[lld] [llvm] [ThinLTO] Skip opt pipeline and summary wrapper pass on empty modules (PR #120143)

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson created https://github.com/llvm/llvm-project/pull/120143

Follow up to PR118508, to avoid unnecessary compile time for an empty
combind regular LTO module if all modules end up being ThinLTO only.


>From 599cde6bd3fd0cbba3d589699170fd95ef10b630 Mon Sep 17 00:00:00 2001
From: Teresa Johnson <tejohnson at google.com>
Date: Mon, 16 Dec 2024 12:48:17 -0800
Subject: [PATCH] [ThinLTO] Skip opt pipeline and summary wrapper pass on empty
 modules

Follow up to PR118508, to avoid unnecessary compile time for an empty
combind regular LTO module if all modules end up being ThinLTO only.
---
 lld/test/ELF/lto/new-pass-manager.ll      |  4 +++
 llvm/lib/LTO/LTOBackend.cpp               | 30 +++++++++++++++++++----
 llvm/test/Feature/load_plugin_error.ll    |  2 +-
 llvm/test/Other/X86/lto-hot-cold-split.ll |  4 +++
 4 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/lld/test/ELF/lto/new-pass-manager.ll b/lld/test/ELF/lto/new-pass-manager.ll
index cc6ff34cd91ae3..77a1d4ed3d27d2 100644
--- a/lld/test/ELF/lto/new-pass-manager.ll
+++ b/lld/test/ELF/lto/new-pass-manager.ll
@@ -9,3 +9,7 @@
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
+
+define void @foo() {
+  ret void
+}
diff --git a/llvm/lib/LTO/LTOBackend.cpp b/llvm/lib/LTO/LTOBackend.cpp
index bdf4ff8960bc82..51bd7cc29c7699 100644
--- a/llvm/lib/LTO/LTOBackend.cpp
+++ b/llvm/lib/LTO/LTOBackend.cpp
@@ -351,6 +351,13 @@ static void runNewPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
   MPM.run(Mod, MAM);
 }
 
+static bool isEmptyModule(const Module &Mod) {
+  // Module is empty if it has no functions, no globals, no inline asm and no
+  // named metadata (aliases and ifuncs require functions or globals so we
+  // don't need to check those explicitly).
+  return Mod.empty() && Mod.global_empty() && Mod.named_metadata_empty() && Mod.getModuleInlineAsm().empty();
+}
+
 bool lto::opt(const Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
               bool IsThinLTO, ModuleSummaryIndex *ExportSummary,
               const ModuleSummaryIndex *ImportSummary,
@@ -372,9 +379,16 @@ bool lto::opt(const Config &Conf, TargetMachine *TM, unsigned Task, Module &Mod,
                                /*EmbedBitcode*/ true, /*EmbedCmdline*/ true,
                                /*Cmdline*/ CmdArgs);
   }
-  // FIXME: Plumb the combined index into the new pass manager.
-  runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
-                 ImportSummary);
+  // No need to run any opt passes if the module is empty.
+  // In theory these passes should take almost no time for an empty
+  // module, however, this guards against doing any unnecessary summary-based
+  // analysis in the case of a ThinLTO build where this might be an empty
+  // regular LTO combined module, with a large combined index from ThinLTO.
+  if (!isEmptyModule(Mod)) {
+    // FIXME: Plumb the combined index into the new pass manager.
+    runNewPMPasses(Conf, Mod, TM, Conf.OptLevel, IsThinLTO, ExportSummary,
+                   ImportSummary);
+  }
   return !Conf.PostOptModuleHook || Conf.PostOptModuleHook(Task, Mod);
 }
 
@@ -422,8 +436,14 @@ static void codegen(const Config &Conf, TargetMachine *TM,
   legacy::PassManager CodeGenPasses;
   TargetLibraryInfoImpl TLII(Triple(Mod.getTargetTriple()));
   CodeGenPasses.add(new TargetLibraryInfoWrapperPass(TLII));
-  CodeGenPasses.add(
-      createImmutableModuleSummaryIndexWrapperPass(&CombinedIndex));
+  // No need to make index available if the module is empty.
+  // In theory these passes should not use the index for an empty
+  // module, however, this guards against doing any unnecessary summary-based
+  // analysis in the case of a ThinLTO build where this might be an empty
+  // regular LTO combined module, with a large combined index from ThinLTO.
+  if (!isEmptyModule(Mod))
+    CodeGenPasses.add(
+        createImmutableModuleSummaryIndexWrapperPass(&CombinedIndex));
   if (Conf.PreCodeGenPassesHook)
     Conf.PreCodeGenPassesHook(CodeGenPasses);
   if (TM->addPassesToEmitFile(CodeGenPasses, *Stream->OS,
diff --git a/llvm/test/Feature/load_plugin_error.ll b/llvm/test/Feature/load_plugin_error.ll
index a112bfbb1cd193..b40dddff1205fc 100644
--- a/llvm/test/Feature/load_plugin_error.ll
+++ b/llvm/test/Feature/load_plugin_error.ll
@@ -5,7 +5,7 @@
 
 ; RUN: opt %s -o %t.o
 ; RUN: not llvm-lto2 run -load-pass-plugin=%t/nonexistent.so %t.o -o %t \
-; RUN:     -r %t.o,test 2>&1 | \
+; RUN:     -r %t.o,test,plx 2>&1 | \
 ; RUN:   FileCheck %s
 
 ; CHECK: Could not load library {{.*}}nonexistent.so
diff --git a/llvm/test/Other/X86/lto-hot-cold-split.ll b/llvm/test/Other/X86/lto-hot-cold-split.ll
index 22c79c7e06bba6..24903f34ed074b 100644
--- a/llvm/test/Other/X86/lto-hot-cold-split.ll
+++ b/llvm/test/Other/X86/lto-hot-cold-split.ll
@@ -10,3 +10,7 @@ target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; OLDPM-ANYLTO-POSTLINK-Os: HotColdSplittingPass
+
+define void @foo() {
+  ret void
+}