[ELF] Support explicitly overriding relocation model in LTO

[phosek]

Summary

lld currently selects the relocation model automatically depending on
the link flags specified, but in some cases it'd be useful to allow
explicitly overriding the relocation model using a flag.

[pcc]

LGTM

lld/test/ELF/lto/relocation-model.ll

41

I think you shouldn't need the --export-dynamic --noinhibit-exec part here, maybe try removing them.

lld/Common/TargetOptionsCommandFlags.cpp

 // Define an externally visible version of
 // initTargetOptionsFromCodeGenFlags, so that its functionality can be
 // used without having to include llvm/CodeGen/CommandFlags.inc, which
 // would lead to multiple definitions of the command line flags.
 llvm::TargetOptions lld::initTargetOptionsFromCodeGenFlags() {
   return ::InitTargetOptionsFromCodeGenFlags();
 }
 
+llvm::Optional<llvm::Reloc::Model> lld::getRelocModelFromCMModel() {
+  return getRelocModel();
+}
+
 llvm::Optional<llvm::CodeModel::Model> lld::getCodeModelFromCMModel() {
   return getCodeModel();
 }
 
 std::string lld::getCPUStr() { return ::getCPUStr(); }
 
 std::vector<std::string> lld::getMAttrs() { return ::MAttrs; }

lld/ELF/LTO.cpp

static lto::Config createConfig() {
   c.Options = initTargetOptionsFromCodeGenFlags();
   c.Options.RelaxELFRelocations = true;
   c.Options.EmitAddrsig = true;
 
   // Always emit a section per function/datum with LTO.
   c.Options.FunctionSections = true;
   c.Options.DataSections = true;
 
-  if (config->relocatable)
+  if (auto relocModel = getRelocModelFromCMModel())
+    c.RelocModel = *relocModel;
+  else if (config->relocatable)
     c.RelocModel = None;
   else if (config->isPic)
     c.RelocModel = Reloc::PIC_;
   else
     c.RelocModel = Reloc::Static;
 
   c.CodeModel = getCodeModelFromCMModel();
   c.DisableVerify = config->disableVerify;

lld/include/lld/Common/TargetOptionsCommandFlags.h

 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/Optional.h"
 #include "llvm/Support/CodeGen.h"
 #include "llvm/Target/TargetOptions.h"
 
 namespace lld {
 llvm::TargetOptions initTargetOptionsFromCodeGenFlags();
+llvm::Optional<llvm::Reloc::Model> getRelocModelFromCMModel();
 llvm::Optional<llvm::CodeModel::Model> getCodeModelFromCMModel();
 std::string getCPUStr();
 std::vector<std::string> getMAttrs();
 }

lld/test/ELF/lto/relocation-model.ll

 
 ; RUN: ld.lld %t.pic.o -o %t-out -save-temps --export-dynamic --noinhibit-exec
 ; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=STATIC
 
 ; RUN: ld.lld %t.pic.o -o %t-out -save-temps -r --export-dynamic
 ; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=PIC
 
 
+;; Explicit flag.
+
+; RUN: ld.lld %t.o -o %t-out -save-temps -r -mllvm -relocation-model=pic
+; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=PIC
+
+; RUN: ld.lld %t.o -o %t-out -save-temps -r -mllvm -relocation-model=static
+; RUN: llvm-readobj -r %t-out.lto.o | FileCheck %s --check-prefix=STATIC
+
+
 ; PIC: R_X86_64_REX_GOTPCRELX foo
 ; STATIC: R_X86_64_PC32 foo
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 @foo = external global i32
 define i32 @main() {
   %t = load i32, i32* @foo
   ret i32 %t
 }