[clang] [Clang][Driver] Revise Cygwin ToolChain to call linker directly (PR #147960)

[via cfe-commits]

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@@ -684,6 +684,8 @@ static StringRef getArchNameForCompilerRTLib(const ToolChain &TC,
 StringRef ToolChain::getOSLibName() const {
   if (Triple.isOSDarwin())
     return "darwin";
+  if (Triple.isWindowsCygwinEnvironment())
+    return "cygwin";
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tyan0 wrote:

I observed following behaviour.

In `ToolChain::getCompilerRT()`, the path `/usr/lib/clang/22/lib/x86_64-pc-windows-cygnus/libclang_rt.builtins.a` is checked first. Then, `/usr/lib/clang/22/lib/windows/libclang_rt.builtins-x86_64.a` is checked.

This algorithm is not affected by `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` setting in clang.

If `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` is `ON` in compiler-rt, the first search will succeed. However, if `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` is `OFF` in compiler-rt, the correct path should not be `/usr/lib/clang/22/lib/windows/libclang_rt.builtins-x86_64.a`, but `/usr/lib/clang/22/lib/cygwin/libclang_rt.builtins-x86_64.a`, therefore, both search will fail. The patch above solves this issue.

As a conclusion, I’ve confirmed that the patch is indeed necessary after all. So, I'll create another PR for that.

https://github.com/llvm/llvm-project/pull/147960