Value one = bcast(floatCst(builder, 1.0f, et));
+  Value onehalf = bcast(floatCst(builder, 0.5f, et));
+  Value neg4 = bcast(floatCst(builder, -4.0f, et));
+  Value neg2 = bcast(floatCst(builder, -2.0f, et));
+  Value pos2 = bcast(floatCst(builder, 2.0f, et));
+  Value posInf = bcast(f32FromBits(builder, 0x7f800000u));
+  Value clampVal = bcast(floatCst(builder, 10.0546875f, et));
+
+  // Get abs(x)
+  Value isNegativeArg =
+      builder.create<arith::CmpFOp>(arith::CmpFPredicate::OLT, x, zero);
+  Value negArg = builder.create<arith::NegFOp>(x);
+  Value a = builder.create<arith::SelectOp>(isNegativeArg, negArg, x);
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bjacob wrote:

Use `math.absf`.

https://github.com/llvm/llvm-project/pull/126439