[llvm] [ORC] Disable `__unw_add_dynamic_eh_frame_section` on Cygwin (PR #227709)

[Tomohiro Kashiwada via llvm-commits]

https://github.com/kikairoya created https://github.com/llvm/llvm-project/pull/227709

Causes a link error by #212260:
```
    ld.lld: error: undefined symbol: __unw_add_dynamic_eh_frame_section
    >>> referenced by libLLVMOrcTargetProcess.a(RegisterEHFrames.cpp.o):(llvm::orc::registerEHFrameSection(void const*, unsigned long))
    >>> referenced by libLLVMOrcTargetProcess.a(RegisterEHFrames.cpp.o):(registerEHFrameWrapper(llvm::orc::ExecutorAddrRange))
```

>From 7d7a4cc1e9eb082eabfd2918bebc9d4b540e26dd Mon Sep 17 00:00:00 2001
From: kikairoya <kikairoya at gmail.com>
Date: Wed, 30 Sep 2026 20:56:31 +0900
Subject: [PATCH] [ORC] Disable __unw_add_dynamic_eh_frame_section on Cygwin

Causes a link error by #212260:
```
    ld.lld: error: undefined symbol: __unw_add_dynamic_eh_frame_section
    >>> referenced by libLLVMOrcTargetProcess.a(RegisterEHFrames.cpp.o):(llvm::orc::registerEHFrameSection(void const*, unsigned long))
    >>> referenced by libLLVMOrcTargetProcess.a(RegisterEHFrames.cpp.o):(registerEHFrameWrapper(llvm::orc::ExecutorAddrRange))
```
---
 .../ExecutionEngine/Orc/TargetProcess/RegisterEHFrames.cpp  | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/ExecutionEngine/Orc/TargetProcess/RegisterEHFrames.cpp b/llvm/lib/ExecutionEngine/Orc/TargetProcess/RegisterEHFrames.cpp
index b64f7664f80f3..e261b3f001ab1 100644
--- a/llvm/lib/ExecutionEngine/Orc/TargetProcess/RegisterEHFrames.cpp
+++ b/llvm/lib/ExecutionEngine/Orc/TargetProcess/RegisterEHFrames.cpp
@@ -22,7 +22,7 @@ using namespace llvm::orc::shared;
 namespace llvm {
 namespace orc {
 
-#if !defined(_WIN32)
+#if !defined(_WIN32) && !defined(__CYGWIN__)
 extern "C" void
 __unw_add_dynamic_eh_frame_section(const void *) LLVM_ATTRIBUTE_WEAK;
 extern "C" void
@@ -130,7 +130,7 @@ Error walkLibunwindEHFrameSection(const char *const SectionStart,
 Error registerEHFrameSection(const void *EHFrameSectionAddr,
                              size_t EHFrameSectionSize) {
   // For libunwind after D111863.
-#if !defined(_WIN32)
+#if !defined(_WIN32) && !defined(__CYGWIN__)
   if (__unw_add_dynamic_eh_frame_section &&
       __unw_remove_dynamic_eh_frame_section) {
     __unw_add_dynamic_eh_frame_section(EHFrameSectionAddr);
@@ -157,7 +157,7 @@ Error registerEHFrameSection(const void *EHFrameSectionAddr,
 
 Error deregisterEHFrameSection(const void *EHFrameSectionAddr,
                                size_t EHFrameSectionSize) {
-#if !defined(_WIN32)
+#if !defined(_WIN32) && !defined(__CYGWIN__)
   if (__unw_add_dynamic_eh_frame_section &&
       __unw_remove_dynamic_eh_frame_section) {
     __unw_remove_dynamic_eh_frame_section(EHFrameSectionAddr);