Skip to content

[mlir][math] Implement alternative decomposition for tanh #85025

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 1 commit into from
Mar 15, 2024
Merged

[mlir][math] Implement alternative decomposition for tanh #85025

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
40 changes: 23 additions & 17 deletions mlir/lib/Dialect/Math/Transforms/ExpandPatterns.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -91,34 +91,40 @@ static LogicalResult convertCoshOp(math::CoshOp op, PatternRewriter &rewriter) {
}

/// Expands tanh op into
/// 1) 1-exp^{-2x} / 1+exp^{-2x}, if x => 0
/// 2) exp^{2x}-1 / exp^{2x}+1 , if x < 0
/// 1-exp^{-2x} / 1+exp^{-2x}
/// To avoid overflow we exploit the reflection symmetry `tanh(-x) = -tanh(x)`.
/// We compute a "signs" value which is -1 if input is negative and +1 if input
/// is positive. Then multiply the input by this value, guaranteeing that the
/// result is positive, which also guarantees `exp^{-2x * sign(x)}` is in (0,
/// 1]. Expand the computation on the input `x * sign(x)`, then multiply the
/// result by `sign(x)` to retain sign of the real result.
static LogicalResult convertTanhOp(math::TanhOp op, PatternRewriter &rewriter) {
auto floatType = op.getOperand().getType();
Location loc = op.getLoc();
Value zero = createFloatConst(loc, floatType, 0.0, rewriter);
Value one = createFloatConst(loc, floatType, 1.0, rewriter);
Value two = createFloatConst(loc, floatType, 2.0, rewriter);
Value doubledX = rewriter.create<arith::MulFOp>(loc, op.getOperand(), two);
Value negTwo = createFloatConst(loc, floatType, -2.0, rewriter);

// Compute sign(x) = cast<float_type>(x < 0) * (-2) + 1
Value sign = rewriter.create<arith::CmpFOp>(loc, arith::CmpFPredicate::OLT,
op.getOperand(), zero);
sign = rewriter.create<arith::SIToFPOp>(loc, floatType, sign);
sign = rewriter.create<arith::MulFOp>(loc, sign, negTwo);
sign = rewriter.create<arith::AddFOp>(loc, sign, one);

// Case 1: tanh(x) = 1-exp^{-2x} / 1+exp^{-2x}
Value negDoubledX = rewriter.create<arith::NegFOp>(loc, doubledX);
// Normalize input to positive value: y = sign(x) * x
Value positiveX = rewriter.create<arith::MulFOp>(loc, sign, op.getOperand());

// Decompose on normalized input
Value negDoubledX = rewriter.create<arith::MulFOp>(loc, negTwo, positiveX);
Value exp2x = rewriter.create<math::ExpOp>(loc, negDoubledX);
Value dividend = rewriter.create<arith::SubFOp>(loc, one, exp2x);
Value divisor = rewriter.create<arith::AddFOp>(loc, one, exp2x);
Value positiveRes = rewriter.create<arith::DivFOp>(loc, dividend, divisor);

// Case 2: tanh(x) = exp^{2x}-1 / exp^{2x}+1
exp2x = rewriter.create<math::ExpOp>(loc, doubledX);
dividend = rewriter.create<arith::SubFOp>(loc, exp2x, one);
divisor = rewriter.create<arith::AddFOp>(loc, exp2x, one);
Value negativeRes = rewriter.create<arith::DivFOp>(loc, dividend, divisor);
// Multiply result by sign(x) to retain signs from negative inputs
rewriter.replaceOpWithNewOp<arith::MulFOp>(op, sign, positiveRes);

// tanh(x) = x >= 0 ? positiveRes : negativeRes
Value zero = createFloatConst(loc, floatType, 0.0, rewriter);
Value cmpRes = rewriter.create<arith::CmpFOp>(loc, arith::CmpFPredicate::OGE,
op.getOperand(), zero);
rewriter.replaceOpWithNewOp<arith::SelectOp>(op, cmpRes, positiveRes,
negativeRes);
return success();
}

Expand Down
19 changes: 9 additions & 10 deletions mlir/test/Dialect/Math/expand-math.mlir
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -7,19 +7,18 @@ func.func @tanh(%arg: f32) -> f32 {
}
// CHECK-DAG: %[[ZERO:.+]] = arith.constant 0.000000e+00 : f32
// CHECK-DAG: %[[ONE:.+]] = arith.constant 1.000000e+00 : f32
// CHECK-DAG: %[[TWO:.+]] = arith.constant 2.000000e+00 : f32
// CHECK: %[[DOUBLEDX:.+]] = arith.mulf %arg0, %[[TWO]] : f32
// CHECK: %[[NEGDOUBLEDX:.+]] = arith.negf %[[DOUBLEDX]] : f32
// CHECK-DAG: %[[TWO:.+]] = arith.constant -2.000000e+00 : f32
// CHECK: %[[VAL0:.+]] = arith.cmpf olt, %arg0, %[[ZERO]] : f32
// CHECK: %[[VAL1:.+]] = arith.sitofp %[[VAL0]] : i1 to f32
// CHECK: %[[VAL2:.+]] = arith.mulf %[[VAL1]], %[[TWO]] : f32
// CHECK: %[[SIGN:.+]] = arith.addf %[[VAL2]], %[[ONE]] : f32
// CHECK: %[[POSX:.+]] = arith.mulf %[[SIGN]], %arg0 : f32
// CHECK: %[[NEGDOUBLEDX:.+]] = arith.mulf %[[POSX]], %[[TWO]] : f32
// CHECK: %[[EXP1:.+]] = math.exp %[[NEGDOUBLEDX]] : f32
// CHECK: %[[DIVIDEND1:.+]] = arith.subf %[[ONE]], %[[EXP1]] : f32
// CHECK: %[[DIVISOR1:.+]] = arith.addf %[[EXP1]], %[[ONE]] : f32
// CHECK: %[[RES1:.+]] = arith.divf %[[DIVIDEND1]], %[[DIVISOR1]] : f32
// CHECK: %[[EXP2:.+]] = math.exp %[[DOUBLEDX]] : f32
// CHECK: %[[DIVIDEND2:.+]] = arith.subf %[[EXP2]], %[[ONE]] : f32
// CHECK: %[[DIVISOR2:.+]] = arith.addf %[[EXP2]], %[[ONE]] : f32
// CHECK: %[[RES2:.+]] = arith.divf %[[DIVIDEND2]], %[[DIVISOR2]] : f32
// CHECK: %[[COND:.+]] = arith.cmpf oge, %arg0, %[[ZERO]] : f32
// CHECK: %[[RESULT:.+]] = arith.select %[[COND]], %[[RES1]], %[[RES2]] : f32
// CHECK: %[[POSRES:.+]] = arith.divf %[[DIVIDEND1]], %[[DIVISOR1]] : f32
// CHECK: %[[RESULT:.+]] = arith.mulf %[[SIGN]], %[[POSRES]] : f32
// CHECK: return %[[RESULT]]

// -----
Expand Down