[clang][driver] Enable '-flto' on bare-metal (llvm#94738)

Pass the linker LTO options enabled by the clang '-flto' command line
options when targeting bare-metal.

---------

Co-authored-by: Keith Walker <[email protected]>

Author: walkerkd

clang/lib/Driver/ToolChains/BareMetal.cpp

@@ -429,6 +429,7 @@ void baremetal::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   ArgStringList CmdArgs;
 
   auto &TC = static_cast<const toolchains::BareMetal &>(getToolChain());
+  const Driver &D = getToolChain().getDriver();
   const llvm::Triple::ArchType Arch = TC.getArch();
   const llvm::Triple &Triple = getToolChain().getEffectiveTriple();
 
@@ -466,6 +467,19 @@ void baremetal::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     TC.AddLinkRuntimeLib(Args, CmdArgs);
   }
 
+  if (D.isUsingLTO()) {
+    assert(!Inputs.empty() && "Must have at least one input.");
+    // Find the first filename InputInfo object.
+    auto Input = llvm::find_if(
+        Inputs, [](const InputInfo &II) -> bool { return II.isFilename(); });
+    if (Input == Inputs.end())
+      // For a very rare case, all of the inputs to the linker are
+      // InputArg. If that happens, just use the first InputInfo.
+      Input = Inputs.begin();
+
+    addLTOOptions(TC, Args, CmdArgs, Output, *Input,
+                  D.getLTOMode() == LTOK_Thin);
+  }
   if (TC.getTriple().isRISCV())
     CmdArgs.push_back("-X");
 

clang/test/Driver/baremetal-ld.c

@@ -0,0 +1,6 @@
+// RUN: %clang -### --target=armv7-unknown-none-eabi -mcpu=cortex-m4 --sysroot= -fuse-ld=ld %s 2>&1 | FileCheck --check-prefix=NOLTO %s
+// NOLTO: {{".*ld.*"}} {{.*}}
+// NOLTO-NOT: "-plugin-opt=mcpu"
+
+// RUN: %clang -### --target=armv7-unknown-none-eabi -mcpu=cortex-m4 --sysroot= -fuse-ld=ld -flto -O3 %s 2>&1 | FileCheck --check-prefix=LTO %s
+// LTO: {{".*ld.*"}} {{.*}} "-plugin-opt=mcpu=cortex-m4" "-plugin-opt=O3"