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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld

Author: Teresa Johnson (teresajohnson)

<details>
<summary>Changes</summary>

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


---
Full diff: https://github.com/llvm/llvm-project/pull/120143.diff


4 Files Affected:

- (modified) lld/test/ELF/lto/new-pass-manager.ll (+4) 
- (modified) llvm/lib/LTO/LTOBackend.cpp (+25-5) 
- (modified) llvm/test/Feature/load_plugin_error.ll (+1-1) 
- (modified) llvm/test/Other/X86/lto-hot-cold-split.ll (+4) 


``````````diff
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
+}

``````````

</details>


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