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

Fixes PR34306.

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 bug).

Based on a patch by Carlo Kok.

Differential Revision: https://reviews.llvm.org/D38769

llvm-svn: 315400

Author: pcc

lld/COFF/LTO.cpp

@@ -64,7 +64,13 @@ static void saveBuffer(StringRef Buffer, const Twine &Path) {
 static std::unique_ptr<lto::LTO> createLTO() {
   lto::Config Conf;
   Conf.Options = InitTargetOptionsFromCodeGenFlags();
-  Conf.RelocModel = Reloc::PIC_;
+  // 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;

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

@@ -0,0 +1,19 @@
+; 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
+}