[llvm] [Support][Cygwin] Fix handling of Process symbol lookup. (PR #143072)

[via llvm-commits]

https://github.com/jeremyd2019 created https://github.com/llvm/llvm-project/pull/143072

In Unix/DynamicLibrary.inc, it was already known that Cygwin required use of `RTLD_DEFAULT` as the `Handle` parameter to `DLSym` to search all modules for a symbol.  Unfortunately, RTLD_DEFAULT is defined as NULL, so the existing checks of the `Process` handle meant `DLSym` would never be called on Cygwin.  Add a `bool` member to indicate whether the Process handle was set instead.

>From 929527a9ddfe427631f13d52ac7ba5a293a7e8ec Mon Sep 17 00:00:00 2001
From: Jeremy Drake <github at jdrake.com>
Date: Thu, 5 Jun 2025 23:01:09 -0700
Subject: [PATCH] [Support][Cygwin] Fix handling of Process symbol lookup.

In Unix/DynamicLibrary.inc, it was already known that Cygwin required
use of RTLD_DEFAULT as the "Handle" parameter to DLSym to search all
modules for a symbol.  Unfortunately RTLD_DEFAULT is defined as NULL, so
the existing checks of the "Process" handle meant DLSym would never be
called on Cygwin.  Add a bool to indicate whether the Process handle was
set instead.
---
 llvm/lib/Support/DynamicLibrary.cpp | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Support/DynamicLibrary.cpp b/llvm/lib/Support/DynamicLibrary.cpp
index 531c035ab9266..34c2dc2e56227 100644
--- a/llvm/lib/Support/DynamicLibrary.cpp
+++ b/llvm/lib/Support/DynamicLibrary.cpp
@@ -26,6 +26,7 @@ class DynamicLibrary::HandleSet {
   typedef std::vector<void *> HandleList;
   HandleList Handles;
   void *Process = nullptr;
+  bool ProcessAdded = false;
 
 public:
   static void *DLOpen(const char *Filename, std::string *Err);
@@ -66,6 +67,7 @@ class DynamicLibrary::HandleSet {
       }
 #endif
       Process = Handle;
+      ProcessAdded = true;
     }
     return true;
   }
@@ -97,11 +99,11 @@ class DynamicLibrary::HandleSet {
     assert(!((Order & SO_LoadedFirst) && (Order & SO_LoadedLast)) &&
            "Invalid Ordering");
 
-    if (!Process || (Order & SO_LoadedFirst)) {
+    if (!ProcessAdded || (Order & SO_LoadedFirst)) {
       if (void *Ptr = LibLookup(Symbol, Order))
         return Ptr;
     }
-    if (Process) {
+    if (ProcessAdded) {
       // Use OS facilities to search the current binary and all loaded libs.
       if (void *Ptr = DLSym(Process, Symbol))
         return Ptr;