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[IRGen] Emit mod summary for full LTO to enable hermetic seal with lld #42366

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Merged
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Aug 11, 2022
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[IRGen] Emit mod summary for full LTO to enable hermetic seal with lld #42366

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22 changes: 21 additions & 1 deletion lib/IRGen/IRGen.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -82,6 +82,7 @@
#include "llvm/Transforms/Instrumentation/SanitizerCoverage.h"
#include "llvm/Transforms/Instrumentation/ThreadSanitizer.h"
#include "llvm/Transforms/ObjCARC.h"
#include "llvm/Transforms/Scalar.h"

#include <thread>

Expand Down Expand Up @@ -601,10 +602,29 @@ bool swift::compileAndWriteLLVM(llvm::Module *module,
EmitPasses.add(createPrintModulePass(out));
break;
case IRGenOutputKind::LLVMBitcode: {
// Emit a module summary by default for Regular LTO except ld64-based ones
// (which use the legacy LTO API).
bool EmitRegularLTOSummary =
targetMachine->getTargetTriple().getVendor() != llvm::Triple::Apple;
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Why the vendor check rather than !targetMachine->getTargetTriple().isOSDarwin()?

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@kubamracek Which form do we prefer here? != Apple or isOSDarwin?

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I followed what clang does.
https://github.com/llvm/llvm-project/blob/a5040860412f3e61151e427af1cff6ff0aebb775/clang/lib/CodeGen/BackendUtil.cpp#L164-L172


if (EmitRegularLTOSummary || opts.LLVMLTOKind == IRGenLLVMLTOKind::Thin) {
// Rename anon globals to be able to export them in the summary.
EmitPasses.add(createNameAnonGlobalPass());
}

if (opts.LLVMLTOKind == IRGenLLVMLTOKind::Thin) {
EmitPasses.add(createWriteThinLTOBitcodePass(out));
} else {
EmitPasses.add(createBitcodeWriterPass(out));
if (EmitRegularLTOSummary) {
module->addModuleFlag(llvm::Module::Error, "ThinLTO", uint32_t(0));
// Assume other sources are compiled with -fsplit-lto-unit (it's enabled
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Can we avoid this assumption? Why do we need this assumption in the first place? Perhaps we should serialize and reference this from the Swift module?

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Other sources mean not only Swiftc-oriented LTO summary but also including Clang-oriented summary. So we can't propagate that information through swiftmodule.
Another option to remove that assumption is adding -split-lto-unit in swiftc also. What do you think?

// by default when -flto is specified on platforms that support regular
// lto summary.)
module->addModuleFlag(llvm::Module::Error, "EnableSplitLTOUnit",
uint32_t(1));
}
EmitPasses.add(createBitcodeWriterPass(
out, /*ShouldPreserveUseListOrder*/ false, EmitRegularLTOSummary));
}
break;
}
Expand Down