Add 'unifiedlto' option to gold plugin (llvm#121336)

Option allows using full LTO when linking bitcode files compiled with
unified LTO pipeline.

Author: eleviant

llvm/test/tools/gold/X86/Inputs/unified-lto-foo.ll

@@ -0,0 +1,26 @@
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-linux-gnu"
+
+@_g = dso_local local_unnamed_addr global i32 0, align 4
+@llvm.compiler.used = appending global [1 x ptr] [ptr @_g], section "llvm.metadata"
+
+define dso_local i32 @foo(i32 noundef %0) #0 {
+  %2 = add nsw i32 %0, 42
+  store i32 %2, ptr @_g, align 4
+  ret i32 %2
+}
+
+attributes #0 = { noinline }
+
+
+!llvm.module.flags = !{!0, !1}
+
+!0 = !{i32 1, !"EnableSplitLTOUnit", i32 1}
+!1 = !{i32 1, !"UnifiedLTO", i32 1}
+
+^0 = module: (path: "unified-lto-foo.o", hash: (1995435377, 1643957463, 3737953841, 2630641917, 1043992145))
+^1 = gv: (name: "foo", summaries: (function: (module: ^0, flags: (linkage: external, visibility: default, notEligibleToImport: 0, live: 0, dsoLocal: 1, canAutoHide: 0), insts: 3, funcFlags: (readNone: 0, readOnly: 0, noRecurse: 1, returnDoesNotAlias: 0, noInline: 1, alwaysInline: 0, noUnwind: 1, mayThrow: 0, hasUnknownCall: 0, mustBeUnreachable: 0), refs: (writeonly ^2)))) ; guid = 6699318081062747564
+^2 = gv: (name: "_g", summaries: (variable: (module: ^0, flags: (linkage: external, visibility: default, notEligibleToImport: 0, live: 0, dsoLocal: 1, canAutoHide: 0), varFlags: (readonly: 1, writeonly: 1, constant: 0)))) ; guid = 9713702464056781075
+^3 = gv: (name: "llvm.compiler.used", summaries: (variable: (module: ^0, flags: (linkage: appending, visibility: default, notEligibleToImport: 0, live: 1, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 0, writeonly: 0, constant: 0), refs: (^3)))) ; guid = 9610627770985738006
+^4 = flags: 520
+^5 = blockcount: 0

llvm/test/tools/gold/X86/unified-lto.ll

@@ -0,0 +1,57 @@
+; Check that we can use full LTO with gold plugin when inputs
+; are compiled using unified LTO pipeline
+; RUN: llvm-as %s -o %t.bc
+; RUN: llvm-as %p/Inputs/unified-lto-foo.ll -o %t-foo.bc
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext \
+; RUN:    -m elf_x86_64 \
+; RUN:    -plugin-opt=unifiedlto \
+; RUN:    -plugin-opt=save-temps \
+; RUN:    -u main \
+; RUN:    %t.bc %t-foo.bc \
+; RUN:    -o %t-out
+; RUN: llvm-dis %t-out.0.5.precodegen.bc -o - | FileCheck %s
+
+; Check thin LTO as well
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext \
+; RUN:    -m elf_x86_64 \
+; RUN:    -plugin-opt=unifiedlto \
+; RUN:    -plugin-opt=thinlto \
+; RUN:    -plugin-opt=save-temps \
+; RUN:    -u main \
+; RUN:    %t.bc %t-foo.bc \
+; RUN:    -o %t-out
+; RUN: llvm-dis %t.bc.5.precodegen.bc -o - | FileCheck %s --check-prefix=THIN
+
+; Constant propagation is not supported by thin LTO.
+; With full LTO we fold argument into constant 43
+; CHECK:       define dso_local noundef i32 @main()
+; CHECK-NEXT:    tail call fastcc void @foo()
+; CHECK-NEXT:    ret i32 43
+
+; CHECK:       define internal fastcc void @foo()
+; CHECK-NEXT:    store i32 43, ptr @_g, align 4
+
+; ThinLTO doesn't import foo, because the latter has noinline attribute
+; THIN:      define dso_local i32 @main()
+; THIN-NEXT:   %1 = tail call i32 @foo(i32 noundef 1)
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-linux-gnu"
+
+define dso_local i32 @main() {
+  %1 = tail call i32 @foo(i32 noundef 1)
+  ret i32 %1
+}
+
+declare i32 @foo(i32 noundef)
+
+!llvm.module.flags = !{!0, !1}
+
+!0 = !{i32 1, !"EnableSplitLTOUnit", i32 1}
+!1 = !{i32 1, !"UnifiedLTO", i32 1}
+
+^0 = module: (path: "unified-lto.o", hash: (2850108895, 1189778381, 479678006, 1191715608, 4095117687))
+^1 = gv: (name: "foo") ; guid = 6699318081062747564
+^2 = gv: (name: "main", summaries: (function: (module: ^0, flags: (linkage: external, visibility: default, notEligibleToImport: 0, live: 0, dsoLocal: 1, canAutoHide: 0), insts: 2, funcFlags: (readNone: 0, readOnly: 0, noRecurse: 0, returnDoesNotAlias: 0, noInline: 0, alwaysInline: 0, noUnwind: 1, mayThrow: 0, hasUnknownCall: 0, mustBeUnreachable: 0), calls: ((callee: ^1, tail: 1))))) ; guid = 15822663052811949562
+^3 = flags: 520
+^4 = blockcount: 0

llvm/tools/gold/gold-plugin.cpp

@@ -154,6 +154,8 @@ namespace options {
   static std::string extra_library_path;
   static std::string triple;
   static std::string mcpu;
+  // Tells plugin to use unified lto
+  static bool unifiedlto = false;
   // When the thinlto plugin option is specified, only read the function
   // the information from intermediate files and write a combined
   // global index for the ThinLTO backends.
@@ -248,6 +250,8 @@ namespace options {
       TheOutputType = OT_DISABLE;
     } else if (opt == "emit-asm") {
       TheOutputType = OT_ASM_ONLY;
+    } else if (opt == "unifiedlto") {
+      unifiedlto = true;
     } else if (opt == "thinlto") {
       thinlto = true;
     } else if (opt == "thinlto-index-only") {
@@ -893,6 +897,7 @@ static std::unique_ptr<LTO> createLTO(IndexWriteCallback OnIndexWrite,
   Conf.OptLevel = options::OptLevel;
   Conf.PTO.LoopVectorization = options::OptLevel > 1;
   Conf.PTO.SLPVectorization = options::OptLevel > 1;
+  Conf.PTO.UnifiedLTO = options::unifiedlto;
   Conf.AlwaysEmitRegularLTOObj = !options::obj_path.empty();
 
   if (options::thinlto_index_only) {
@@ -972,8 +977,13 @@ static std::unique_ptr<LTO> createLTO(IndexWriteCallback OnIndexWrite,
   Conf.TimeTraceEnabled = !options::time_trace_file.empty();
   Conf.TimeTraceGranularity = options::time_trace_granularity;
 
+  LTO::LTOKind ltoKind = LTO::LTOK_Default;
+  if (options::unifiedlto)
+    ltoKind =
+        options::thinlto ? LTO::LTOK_UnifiedThin : LTO::LTOK_UnifiedRegular;
   return std::make_unique<LTO>(std::move(Conf), Backend,
-                                options::ParallelCodeGenParallelismLevel);
+                               options::ParallelCodeGenParallelismLevel,
+                               ltoKind);
 }
 
 // Write empty files that may be expected by a distributed build