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Fix numerically unstable tanh derivative rule #2908

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Fix numerically unstable tanh derivative rule #2908

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Copilot Jun 26, 2026
d2ddc6a
Fix tanh derivative: use 1-tanh(x)^2 instead of 1/cosh(x)^2 for numer…
Copilot Jun 26, 2026
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8 changes: 4 additions & 4 deletions enzyme/Enzyme/InstructionDerivatives.td
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Original file line number Diff line number Diff line change
Expand Up @@ -352,19 +352,19 @@ def : CallPattern<(Op $x, $y),

def : CallPattern<(Op $x),
["tanh"],
[(FDiv (DiffeRet), (FMul(Call<(SameTypesFunc<"cosh">), [ReadNone,NoUnwind]> $x):$c, $c))],
[(FMul (DiffeRet), (FSub (ConstantFP<"1.0"> $x), (FMul(Call<(SameFunc), [ReadNone,NoUnwind]> $x):$c, $c)))],
(ForwardFromSummedReverse),
[ReadNone, NoUnwind]
>;
def : CallPattern<(Op $x),
["tanhf"],
[(FDiv (DiffeRet), (FMul(Call<(SameTypesFunc<"coshf">), [ReadNone,NoUnwind]> $x):$c, $c))],
[(FMul (DiffeRet), (FSub (ConstantFP<"1.0"> $x), (FMul(Call<(SameFunc), [ReadNone,NoUnwind]> $x):$c, $c)))],
(ForwardFromSummedReverse),
[ReadNone, NoUnwind]
>;
def : CallPattern<(Op $x),
["tanhl"],
[(FDiv (DiffeRet), (FMul(Call<(SameTypesFunc<"coshl">), [ReadNone,NoUnwind]> $x):$c, $c))],
[(FMul (DiffeRet), (FSub (ConstantFP<"1.0"> $x), (FMul(Call<(SameFunc), [ReadNone,NoUnwind]> $x):$c, $c)))],
(ForwardFromSummedReverse),
[ReadNone, NoUnwind]
>;
Expand Down Expand Up @@ -931,7 +931,7 @@ def : CallPattern<(Op (Op $x, $y):$z),

def : IntrPattern<(Op $x),
[["tanh", "19", ""]],
[(FDiv (DiffeRet), (FMul(Intrinsic<"cosh"> $x):$c, $c))],
[(FMul (DiffeRet), (FSub (ConstantFP<"1.0"> $x), (FMul(Intrinsic<"tanh"> $x):$c, $c)))],
(ForwardFromSummedReverse)
>;

Expand Down
9 changes: 5 additions & 4 deletions enzyme/test/Enzyme/ReverseMode/tanh19.ll
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Expand Up @@ -21,9 +21,10 @@ declare double @__enzyme_autodiff(ptr, ...)

; CHECK: define internal { double } @diffetester(double %x, double %differeturn)
; CHECK-NEXT: entry:
; CHECK-NEXT: %0 = call fast double @llvm.cosh.f64(double %x)
; CHECK-NEXT: %0 = call fast double @llvm.tanh.f64(double %x)
; CHECK-NEXT: %1 = fmul fast double %0, %0
; CHECK-NEXT: %2 = fdiv fast double %differeturn, %1
; CHECK-NEXT: %3 = insertvalue { double } undef, double %2, 0
; CHECK-NEXT: ret { double } %3
; CHECK-NEXT: %2 = fsub fast double 1.000000e+00, %1
; CHECK-NEXT: %3 = fmul fast double %differeturn, %2
; CHECK-NEXT: %4 = insertvalue { double } undef, double %3, 0
; CHECK-NEXT: ret { double } %4
; CHECK-NEXT: }
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