[Mlir-commits] [mlir] [mlir][math] `powf(a, b)` drop support when a < 0 (PR #126338)

[Hyunsung Lee]

================
@@ -311,40 +312,71 @@ static LogicalResult convertFPowIOp(math::FPowIOp op,
   return success();
 }
 
-// Converts  Powf(float a, float b) (meaning a^b) to exp^(b * ln(a))
+// Converts Powf(float a, float b) (meaning a^b) to exp^(b * ln(a))
+// Some special cases where b is constant are handled separately:
+// when b == 0, or |b| == 0.5, 1.0, or 2.0.
 static LogicalResult convertPowfOp(math::PowFOp op, PatternRewriter &rewriter) {
   ImplicitLocOpBuilder b(op->getLoc(), rewriter);
   Value operandA = op.getOperand(0);
   Value operandB = op.getOperand(1);
-  Type opType = operandA.getType();
-  Value zero = createFloatConst(op->getLoc(), opType, 0.00, rewriter);
-  Value one = createFloatConst(op->getLoc(), opType, 1.00, rewriter);
-  Value two = createFloatConst(op->getLoc(), opType, 2.00, rewriter);
-  Value negOne = createFloatConst(op->getLoc(), opType, -1.00, rewriter);
-  Value opASquared = b.create<arith::MulFOp>(opType, operandA, operandA);
-  Value opBHalf = b.create<arith::DivFOp>(opType, operandB, two);
-
-  Value logA = b.create<math::LogOp>(opType, opASquared);
-  Value mult = b.create<arith::MulFOp>(opType, opBHalf, logA);
-  Value expResult = b.create<math::ExpOp>(opType, mult);
-  Value negExpResult = b.create<arith::MulFOp>(opType, expResult, negOne);
-  Value remainder = b.create<arith::RemFOp>(opType, operandB, two);
-  Value negCheck =
-      b.create<arith::CmpFOp>(arith::CmpFPredicate::OLT, operandA, zero);
-  Value oddPower =
-      b.create<arith::CmpFOp>(arith::CmpFPredicate::ONE, remainder, zero);
-  Value oddAndNeg = b.create<arith::AndIOp>(op->getLoc(), oddPower, negCheck);
-
-  // First, we select between the exp value and the adjusted value for odd
-  // powers of negatives. Then, we ensure that one is produced if `b` is zero.
-  // This corresponds to `libm` behavior, even for `0^0`. Without this check,
-  // `exp(0 * ln(0)) = exp(0 *-inf) = exp(-nan) = -nan`.
-  Value zeroCheck =
-      b.create<arith::CmpFOp>(arith::CmpFPredicate::OEQ, operandB, zero);
-  Value res = b.create<arith::SelectOp>(op->getLoc(), oddAndNeg, negExpResult,
-                                        expResult);
-  res = b.create<arith::SelectOp>(op->getLoc(), zeroCheck, one, res);
-  rewriter.replaceOp(op, res);
+  auto typeA = operandA.getType();
+  auto typeB = operandB.getType();
+
+  auto &sem =
+      cast<mlir::FloatType>(getElementTypeOrSelf(typeB)).getFloatSemantics();
+  APFloat valueB(sem);
----------------
ita9naiwa wrote:

https://github.com/ita9naiwa/llvm-project/blob/d5ee522a217359264d35516a5b506149b5d1ab68/mlir/lib/Dialect/Math/Transforms/ExpandPatterns.cpp#L286-L288
other parts the same file explicitly use sem so I followed. It would work without it, should we remove? 

https://github.com/llvm/llvm-project/pull/126338