Pass split-machine-functions to code generator when flto is used

[junfd]

Summary

-fsplit-machine-functions is an optimization in codegen phase. when -flto is use, clang generate IR bitcode in .o files, and linker will call into these codegen optimization passes. Current clang driver doesn't pass this option to linker when both -fsplit-machine-functions and -flto are used, so the optimization is silently ignored. My fix generates linker option -plugin-opt=-split-machine-functions for this case. It allows the linker to pass "split-machine-functions" to code generator to turn on that optimization. It works for both gold and lld.

[junfd]

Follow clang format.

[junfd]

Disable test on Windows, because -fsplit-machine-functions only works for ELF.

[wenlei]

clang/test/Driver/split-machine-functions.c

2 ↗

(On Diff #416555)

My understanding is that if you specify target, e.g. -target x86_64-unknown-linux, the test should still be runnable on windows host platform.

[junfd]

Refine the test case.

[hoy]

clang/lib/Driver/ToolChains/CommonArgs.cpp

577

Should also check OPT_fno_split_machine_functions, i.e,

if (Args.getLastArg(options::OPT_fsplit_machine_functions,
                           options::OPT_fno_split_machine_functions))

clang/test/Driver/fsplit-machine-functions2.c

6

Please add a test case for mixed -fsplit-machine-functions and -fno-split-machine-functions

[junfd]

Check the mixing -fsplit-machine-functions and -fno-split-machine-functions.

[hoy]

lgtm, thanks.

[wenlei]

The linter warning seems legit, otherwise looks good.

[junfd]

Apply clang format.

clang/lib/Driver/ToolChains/CommonArgs.cpp

if (Args.hasFlag(options::OPT_ffunction_sections,
     CmdArgs.push_back("-plugin-opt=-function-sections");
   }
 
   if (Args.hasFlag(options::OPT_fdata_sections, options::OPT_fno_data_sections,
                    UseSeparateSections)) {
     CmdArgs.push_back("-plugin-opt=-data-sections");
   }
 
+  // Pass an option to enable split machine functions.
+  if (auto *A = Args.getLastArg(options::OPT_fsplit_machine_functions,
+                                options::OPT_fno_split_machine_functions)) {
+    if (A->getOption().matches(options::OPT_fsplit_machine_functions))
+      CmdArgs.push_back("-plugin-opt=-split-machine-functions");
+  }
+
   if (Arg *A = getLastProfileSampleUseArg(Args)) {
     StringRef FName = A->getValue();
     if (!llvm::sys::fs::exists(FName))
       D.Diag(diag::err_drv_no_such_file) << FName;
     else
       CmdArgs.push_back(
           Args.MakeArgString(Twine("-plugin-opt=sample-profile=") + FName));
   }

clang/test/Driver/fsplit-machine-functions2.c

+// Test -fsplit-machine-functions option pass-through with lto
+// RUN: %clang -### -target x86_64-unknown-linux -flto -fsplit-machine-functions %s 2>&1 | FileCheck %s -check-prefix=CHECK-PASS
+
+// Test no pass-through to ld without lto
+// RUN: %clang -### -target x86_64-unknown-linux -fsplit-machine-functions %s 2>&1 | FileCheck %s -check-prefix=CHECK-NOPASS
+
+// Test the mix of -fsplit-machine-functions and -fno-split-machine-functions
+// RUN: %clang -### -target x86_64-unknown-linux -flto -fsplit-machine-functions -fno-split-machine-functions %s 2>&1 | FileCheck %s -check-prefix=CHECK-NOPASS
+// RUN: %clang -### -target x86_64-unknown-linux -flto -fno-split-machine-functions -fsplit-machine-functions %s 2>&1 | FileCheck %s -check-prefix=CHECK-PASS
+
+// CHECK-PASS:          "-plugin-opt=-split-machine-functions"
+// CHECK-NOPASS-NOT:    "-plugin-opt=-split-machine-functions"