[OpenMP][AMDGPU] Optimize the linked in math libraries

[jdoerfert]

Summary

Once we linked in math files, potentially even if we link in only other
"system libraries", we want to optimize the code again. This is not only
reasonable but also helps to hide various problems with the missing
attribute annotations in the math libraries.

[jdoerfert]

This fixes/hides the last remaining OvO failures with upstream on an AMD MI100 compiled with -O3.

[JonChesterfield]

Using the optimiser to hide errors in how we're pulling in libm is clearly not right, but it leaves us less obviously broken than we are at present.

clang/lib/Driver/ToolChains/AMDGPUOpenMP.cpp

 #include "CommonArgs.h"
 #include "ToolChains/ROCm.h"
 #include "clang/Basic/DiagnosticDriver.h"
 #include "clang/Driver/Compilation.h"
 #include "clang/Driver/Driver.h"
 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/InputInfo.h"
 #include "clang/Driver/Options.h"
+#include "clang/Driver/Tool.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/FormatAdapters.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/Path.h"
 
 using namespace clang::driver;
 using namespace clang::driver::toolchains;
const char *AMDGCN::OpenMPLinker::constructLLVMLinkCommand(
     const JobAction &JA, const InputInfoList &Inputs, const ArgList &Args,
     StringRef SubArchName, StringRef OutputFilePrefix) const {
   ArgStringList CmdArgs;
 
   for (const auto &II : Inputs)
     if (II.isFilename())
       CmdArgs.push_back(II.getFilename());
 
+  bool HasLibm = false;
   if (Args.hasArg(options::OPT_l)) {
     auto Lm = Args.getAllArgValues(options::OPT_l);
-    bool HasLibm = false;
     for (auto &Lib : Lm) {
       if (Lib == "m") {
         HasLibm = true;
         break;
       }
     }
 
     if (HasLibm) {
const char *AMDGCN::OpenMPLinker::constructLLVMLinkCommand(
   const char *OutputFileName =
       getOutputFileName(C, OutputFilePrefix, "-linked", "bc");
   CmdArgs.push_back(OutputFileName);
   const char *Exec =
       Args.MakeArgString(getToolChain().GetProgramPath("llvm-link"));
   C.addCommand(std::make_unique<Command>(
       JA, *this, ResponseFileSupport::AtFileCurCP(), Exec, CmdArgs, Inputs,
       InputInfo(&JA, Args.MakeArgString(OutputFileName))));
+
+  // If we linked in libm definitions late we run another round of optimizations
+  // to inline the definitions and fold what is foldable.
+  if (HasLibm) {
+    ArgStringList OptCmdArgs;
+    const char *OptOutputFileName =
+        getOutputFileName(C, OutputFilePrefix, "-linked-opt", "bc");
+    addLLCOptArg(Args, OptCmdArgs);
+    OptCmdArgs.push_back(OutputFileName);
+    OptCmdArgs.push_back("-o");
+    OptCmdArgs.push_back(OptOutputFileName);
+    const char *OptExec =
+        Args.MakeArgString(getToolChain().GetProgramPath("opt"));
+    C.addCommand(std::make_unique<Command>(
+        JA, *this, ResponseFileSupport::AtFileCurCP(), OptExec, OptCmdArgs,
+        InputInfo(&JA, Args.MakeArgString(OutputFileName)),
+        InputInfo(&JA, Args.MakeArgString(OptOutputFileName))));
+    OutputFileName = OptOutputFileName;
+  }
+
   return OutputFileName;
 }
 
 const char *AMDGCN::OpenMPLinker::constructLlcCommand(
     Compilation &C, const JobAction &JA, const InputInfoList &Inputs,
     const llvm::opt::ArgList &Args, llvm::StringRef SubArchName,
     llvm::StringRef OutputFilePrefix, const char *InputFileName,
     bool OutputIsAsm) const {