[clang][driver] Always add LTO options when using GNU toolchain

[tbaeder]

Summary

This is what the GCC driver does as well.

This way, ld.bfd can properly handle inputs from a clang LTO build,
without -flto being specified at link time explicitly.

[tbaeder]

The test failures are pretty obvious about when this fails. All affected builds have an implicit dependency on the LLVMgold.so like this.

[tejohnson]

clang/lib/Driver/ToolChains/Gnu.cpp

558

This will add a whole lot of options besides -plugin, which is the only thing guarded against for lld. But in general I'm not in favor of adding a dependence on having the LTO plugin available to all non-lld links by default.

[serge-sans-paille]

This comment was removed by serge-sans-paille.

[MaskRay]

Request changes according to the outstanding comment "But in general I'm not in favor of adding a dependence on having the LTO plugin available to all non-lld links by default."

Agree with tejohnson that this looks weird.

clang/lib/Driver/ToolChains/Gnu.cpp

if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles)) {
     ToolChain.addFastMathRuntimeIfAvailable(Args, CmdArgs);
   }
 
   Args.AddAllArgs(CmdArgs, options::OPT_L);
   Args.AddAllArgs(CmdArgs, options::OPT_u);
 
   ToolChain.AddFilePathLibArgs(Args, CmdArgs);
 
-  if (D.isUsingLTO()) {
+  // If no LTO option has been specified, add the LTO options anyway.
+  // This has the upside of making the linking automatically work with
+  // LLVM bitcode files created from an LTO compile. If the input is not
+  // an LTO file, the LTO plugin will not be invoked.
+  // This is the same behavior the GCC driver uses.
+  if ((!Args.hasArg(options::OPT_flto_EQ) &&
+       !Args.hasArg(options::OPT_fno_lto)) ||
+      D.isUsingLTO()) {
     assert(!Inputs.empty() && "Must have at least one input.");
     addLTOOptions(ToolChain, Args, CmdArgs, Output, Inputs[0],
                   D.getLTOMode() == LTOK_Thin);
   }
 
   if (Args.hasArg(options::OPT_Z_Xlinker__no_demangle))
     CmdArgs.push_back("--no-demangle");
 
   bool NeedsSanitizerDeps = addSanitizerRuntimes(ToolChain, Args, CmdArgs);
   bool NeedsXRayDeps = addXRayRuntime(ToolChain, Args, CmdArgs);

clang/test/Driver/lto.c

 // FLTO-FULL-NOT: -flto=thin
 // FLTO-FULL: -flto=full
 // FLTO-FULL-NOT: -flto=thin
 //
 // FLTO-THIN-NOT: -flto=full
 // FLTO-THIN-NOT: "-flto"
 // FLTO-THIN: -flto=thin
 // FLTO-THIN-NOT: "-flto"
-// FLTO-THIN-NOT: -flto=full
+// FLTO-THIN-NOT: -flto=full
+
+// When ld.bfd is being used and no lto option has been specified, the driver
+// should pass the lto plugin options to the linker anyway, so LTO
+// input files can be linked.
+// RUN: %clang -target x86_64-unknown-linux -fuse-ld=bfd -### %s 2>&1 | \
+// RUN: FileCheck --check-prefix=FLTO-NOT-SPECIFIED-BFD %s
+//
+// FLTO-NOT-SPECIFIED-BFD: "-plugin" "{{.*}}{{[/\\]}}LLVMgold.{{dll|dylib|so}}"
+//
+//
+// But not when LLD is used:
+// RUN: %clang -target x86_64-unknown-linux -fuse-ld=lld -### %s 2>&1 | \
+// RUN: FileCheck --check-prefix=FLTO-NOT-SPECIFIED-LLD %s
+//
+// FLTO-NOT-SPECIFIED-LLD-NOT: "-plugin" "{{.*}}{{[/\\]}}LLVMgold.{{dll|dylib|so}}"