[clang] [llvm] [clang][llvm][fatlto] Avoid cloning modules in FatLTO (PR #72180)
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Mon Nov 13 16:48:40 PST 2023
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-lto
@llvm/pr-subscribers-clang
Author: Paul Kirth (ilovepi)
<details>
<summary>Changes</summary>
https://github.com/llvm/llvm-project/issues/70703 pointed out that cloning LLVM modules could lead to miscompiles when using FatLTO.
This is due to an existing issue when cloning modules with labels (see #<!-- -->55991 and #<!-- -->47769). Since this can lead to miscompilation, we can avoid cloning the LLVM modules, which was desirable anyway.
This patch modifies the EmbedBitcodePass to no longer clone the module or run an input pipeline over it. Further, it make FatLTO always perform UnifiedLTO, so we can still defer the Thin/Full LTO decision to link-time. Lastly, it removes dead/obsolete code related to now defunct options that do not work with the EmbedBitcodePass implementation any longer.
---
Full diff: https://github.com/llvm/llvm-project/pull/72180.diff
12 Files Affected:
- (modified) clang/lib/CodeGen/BackendUtil.cpp (+4-5)
- (modified) clang/lib/Driver/ToolChains/Clang.cpp (+3-1)
- (modified) clang/test/CodeGen/fat-lto-objects.c (+5-5)
- (modified) llvm/docs/FatLTO.rst (+18-17)
- (modified) llvm/include/llvm/Passes/PassBuilder.h (+1-2)
- (modified) llvm/include/llvm/Transforms/IPO/EmbedBitcodePass.h (+1-16)
- (modified) llvm/lib/Passes/PassBuilder.cpp (-20)
- (modified) llvm/lib/Passes/PassBuilderPipelines.cpp (+4-7)
- (modified) llvm/lib/Passes/PassRegistry.def (+1-7)
- (modified) llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp (+1-15)
- (modified) llvm/test/CodeGen/X86/fat-lto-section.ll (+1-1)
- (modified) llvm/test/Transforms/EmbedBitcode/embed.ll (-3)
``````````diff
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index a7a47d17723cb73..4114860545ade1b 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -810,7 +810,7 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
// Only enable CGProfilePass when using integrated assembler, since
// non-integrated assemblers don't recognize .cgprofile section.
PTO.CallGraphProfile = !CodeGenOpts.DisableIntegratedAS;
- PTO.UnifiedLTO = CodeGenOpts.UnifiedLTO;
+ PTO.UnifiedLTO = CodeGenOpts.UnifiedLTO || CodeGenOpts.FatLTO;
LoopAnalysisManager LAM;
FunctionAnalysisManager FAM;
@@ -996,9 +996,7 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
}
if (CodeGenOpts.FatLTO) {
- MPM.addPass(PB.buildFatLTODefaultPipeline(
- Level, PrepareForThinLTO,
- PrepareForThinLTO || shouldEmitRegularLTOSummary()));
+ MPM.addPass(PB.buildFatLTODefaultPipeline(Level));
} else if (PrepareForThinLTO) {
MPM.addPass(PB.buildThinLTOPreLinkDefaultPipeline(Level));
} else if (PrepareForLTO) {
@@ -1073,7 +1071,8 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
if (!TheModule->getModuleFlag("EnableSplitLTOUnit"))
TheModule->addModuleFlag(llvm::Module::Error, "EnableSplitLTOUnit",
uint32_t(CodeGenOpts.EnableSplitLTOUnit));
- if (CodeGenOpts.UnifiedLTO && !TheModule->getModuleFlag("UnifiedLTO"))
+ // FatLTO always means UnifiedLTO
+ if (!TheModule->getModuleFlag("UnifiedLTO"))
TheModule->addModuleFlag(llvm::Module::Error, "UnifiedLTO", uint32_t(1));
}
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 3b98c7ae6e6ec66..f4cd9cbc5eccdec 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -4845,7 +4845,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
bool UnifiedLTO = false;
if (IsUsingLTO) {
UnifiedLTO = Args.hasFlag(options::OPT_funified_lto,
- options::OPT_fno_unified_lto, Triple.isPS());
+ options::OPT_fno_unified_lto, Triple.isPS()) ||
+ Args.hasFlag(options::OPT_ffat_lto_objects,
+ options::OPT_fno_fat_lto_objects, false);
if (UnifiedLTO)
CmdArgs.push_back("-funified-lto");
}
diff --git a/clang/test/CodeGen/fat-lto-objects.c b/clang/test/CodeGen/fat-lto-objects.c
index 2c3a4ef9c615529..6085762fa5a2467 100644
--- a/clang/test/CodeGen/fat-lto-objects.c
+++ b/clang/test/CodeGen/fat-lto-objects.c
@@ -9,22 +9,22 @@
// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=full -ffat-lto-objects -fsplit-lto-unit -emit-obj < %s -o %t.full.split.o
// RUN: llvm-readelf -S %t.full.split.o | FileCheck %s --check-prefixes=ELF
// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.full.split.bc %t.full.split.o
-// RUN: llvm-dis %t.full.split.bc -o - | FileCheck %s --check-prefixes=FULL,SPLIT,NOUNIFIED
+// RUN: llvm-dis %t.full.split.bc -o - | FileCheck %s --check-prefixes=FULL,SPLIT,UNIFIED
// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=full -ffat-lto-objects -emit-obj < %s -o %t.full.nosplit.o
// RUN: llvm-readelf -S %t.full.nosplit.o | FileCheck %s --check-prefixes=ELF
// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.full.nosplit.bc %t.full.nosplit.o
-// RUN: llvm-dis %t.full.nosplit.bc -o - | FileCheck %s --check-prefixes=FULL,NOSPLIT,NOUNIFIED
+// RUN: llvm-dis %t.full.nosplit.bc -o - | FileCheck %s --check-prefixes=FULL,NOSPLIT,UNIFIED
// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=thin -fsplit-lto-unit -ffat-lto-objects -emit-obj < %s -o %t.thin.split.o
// RUN: llvm-readelf -S %t.thin.split.o | FileCheck %s --check-prefixes=ELF
// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.thin.split.bc %t.thin.split.o
-// RUN: llvm-dis %t.thin.split.bc -o - | FileCheck %s --check-prefixes=THIN,SPLIT,NOUNIFIED
+// RUN: llvm-dis %t.thin.split.bc -o - | FileCheck %s --check-prefixes=THIN,SPLIT,UNIFIED
// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=thin -ffat-lto-objects -emit-obj < %s -o %t.thin.nosplit.o
// RUN: llvm-readelf -S %t.thin.nosplit.o | FileCheck %s --check-prefixes=ELF
// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.thin.nosplit.bc %t.thin.nosplit.o
-// RUN: llvm-dis %t.thin.nosplit.bc -o - | FileCheck %s --check-prefixes=THIN,NOSPLIT,NOUNIFIED
+// RUN: llvm-dis %t.thin.nosplit.bc -o - | FileCheck %s --check-prefixes=THIN,NOSPLIT,UNIFIED
// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=thin -funified-lto -ffat-lto-objects -emit-obj < %s -o %t.unified.o
// RUN: llvm-readelf -S %t.unified.o | FileCheck %s --check-prefixes=ELF
@@ -42,8 +42,8 @@
// SPLIT: ![[#]] = !{i32 1, !"EnableSplitLTOUnit", i32 1}
// NOSPLIT: ![[#]] = !{i32 1, !"EnableSplitLTOUnit", i32 0}
+/// FatLTO always uses UnifiedLTO
// UNIFIED: ![[#]] = !{i32 1, !"UnifiedLTO", i32 1}
-// NOUNIFIED-NOT: ![[#]] = !{i32 1, !"UnifiedLTO", i32 1}
// ELF: .llvm.lto
diff --git a/llvm/docs/FatLTO.rst b/llvm/docs/FatLTO.rst
index b505bb2a96fe160..0e424f694b1bf09 100644
--- a/llvm/docs/FatLTO.rst
+++ b/llvm/docs/FatLTO.rst
@@ -29,30 +29,31 @@ Overview
Within LLVM, FatLTO is supported by choosing the ``FatLTODefaultPipeline``.
This pipeline will:
-#) Clone the IR module.
-#) Run the pre-link (Thin)LTO pipeline using the cloned module.
+#) Run the pre-link UnifiedLTO pipeline on the current module.
#) Embed the pre-link bitcode in a special ``.llvm.lto`` section.
-#) Optimize the unmodified copy of the module using the normal compilation pipeline.
+#) Finish optimizing the module using the post-link ThinLTO pipeline.
#) Emit the object file, including the new ``.llvm.lto`` section.
.. NOTE
- At the time of writing, we conservatively run independent pipelines to
- generate the bitcode section and the object code, which happen to be
- identical to those used outside of FatLTO. This results in compiled
- artifacts that are identical to those produced by the default and (Thin)LTO
- pipelines. However, this is not a guarantee, and we reserve the right to
- change this at any time. Explicitly, users should not rely on the produced
- bitcode or object code to mach their non-LTO counterparts precisely. They
- will exhibit similar performance characteristics, but may not be bit-for-bit
- the same.
+ Previously, we conservatively ran independent pipelines on separate copies
+ of the LLVM module to generate the bitcode section and the object code,
+ which happen to be identical to those used outside of FatLTO. While that
+ resulted in compiled artifacts that were identical to those produced by the
+ default and (Thin)LTO pipelines, module cloning led to some cases of
+ miscompilation, and we have moved away from trying to keep bitcode
+ generation and optimization completely disjoint.
+
+ Bit-for-bit compatibility is not (and never was) a guarantee, and we reserve
+ the right to change this at any time. Explicitly, users should not rely on
+ the produced bitcode or object code to mach their non-LTO counterparts
+ precisely. They will exhibit similar performance characteristics, but may
+ not be bit-for-bit the same.
Internally, the ``.llvm.lto`` section is created by running the
-``EmbedBitcodePass`` at the start of the ``PerModuleDefaultPipeline``. This
-pass is responsible for cloning and optimizing the module with the appropriate
-LTO pipeline and emitting the ``.llvm.lto`` section. Afterwards, the
-``PerModuleDefaultPipeline`` runs normally and the compiler can emit the fat
-object file.
+``EmbedBitcodePass`` after the ``ThinLTOPreLinkDefaultPipeline``. This pass is
+responsible for emitting the ``.llvm.lto`` section. Afterwards, the
+``ThinLTODefaultPipeline`` runs and the compiler can emit the fat object file.
Limitations
===========
diff --git a/llvm/include/llvm/Passes/PassBuilder.h b/llvm/include/llvm/Passes/PassBuilder.h
index 23bc891a8f1e97c..19ac90842bcb08d 100644
--- a/llvm/include/llvm/Passes/PassBuilder.h
+++ b/llvm/include/llvm/Passes/PassBuilder.h
@@ -246,8 +246,7 @@ class PassBuilder {
/// separately to avoid any inconsistencies with an ad-hoc pipeline that tries
/// to approximate the PerModuleDefaultPipeline from the pre-link LTO
/// pipelines.
- ModulePassManager buildFatLTODefaultPipeline(OptimizationLevel Level,
- bool ThinLTO, bool EmitSummary);
+ ModulePassManager buildFatLTODefaultPipeline(OptimizationLevel Level);
/// Build a pre-link, ThinLTO-targeting default optimization pipeline to
/// a pass manager.
diff --git a/llvm/include/llvm/Transforms/IPO/EmbedBitcodePass.h b/llvm/include/llvm/Transforms/IPO/EmbedBitcodePass.h
index f323c61483fd30a..c35048c91aba207 100644
--- a/llvm/include/llvm/Transforms/IPO/EmbedBitcodePass.h
+++ b/llvm/include/llvm/Transforms/IPO/EmbedBitcodePass.h
@@ -25,28 +25,13 @@ class Module;
class ModulePass;
class Pass;
-struct EmbedBitcodeOptions {
- EmbedBitcodeOptions() : EmbedBitcodeOptions(false, false) {}
- EmbedBitcodeOptions(bool IsThinLTO, bool EmitLTOSummary)
- : IsThinLTO(IsThinLTO), EmitLTOSummary(EmitLTOSummary) {}
- bool IsThinLTO;
- bool EmitLTOSummary;
-};
-
/// Pass embeds a copy of the module optimized with the provided pass pipeline
/// into a global variable.
class EmbedBitcodePass : public PassInfoMixin<EmbedBitcodePass> {
- bool IsThinLTO;
- bool EmitLTOSummary;
ModulePassManager MPM;
public:
- EmbedBitcodePass(EmbedBitcodeOptions Opts)
- : EmbedBitcodePass(Opts.IsThinLTO, Opts.EmitLTOSummary,
- ModulePassManager()) {}
- EmbedBitcodePass(bool IsThinLTO, bool EmitLTOSummary, ModulePassManager &&MPM)
- : IsThinLTO(IsThinLTO), EmitLTOSummary(EmitLTOSummary),
- MPM(std::move(MPM)) {}
+ EmbedBitcodePass() {}
PreservedAnalyses run(Module &M, ModuleAnalysisManager &);
diff --git a/llvm/lib/Passes/PassBuilder.cpp b/llvm/lib/Passes/PassBuilder.cpp
index dd9d799f9d55dcc..b2e45b50898640f 100644
--- a/llvm/lib/Passes/PassBuilder.cpp
+++ b/llvm/lib/Passes/PassBuilder.cpp
@@ -757,26 +757,6 @@ Expected<HWAddressSanitizerOptions> parseHWASanPassOptions(StringRef Params) {
return Result;
}
-Expected<EmbedBitcodeOptions> parseEmbedBitcodePassOptions(StringRef Params) {
- EmbedBitcodeOptions Result;
- while (!Params.empty()) {
- StringRef ParamName;
- std::tie(ParamName, Params) = Params.split(';');
-
- if (ParamName == "thinlto") {
- Result.IsThinLTO = true;
- } else if (ParamName == "emit-summary") {
- Result.EmitLTOSummary = true;
- } else {
- return make_error<StringError>(
- formatv("invalid EmbedBitcode pass parameter '{0}' ", ParamName)
- .str(),
- inconvertibleErrorCode());
- }
- }
- return Result;
-}
-
Expected<MemorySanitizerOptions> parseMSanPassOptions(StringRef Params) {
MemorySanitizerOptions Result;
while (!Params.empty()) {
diff --git a/llvm/lib/Passes/PassBuilderPipelines.cpp b/llvm/lib/Passes/PassBuilderPipelines.cpp
index f3d280316e04077..80c191f880087c5 100644
--- a/llvm/lib/Passes/PassBuilderPipelines.cpp
+++ b/llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -1530,14 +1530,11 @@ PassBuilder::buildPerModuleDefaultPipeline(OptimizationLevel Level,
}
ModulePassManager
-PassBuilder::buildFatLTODefaultPipeline(OptimizationLevel Level, bool ThinLTO,
- bool EmitSummary) {
+PassBuilder::buildFatLTODefaultPipeline(OptimizationLevel Level) {
ModulePassManager MPM;
- MPM.addPass(EmbedBitcodePass(ThinLTO, EmitSummary,
- ThinLTO
- ? buildThinLTOPreLinkDefaultPipeline(Level)
- : buildLTOPreLinkDefaultPipeline(Level)));
- MPM.addPass(buildPerModuleDefaultPipeline(Level));
+ MPM.addPass(buildThinLTOPreLinkDefaultPipeline(Level));
+ MPM.addPass(EmbedBitcodePass());
+ MPM.addPass(buildThinLTODefaultPipeline(Level, /*ImportSummary=*/nullptr));
return MPM;
}
diff --git a/llvm/lib/Passes/PassRegistry.def b/llvm/lib/Passes/PassRegistry.def
index 2067fc473b522db..9961312b7a59022 100644
--- a/llvm/lib/Passes/PassRegistry.def
+++ b/llvm/lib/Passes/PassRegistry.def
@@ -59,6 +59,7 @@ MODULE_PASS("debugify", NewPMDebugifyPass())
MODULE_PASS("dot-callgraph", CallGraphDOTPrinterPass())
MODULE_PASS("dxil-upgrade", DXILUpgradePass())
MODULE_PASS("elim-avail-extern", EliminateAvailableExternallyPass())
+MODULE_PASS("embed-bitcode", EmbedBitcodePass())
MODULE_PASS("extract-blocks", BlockExtractorPass({}, false))
MODULE_PASS("forceattrs", ForceFunctionAttrsPass())
MODULE_PASS("function-import", FunctionImportPass())
@@ -181,13 +182,6 @@ MODULE_PASS_WITH_PARAMS("ipsccp",
},
parseIPSCCPOptions,
"no-func-spec;func-spec")
-MODULE_PASS_WITH_PARAMS("embed-bitcode",
- "EmbedBitcodePass",
- [](EmbedBitcodeOptions Opts) {
- return EmbedBitcodePass(Opts);
- },
- parseEmbedBitcodePassOptions,
- "thinlto;emit-summary")
MODULE_PASS_WITH_PARAMS("memprof-use",
"MemProfUsePass",
[](std::string Opts) {
diff --git a/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp b/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp
index fa56a5b564ae668..93a246e3a480320 100644
--- a/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp
+++ b/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp
@@ -7,8 +7,6 @@
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO/EmbedBitcodePass.h"
-#include "llvm/Bitcode/BitcodeWriter.h"
-#include "llvm/Bitcode/BitcodeWriterPass.h"
#include "llvm/IR/PassManager.h"
#include "llvm/Pass.h"
#include "llvm/Support/ErrorHandling.h"
@@ -16,10 +14,8 @@
#include "llvm/Support/raw_ostream.h"
#include "llvm/TargetParser/Triple.h"
#include "llvm/Transforms/IPO/ThinLTOBitcodeWriter.h"
-#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/ModuleUtils.h"
-#include <memory>
#include <string>
using namespace llvm;
@@ -34,19 +30,9 @@ PreservedAnalyses EmbedBitcodePass::run(Module &M, ModuleAnalysisManager &AM) {
report_fatal_error(
"EmbedBitcode pass currently only supports ELF object format",
/*gen_crash_diag=*/false);
-
- std::unique_ptr<Module> NewModule = CloneModule(M);
- MPM.run(*NewModule, AM);
-
std::string Data;
raw_string_ostream OS(Data);
- if (IsThinLTO)
- ThinLTOBitcodeWriterPass(OS, /*ThinLinkOS=*/nullptr).run(*NewModule, AM);
- else
- BitcodeWriterPass(OS, /*ShouldPreserveUseListOrder=*/false, EmitLTOSummary)
- .run(*NewModule, AM);
-
+ ThinLTOBitcodeWriterPass(OS, /*ThinLinkOS=*/nullptr).run(M, AM);
embedBufferInModule(M, MemoryBufferRef(Data, "ModuleData"), ".llvm.lto");
-
return PreservedAnalyses::all();
}
diff --git a/llvm/test/CodeGen/X86/fat-lto-section.ll b/llvm/test/CodeGen/X86/fat-lto-section.ll
index 30c56229a0e2a31..9a4359bab6b5ddc 100644
--- a/llvm/test/CodeGen/X86/fat-lto-section.ll
+++ b/llvm/test/CodeGen/X86/fat-lto-section.ll
@@ -1,5 +1,5 @@
;; Ensure that the .llvm.lto section has SHT_EXCLUDE set.
-; RUN: opt --mtriple x86_64-unknown-linux-gnu < %s -passes="embed-bitcode<thinlto;emit-summary>" -S \
+; RUN: opt --mtriple x86_64-unknown-linux-gnu < %s -passes="embed-bitcode" -S \
; RUN: | llc --mtriple x86_64-unknown-linux-gnu -filetype=obj \
; RUN: | llvm-readelf - --sections \
; RUN: | FileCheck %s --check-prefix=EXCLUDE
diff --git a/llvm/test/Transforms/EmbedBitcode/embed.ll b/llvm/test/Transforms/EmbedBitcode/embed.ll
index dffb5cf7554772a..734bf5274a5f2e5 100644
--- a/llvm/test/Transforms/EmbedBitcode/embed.ll
+++ b/llvm/test/Transforms/EmbedBitcode/embed.ll
@@ -1,7 +1,4 @@
; RUN: opt --mtriple x86_64-unknown-linux-gnu < %s -passes="embed-bitcode" -S | FileCheck %s
-; RUN: opt --mtriple x86_64-unknown-linux-gnu < %s -passes="embed-bitcode<thinlto>" -S | FileCheck %s
-; RUN: opt --mtriple x86_64-unknown-linux-gnu < %s -passes="embed-bitcode<emit-summary>" -S | FileCheck %s
-; RUN: opt --mtriple x86_64-unknown-linux-gnu < %s -passes="embed-bitcode<thinlto;emit-summary>" -S | FileCheck %s
@a = global i32 1
``````````
</details>
https://github.com/llvm/llvm-project/pull/72180
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