From 696a1537992d63f3a3edaa37db935079e176df28 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 26 Jun 2026 18:24:20 +0000
Subject: [PATCH 1/2] Initial plan


From d2ddc6ab85b71a968ea15234bd7cc3473b977d5a Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 26 Jun 2026 18:34:53 +0000
Subject: [PATCH 2/2] Fix tanh derivative: use 1-tanh(x)^2 instead of
 1/cosh(x)^2 for numerical stability

---
 enzyme/Enzyme/InstructionDerivatives.td  | 8 ++++----
 enzyme/test/Enzyme/ReverseMode/tanh19.ll | 9 +++++----
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/enzyme/Enzyme/InstructionDerivatives.td b/enzyme/Enzyme/InstructionDerivatives.td
index c0eeb99dcc75..60e626859da0 100644
--- a/enzyme/Enzyme/InstructionDerivatives.td
+++ b/enzyme/Enzyme/InstructionDerivatives.td
@@ -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]
                   >;
@@ -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)
                   >;
 
diff --git a/enzyme/test/Enzyme/ReverseMode/tanh19.ll b/enzyme/test/Enzyme/ReverseMode/tanh19.ll
index 5fde75e700c7..2f516cd0c6b0 100644
--- a/enzyme/test/Enzyme/ReverseMode/tanh19.ll
+++ b/enzyme/test/Enzyme/ReverseMode/tanh19.ll
@@ -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: }