Add function merger to be run during LTO link with gold plugin (#121343)

Patch adds 'merge-functions' plugin option for this purpose.

Author: eleviant

llvm/test/tools/gold/X86/Inputs/merge-functions-foo.ll

@@ -0,0 +1,25 @@
+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 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, !"ThinLTO", i32 0}
+!1 = !{i32 1, !"EnableSplitLTOUnit", i32 1}
+
+^0 = module: (path: "/tmp/func2.o", hash: (0, 0, 0, 0, 0))
+^1 = gv: (name: "foo", summaries: (function: (module: ^0, flags: (linkage: external, visibility: default, notEligibleToImport: 1, 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: (^3)))) ; guid = 6699318081062747564
+^2 = gv: (name: "llvm.compiler.used", summaries: (variable: (module: ^0, flags: (linkage: appending, visibility: default, notEligibleToImport: 1, live: 1, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 0, writeonly: 0, constant: 0), refs: (^3)))) ; guid = 9610627770985738006
+^3 = gv: (name: "_g", summaries: (variable: (module: ^0, flags: (linkage: external, visibility: default, notEligibleToImport: 1, live: 0, dsoLocal: 1, canAutoHide: 0), varFlags: (readonly: 1, writeonly: 1, constant: 0)))) ; guid = 9713702464056781075
+^4 = flags: 8
+^5 = blockcount: 0

llvm/test/tools/gold/X86/merge-functions.ll

@@ -0,0 +1,49 @@
+; RUN: llvm-as %s -o %t.bc
+; RUN: llvm-as %p/Inputs/merge-functions-foo.ll -o %t-foo.bc
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold%shlibext \
+; RUN:    -m elf_x86_64 \
+; RUN:    -plugin-opt=merge-functions \
+; 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 that we've merged foo and bar
+; CHECK:      define dso_local noundef i32 @main()
+; CHECK-NEXT:   tail call fastcc void @bar()
+; CHECK-NEXT:   tail call fastcc void @bar()
+
+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 = external local_unnamed_addr global i32, align 4
+
+define dso_local i32 @bar(i32 noundef %0) #0 {
+  %2 = add nsw i32 %0, 42
+  store i32 %2, ptr @_g, align 4
+  ret i32 %2
+}
+
+define dso_local noundef i32 @main() {
+  %1 = tail call i32 @foo(i32 noundef 1)
+  %2 = tail call i32 @bar(i32 noundef 1)
+  ret i32 0
+}
+
+declare i32 @foo(i32 noundef) local_unnamed_addr #2
+
+attributes #0 = { noinline }
+
+!llvm.module.flags = !{!0, !1}
+
+!0 = !{i32 1, !"ThinLTO", i32 0}
+!1 = !{i32 1, !"EnableSplitLTOUnit", i32 1}
+
+^0 = module: (path: "merge-functions.o", hash: (0, 0, 0, 0, 0))
+^1 = gv: (name: "foo") ; guid = 6699318081062747564
+^2 = gv: (name: "_g") ; guid = 9713702464056781075
+^3 = gv: (name: "main", summaries: (function: (module: ^0, flags: (linkage: external, visibility: default, notEligibleToImport: 1, live: 0, dsoLocal: 1, canAutoHide: 0), insts: 3, 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), (callee: ^4, tail: 1))))) ; guid = 15822663052811949562
+^4 = gv: (name: "bar", summaries: (function: (module: ^0, flags: (linkage: external, visibility: default, notEligibleToImport: 1, 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: (^2)))) ; guid = 16434608426314478903
+^5 = flags: 8
+^6 = blockcount: 0

llvm/tools/gold/gold-plugin.cpp

@@ -224,6 +224,9 @@ namespace options {
   static std::string cs_profile_path;
   static bool cs_pgo_gen = false;
 
+  // When true, MergeFunctions pass is used in LTO link pipeline.
+  static bool merge_functions = false;
+
   // Time trace options.
   static std::string time_trace_file;
   static unsigned time_trace_granularity = 500;
@@ -292,6 +295,8 @@ namespace options {
       sample_profile = std::string(opt);
     } else if (opt == "cs-profile-generate") {
       cs_pgo_gen = true;
+    } else if (opt == "merge-functions") {
+      merge_functions = true;
     } else if (opt.consume_front("cs-profile-path=")) {
       cs_profile_path = std::string(opt);
     } else if (opt == "new-pass-manager") {
@@ -897,6 +902,7 @@ 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.MergeFunctions = options::merge_functions;
   Conf.PTO.UnifiedLTO = options::unifiedlto;
   Conf.AlwaysEmitRegularLTOObj = !options::obj_path.empty();