[LLD][Cygwin] Implement --dll-search-prefix (llvm#143263)

GCC on Cygwin environment invokes linker with passing
`--dll-search-prefix=cyg`.
Implementing this option makes lld-mingw invokable by `gcc -fuse-ld=lld`.

---------

Co-authored-by: jeremyd2019 <[email protected]>

Author: kikairoya

lld/MinGW/Driver.cpp

@@ -139,8 +139,9 @@ static std::optional<std::string> findFile(StringRef path1,
 }
 
 // This is for -lfoo. We'll look for libfoo.dll.a or libfoo.a from search paths.
-static std::string
-searchLibrary(StringRef name, ArrayRef<StringRef> searchPaths, bool bStatic) {
+static std::string searchLibrary(StringRef name,
+                                 ArrayRef<StringRef> searchPaths, bool bStatic,
+                                 StringRef prefix) {
   if (name.starts_with(":")) {
     for (StringRef dir : searchPaths)
       if (std::optional<std::string> s = findFile(dir, name.substr(1)))
@@ -161,7 +162,7 @@ searchLibrary(StringRef name, ArrayRef<StringRef> searchPaths, bool bStatic) {
     if (std::optional<std::string> s = findFile(dir, name + ".lib"))
       return *s;
     if (!bStatic) {
-      if (std::optional<std::string> s = findFile(dir, "lib" + name + ".dll"))
+      if (std::optional<std::string> s = findFile(dir, prefix + name + ".dll"))
         return *s;
       if (std::optional<std::string> s = findFile(dir, name + ".dll"))
         return *s;
@@ -545,6 +546,10 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
     add("-libpath:" + StringRef(a->getValue()));
   }
 
+  StringRef dllPrefix = "lib";
+  if (auto *arg = args.getLastArg(OPT_dll_search_prefix))
+    dllPrefix = arg->getValue();
+
   StringRef prefix = "";
   bool isStatic = false;
   for (auto *a : args) {
@@ -556,7 +561,8 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
         add(prefix + StringRef(a->getValue()));
       break;
     case OPT_l:
-      add(prefix + searchLibrary(a->getValue(), searchPaths, isStatic));
+      add(prefix +
+          searchLibrary(a->getValue(), searchPaths, isStatic, dllPrefix));
       break;
     case OPT_whole_archive:
       prefix = "-wholearchive:";

lld/MinGW/Options.td

@@ -79,6 +79,8 @@ defm exclude_symbols: Eq<"exclude-symbols",
     "Exclude symbols from automatic export">, MetaVarName<"<symbol[,symbol,...]>">;
 def export_all_symbols: F<"export-all-symbols">,
     HelpText<"Export all symbols even if a def file or dllexport attributes are used">;
+defm dll_search_prefix:Eq<"dll-search-prefix", "Specify DLL prefix instead of 'lib'">,
+    MetaVarName<"<dll_search_prefix>">;
 defm fatal_warnings: B<"fatal-warnings",
     "Treat warnings as errors",
     "Do not treat warnings as errors (default)">;

lld/test/MinGW/lib.test

@@ -5,6 +5,7 @@ LIB1: unable to find library -lfoo
 
 RUN: echo > %t/lib/libfoo.dll.a
 RUN: ld.lld -### -m i386pep -lfoo -L%t/lib 2>&1 | FileCheck -check-prefix=LIB2 %s
+RUN: ld.lld -### -m i386pep -lfoo --dll-search-prefix=cyg -L%t/lib 2>&1 | FileCheck -check-prefix=LIB2 %s
 LIB2: libfoo.dll.a
 
 RUN: not ld.lld -### -m i386pep -l:barefilename -L%t/lib 2>&1 | FileCheck -check-prefix=LIB-LITERAL-FAIL %s
@@ -22,6 +23,7 @@ LIB3: unable to find library -lfoo
 
 RUN: echo > %t/lib/libfoo.a
 RUN: ld.lld -### -m i386pep -Bstatic -lfoo -L%t/lib 2>&1 | FileCheck -check-prefix=LIB4 %s
+RUN: ld.lld -### -m i386pep -Bstatic -lfoo --dll-search-prefix=cyg -L%t/lib 2>&1 | FileCheck -check-prefix=LIB4 %s
 LIB4: libfoo.a
 
 RUN: echo > %t/lib/libbar.dll.a
@@ -46,12 +48,17 @@ MSVCSTYLE: msvcstyle.lib
 
 RUN: echo > %t/lib/libnoimplib.dll
 RUN: echo > %t/lib/noprefix_noimplib.dll
+RUN: echo > %t/lib/cygnoimplib2.dll
 RUN: ld.lld -### -m i386pep -L%t/lib -lnoimplib 2>&1 | FileCheck -check-prefix=DLL1 %s
 RUN: ld.lld -### -m i386pep -L%t/lib -lnoprefix_noimplib 2>&1 | FileCheck -check-prefix=DLL2 %s
+RUN: ld.lld -### -m i386pep -L%t/lib -lnoimplib2 --dll-search-prefix=cyg 2>&1 | FileCheck -check-prefix=DLL3 %s
 DLL1: libnoimplib.dll
 DLL2: noprefix_noimplib.dll
+DLL3: cygnoimplib2.dll
 
 RUN: not ld.lld -### -m i386pep -L%t/lib -static -lnoimplib 2>&1 | FileCheck -check-prefix=ERROR-NOIMPLIB %s
 RUN: not ld.lld -### -m i386pep -L%t/lib -static -lnoprefix_noimplib 2>&1 | FileCheck -check-prefix=ERROR-NOPREFIX-NOIMPLIB %s
+RUN: not ld.lld -### -m i386pep -L%t/lib -static -lnoimplib2 --dll-search-prefix=cyg 2>&1 | FileCheck -check-prefix=ERROR-CYG-NOIMPLIB %s
 ERROR-NOIMPLIB: unable to find library -lnoimplib
 ERROR-NOPREFIX-NOIMPLIB: unable to find library -lnoprefix_noimplib
+ERROR-CYG-NOIMPLIB: unable to find library -lnoimplib2