COFF: When generating code for LTO, use static reloc model on 32-bit x86.

[pcc]

Summary

This is because it usually results in more compact code, and because
there are also known code generation bugs when using the PIC model
(see PR34306).

Based on a patch by Carlo Kok.

[rnk]

lgtm, clang uses static as well:
$ clang -m32 -c t.c -### |& sed -e 's/" "/\n/g' | grep -A1 reloc
-mrelocation-model
static

lld/COFF/LTO.cpp

static void saveBuffer(StringRef Buffer, const Twine &Path) {
   if (EC)
     error("cannot create " + Path + ": " + EC.message());
   OS << Buffer;
 }
 
 static std::unique_ptr<lto::LTO> createLTO() {
   lto::Config Conf;
   Conf.Options = InitTargetOptionsFromCodeGenFlags();
+  // Use static reloc model on 32-bit x86 because it usually results in more
+  // compact code, and because there are also known code generation bugs when
+  // using the PIC model (see PR34306).
+  if (Config->Machine == COFF::IMAGE_FILE_MACHINE_I386)
+    Conf.RelocModel = Reloc::Static;
+  else
     Conf.RelocModel = Reloc::PIC_;
   Conf.DisableVerify = true;
   Conf.DiagHandler = diagnosticHandler;
   Conf.OptLevel = Config->LTOOptLevel;
   if (Config->SaveTemps)
     checkError(Conf.addSaveTemps(std::string(Config->OutputFile) + ".",
                                  /*UseInputModulePath*/ true));
   lto::ThinBackend Backend;
   if (Config->LTOJobs != 0)

lld/test/COFF/lto-reloc-model.ll

+; RUN: llvm-as -o %t %s
+; RUN: lld-link /entry:main /subsystem:console /out:%t.exe %t
+; RUN: llvm-objdump -d %t.exe | FileCheck %s
+
+target datalayout = "e-m:x-p:32:32-i64:64-f80:32-n8:16:32-a:0:32-S32"
+target triple = "i686-pc-windows-msvc"
+
+@foo = thread_local global i8 0
+
+module asm "__tls_index = 1"
+module asm "__tls_array = 2"
+
+define i8* @main() {
+  ; CHECK: movl 1, %eax
+  ; CHECK: movl %fs:2, %ecx
+  ; CHECK: movl (%ecx,%eax,4), %eax
+  ; CHECK: leal (%eax), %eax
+  ret i8* @foo
+}