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

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.

Reviewed By: phosek, vit9696

Differential Revision: https://reviews.llvm.org/D147834

(cherry picked from commit a78816a)

Author: ilovepi

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -757,6 +757,13 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
       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(

clang/test/Driver/emulated-tls.cpp

@@ -29,6 +29,16 @@
 // 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"
@@ -40,3 +50,10 @@
 
 // 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