[clang][driver] Pass `-femulated-tls` through to the linker in LTO mode

[paulkirth]

Summary

Currently the driver does not propagate the -f[no-]emulated-tls flags
to the linker under LTO. This can be surprising when the platform
defaults differ from the flags being passed. A related discussion can be
found in https://reviews.llvm.org/D143619. While the focus there was
RISC-V support, the root cause was that setting -femualted-tls and
-flto when compiling with Clang resulted in missing symbols because
the platform defaults for Android differed from the flags being passed
to Clang.

This patch changes the Clang driver's behavior to pass the emulated-tls
flags through to the linker when compiling with LTO/ThinLTO.

[vit9696]

Looks good to me, thank you!

clang/test/Driver/emulated-tls.cpp

55

Maybe add a space before LTO here?

[paulkirth]

Fix formatting in CHECK lines.

[paulkirth]

clang/test/Driver/emulated-tls.cpp

55

Good catch. For whatever reason I thought clang-format had fixed that, but obviously not. :)

[paulkirth]

Remove extra newlines in emulated-tls.cpp

[phosek]

LGTM

clang/lib/Driver/ToolChains/CommonArgs.cpp

#endif
   if (Args.hasFlag(options::OPT_fjmc, options::OPT_fno_jmc, false)) {
     if (ToolChain.getEffectiveTriple().isOSBinFormatELF())
       CmdArgs.push_back(Args.MakeArgString(Twine(PluginOptPrefix) +
                                            "-enable-jmc-instrument"));
     else
       D.Diag(clang::diag::warn_drv_fjmc_for_elf_only);
   }
 
+  if (Arg *A = Args.getLastArg(options::OPT_femulated_tls,
+                               options::OPT_fno_emulated_tls)) {
+    bool Enable = A->getOption().getID() == options::OPT_femulated_tls;
+    CmdArgs.push_back(Args.MakeArgString(
+        Twine(PluginOptPrefix) + "-emulated-tls=" + (Enable ? "1" : "0")));
+  }
+
   if (Args.hasFlag(options::OPT_fstack_size_section,
                    options::OPT_fno_stack_size_section, false))
     CmdArgs.push_back(
         Args.MakeArgString(Twine(PluginOptPrefix) + "-stack-size-section"));
 
   // Setup statistics file output.
   SmallString<128> StatsFile = getStatsFileName(Args, Output, Input, D);
   if (!StatsFile.empty())

clang/test/Driver/emulated-tls.cpp

 // RUN: | FileCheck -check-prefix=NOEMU %s
 // RUN: %clang -### -target arm-linux-gnu %s -femulated-tls -fno-emulated-tls 2>&1 \
 // RUN: | FileCheck -check-prefix=NOEMU %s
 // RUN: %clang -### -target i686-pc-cygwin %s -femulated-tls -fno-emulated-tls 2>&1 \
 // RUN: | FileCheck -check-prefix=NOEMU %s
 // RUN: %clang -### -target i686-pc-openbsd %s -femulated-tls -fno-emulated-tls 2>&1 \
 // RUN: | FileCheck -check-prefix=NOEMU %s
 
+// Test that when lto is used any -emualted-tls flags are passed to the linker
+// LINUX and Android have different defaults for EmulatedTLS
+// RUN: %clang -### -flto --target=riscv64-linux -fno-emulated-tls %s 2>&1 \
+// RUN: | FileCheck %s --check-prefix=LTO_NOEMUTLS
+// RUN: %clang -### -flto --target=riscv64-linux-android10000 -femulated-tls %s 2>&1 \
+// RUN: | FileCheck %s --check-prefix=LTO_EMUTLS
+// RUN: %clang -### -flto --target=riscv64-linux -femulated-tls %s 2>&1 \
+// RUN: | FileCheck %s --check-prefix=LTO_EMUTLS
+// RUN: %clang -### -flto --target=riscv64-linux-android10000 -fno-emulated-tls %s 2>&1 \
+// RUN: | FileCheck %s --check-prefix=LTO_NOEMUTLS
 
 // Default without -f[no-]emulated-tls, will be decided by the target triple.
 // DEFAULT-NOT: "-cc1" {{.*}}"-femulated-tls"
 // DEFAULT-NOT: "-cc1" {{.*}}"-fno-emulated-tls"
 
 // Explicit and last -f[no-]emulated-tls flag will be passed to cc1.
 // EMU: "-cc1" {{.*}}"-femulated-tls"
 // EMU-NOT: "-cc1" {{.*}}"-fno-emulated-tls"
 
 // NOEMU: "-cc1" {{.*}}"-fno-emulated-tls"
 // NOEMU-NOT: "-cc1" {{.*}}"-femulated-tls"
+
+// LTO related checks
+// LTO_NOEMUTLS: plugin-opt=-emulated-tls=0
+// LTO_NOEMUTLS-NOT: plugin-opt=-emulated-tls=1
+
+// LTO_EMUTLS: plugin-opt=-emulated-tls=1
+// LTO_EMUTLS-NOT: plugin-opt=-emulated-tls=0