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[mlir][math] Simplify pow(2^n, y) to exp2(y) #166183

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[mlir][math] Simplify pow(2^n, y) to exp2(y) #166183

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34 changes: 28 additions & 6 deletions mlir/lib/Dialect/Math/Transforms/AlgebraicSimplification.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -42,20 +42,24 @@ LogicalResult
PowFStrengthReduction::matchAndRewrite(math::PowFOp op,
PatternRewriter &rewriter) const {
Location loc = op.getLoc();
// pow(x, y)
Value x = op.getLhs();
Value y = op.getRhs();

FloatAttr scalarExponent;
DenseFPElementsAttr vectorExponent;
FloatAttr scalarBase, scalarExponent;
DenseFPElementsAttr vectorBase, vectorExponent;

bool isScalar = matchPattern(op.getRhs(), m_Constant(&scalarExponent));
bool isVector = matchPattern(op.getRhs(), m_Constant(&vectorExponent));
bool isScalarBase = matchPattern(x, m_Constant(&scalarBase));
bool isVectorBase = matchPattern(x, m_Constant(&vectorBase));
bool isScalarExponent = matchPattern(y, m_Constant(&scalarExponent));
bool isVectorExponent = matchPattern(y, m_Constant(&vectorExponent));

// Returns true if exponent is a constant equal to `value`.
auto isExponentValue = [&](double value) -> bool {
if (isScalar)
if (isScalarExponent)
return scalarExponent.getValue().isExactlyValue(value);

if (isVector && vectorExponent.isSplat())
if (isVectorExponent && vectorExponent.isSplat())
return vectorExponent.getSplatValue<FloatAttr>()
.getValue()
.isExactlyValue(value);
Expand Down Expand Up @@ -120,6 +124,24 @@ PowFStrengthReduction::matchAndRewrite(math::PowFOp op,
return success();
}

// Replace `pow(2.0^n, y)` with `exp2(n * y)`
if (isScalarBase || (isVectorBase && vectorBase.isSplat())) {
APFloat baseValue = isScalarBase
? scalarBase.getValue()
: vectorBase.getSplatValue<FloatAttr>().getValue();
// Check if base is an exact power of 2
int n = baseValue.getExactLog2();
if (n != INT_MIN) {
Type opType = getElementTypeOrSelf(op.getType());
Value nValue = arith::ConstantOp::create(
rewriter, loc, rewriter.getFloatAttr(opType, n));
Value nTimesY =
arith::MulFOp::create(rewriter, loc, ValueRange({bcast(nValue), y}));
rewriter.replaceOpWithNewOp<math::Exp2Op>(op, nTimesY);
return success();
}
}

return failure();
}

Expand Down
60 changes: 60 additions & 0 deletions mlir/test/Dialect/Math/algebraic-simplification.mlir
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Original file line number Diff line number Diff line change
Expand Up @@ -90,6 +90,66 @@ func.func @pow_0_75(%arg0: f32, %arg1 : vector<4xf32>) -> (f32, vector<4xf32>) {
return %0, %1 : f32, vector<4xf32>
}

// CHECK-LABEL: @pow_of_two
func.func @pow_of_two(%arg0: f32, %arg1 : vector<4xf32>) -> (f32, vector<4xf32>) {
// CHECK: %[[SCALAR:.*]] = math.exp2 %arg0
// CHECK: %[[VECTOR:.*]] = math.exp2 %arg1
// CHECK: return %[[SCALAR]], %[[VECTOR]]
%c = arith.constant 2.0 : f32
%v = arith.constant dense <2.0> : vector<4xf32>
%0 = math.powf %c, %arg0 : f32
%1 = math.powf %v, %arg1 : vector<4xf32>
return %0, %1 : f32, vector<4xf32>
}

// CHECK-LABEL: @pow_of_four
func.func @pow_of_four(%arg0: f32, %arg1 : vector<4xf32>) -> (f32, vector<4xf32>) {
// CHECK-DAG: %[[CST_V:.*]] = arith.constant dense<2.000000e+00> : vector<4xf32>
// CHECK-DAG: %[[CST_S:.*]] = arith.constant 2.000000e+00 : f32
// CHECK: %[[MUL_S:.*]] = arith.mulf %arg0, %[[CST_S]]
// CHECK: %[[SCALAR:.*]] = math.exp2 %[[MUL_S]]
// CHECK: %[[MUL_V:.*]] = arith.mulf %arg1, %[[CST_V]]
// CHECK: %[[VECTOR:.*]] = math.exp2 %[[MUL_V]]
// CHECK: return %[[SCALAR]], %[[VECTOR]]
%c = arith.constant 4.0 : f32
%v = arith.constant dense <4.0> : vector<4xf32>
%0 = math.powf %c, %arg0 : f32
%1 = math.powf %v, %arg1 : vector<4xf32>
return %0, %1 : f32, vector<4xf32>
}

// CHECK-LABEL: @pow_of_half
func.func @pow_of_half(%arg0: f32, %arg1 : vector<4xf32>) -> (f32, vector<4xf32>) {
// CHECK-DAG: %[[CST_V:.*]] = arith.constant dense<-1.000000e+00> : vector<4xf32>
// CHECK-DAG: %[[CST_S:.*]] = arith.constant -1.000000e+00 : f32
// CHECK: %[[MUL_S:.*]] = arith.mulf %arg0, %[[CST_S]]
// CHECK: %[[SCALAR:.*]] = math.exp2 %[[MUL_S]]
// CHECK: %[[MUL_V:.*]] = arith.mulf %arg1, %[[CST_V]]
// CHECK: %[[VECTOR:.*]] = math.exp2 %[[MUL_V]]
// CHECK: return %[[SCALAR]], %[[VECTOR]]
%c = arith.constant 0.5 : f32
%v = arith.constant dense <0.5> : vector<4xf32>
%0 = math.powf %c, %arg0 : f32
%1 = math.powf %v, %arg1 : vector<4xf32>
return %0, %1 : f32, vector<4xf32>
}

// CHECK-LABEL: @pow_of_quarter
func.func @pow_of_quarter(%arg0: f32, %arg1 : vector<4xf32>) -> (f32, vector<4xf32>) {
// CHECK-DAG: %[[CST_V:.*]] = arith.constant dense<-2.000000e+00> : vector<4xf32>
// CHECK-DAG: %[[CST_S:.*]] = arith.constant -2.000000e+00 : f32
// CHECK: %[[MUL_S:.*]] = arith.mulf %arg0, %[[CST_S]]
// CHECK: %[[SCALAR:.*]] = math.exp2 %[[MUL_S]]
// CHECK: %[[MUL_V:.*]] = arith.mulf %arg1, %[[CST_V]]
// CHECK: %[[VECTOR:.*]] = math.exp2 %[[MUL_V]]
// CHECK: return %[[SCALAR]], %[[VECTOR]]
%c = arith.constant 0.25 : f32
%v = arith.constant dense <0.25> : vector<4xf32>
%0 = math.powf %c, %arg0 : f32
%1 = math.powf %v, %arg1 : vector<4xf32>
return %0, %1 : f32, vector<4xf32>
}

// CHECK-LABEL: @ipowi_zero_exp(
// CHECK-SAME: %[[ARG0:.+]]: i32
// CHECK-SAME: %[[ARG1:.+]]: vector<4xi32>
Expand Down
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