[FatLTO] Detect LLD linker more reliably (llvm#128285)

It's possible to have an `ld-path` point to a linker that doesn't have
the `ld.lld` filename (e.g. linker wrapper that may emit telemetry
before invoking the linker). This was causing mis-compilations with
fatLTO since the check couldn't reliably detect that it was using lld.
Instead, rely on the value from `-fuse-ld` to determine whether lld is
enabled.

Author: thevinster

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -862,13 +862,15 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
   const llvm::Triple &Triple = ToolChain.getTriple();
   const bool IsOSAIX = Triple.isOSAIX();
   const bool IsAMDGCN = Triple.isAMDGCN();
-  const char *Linker = Args.MakeArgString(ToolChain.GetLinkerPath());
+  StringRef Linker = Args.getLastArgValue(options::OPT_fuse_ld_EQ);
+  const char *LinkerPath = Args.MakeArgString(ToolChain.GetLinkerPath());
   const Driver &D = ToolChain.getDriver();
   const bool IsFatLTO = Args.hasFlag(options::OPT_ffat_lto_objects,
                                      options::OPT_fno_fat_lto_objects, false);
   const bool IsUnifiedLTO = Args.hasArg(options::OPT_funified_lto);
-  if (llvm::sys::path::filename(Linker) != "ld.lld" &&
-      llvm::sys::path::stem(Linker) != "ld.lld" && !Triple.isOSOpenBSD()) {
+  if (Linker != "lld" && Linker != "lld-link" &&
+      llvm::sys::path::filename(LinkerPath) != "ld.lld" &&
+      llvm::sys::path::stem(LinkerPath) != "ld.lld" && !Triple.isOSOpenBSD()) {
     // Tell the linker to load the plugin. This has to come before
     // AddLinkerInputs as gold requires -plugin and AIX ld requires -bplugin to
     // come before any -plugin-opt/-bplugin_opt that -Wl might forward.

clang/test/Driver/Inputs/basic_cross_linux_tree/usr/x86_64-unknown-linux-gnu/bin/lld-wrapper

@@ -0,0 +1 @@
+ld.lld

clang/test/Driver/fat-lto-objects.c

@@ -49,5 +49,8 @@
 // RUN:   -fuse-ld=lld -flto -ffat-lto-objects -### 2>&1 | FileCheck --check-prefix=LTO %s
 // RUN: %clang --target=x86_64-unknown-linux-gnu --sysroot=%S/Inputs/basic_cross_linux_tree %s \
 // RUN:   -fuse-ld=lld -fno-lto -ffat-lto-objects -### 2>&1 | FileCheck --check-prefix=NOLTO %s
+// RUN: %clang --target=x86_64-unknown-linux-gnu --sysroot=%S/Inputs/basic_cross_linux_tree %s \
+// RUN:   -fuse-ld=lld --ld-path=%S/Inputs/basic_cross_linux_tree/usr/x86_64-unknown-linux-gnu/bin/lld-wrapper \
+// RUN:   -flto -ffat-lto-objects -### 2>&1 | FileCheck --check-prefix=LTO %s
 // LTO: "--fat-lto-objects"
 // NOLTO-NOT: "--fat-lto-objects"