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LTO: Mark undefined module asm symbols as used.

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Marking them as used causes them to be considered visible outside of LTO. This
prevents the symbols from being internalized or discarded, either by GlobalDCE
or by summary-based dead stripping in ThinLTO.

This change makes it unnecessary to add these symbols to llvm.compiler.used
in the backend, as the symbols are kept alive by virtue of being external,
so remove the backend code that handles that.

Fixes PR32798.

Differential Revision: https://reviews.llvm.org/D32544

llvm-svn: 301438
This commit is contained in:
Peter Collingbourne 2017-04-26 17:53:39 +00:00
parent 409b9323b6
commit a2ab4a5c5d
6 changed files with 21 additions and 21 deletions
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3
lib/LTO/LTO.cpp
View File

@ -415,7 +415,8 @@ void LTO::addSymbolToGlobalRes(const InputFile::Symbol &Sym,
// Flag as visible outside of ThinLTO if visible from a regular object or
// if this is a reference in the regular LTO partition.
GlobalRes.VisibleOutsideThinLTO |=
(Res.VisibleToRegularObj || (Partition == GlobalResolution::RegularLTO));
(Res.VisibleToRegularObj || Sym.isUsed() ||
Partition == GlobalResolution::RegularLTO);
}
static void writeToResolutionFile(raw_ostream &OS, InputFile *Input,

18
lib/LTO/LTOBackend.cpp
View File

@ -25,7 +25,6 @@
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Verifier.h"
#include "llvm/LTO/LTO.h"
#include "llvm/LTO/legacy/UpdateCompilerUsed.h"
#include "llvm/MC/SubtargetFeature.h"
#include "llvm/Object/ModuleSymbolTable.h"
#include "llvm/Passes/PassBuilder.h"
@ -353,19 +352,6 @@ finalizeOptimizationRemarks(std::unique_ptr<tool_output_file> DiagOutputFile) {
DiagOutputFile->os().flush();
}
static void handleAsmUndefinedRefs(Module &Mod, TargetMachine &TM) {
// Collect the list of undefined symbols used in asm and update
// llvm.compiler.used to prevent optimization to drop these from the output.
StringSet<> AsmUndefinedRefs;
ModuleSymbolTable::CollectAsmSymbols(
Mod,
[&AsmUndefinedRefs](StringRef Name, object::BasicSymbolRef::Flags Flags) {
if (Flags & object::BasicSymbolRef::SF_Undefined)
AsmUndefinedRefs.insert(Name);
});
updateCompilerUsed(Mod, TM, AsmUndefinedRefs);
}
Error lto::backend(Config &C, AddStreamFn AddStream,
unsigned ParallelCodeGenParallelismLevel,
std::unique_ptr<Module> Mod,
@ -377,8 +363,6 @@ Error lto::backend(Config &C, AddStreamFn AddStream,
std::unique_ptr<TargetMachine> TM =
createTargetMachine(C, Mod->getTargetTriple(), *TOrErr);
handleAsmUndefinedRefs(*Mod, *TM);
// Setup optimization remarks.
auto DiagFileOrErr = lto::setupOptimizationRemarks(
Mod->getContext(), C.RemarksFilename, C.RemarksWithHotness);
@ -416,8 +400,6 @@ Error lto::thinBackend(Config &Conf, unsigned Task, AddStreamFn AddStream,
std::unique_ptr<TargetMachine> TM =
createTargetMachine(Conf, Mod.getTargetTriple(), *TOrErr);
handleAsmUndefinedRefs(Mod, *TM);
if (Conf.CodeGenOnly) {
codegen(Conf, TM.get(), AddStream, Task, Mod);
return Error::success();

3
lib/Object/IRSymtab.cpp
View File

@ -163,6 +163,9 @@ Error Builder::addSymbol(const ModuleSymbolTable &Msymtab,
Sym.ComdatIndex = -1;
auto *GV = Msym.dyn_cast<GlobalValue *>();
if (!GV) {
// Undefined module asm symbols act as GC roots and are implicitly used.
if (Flags & object::BasicSymbolRef::SF_Undefined)
Sym.Flags |= 1 << storage::Symbol::FB_used;
setStr(Sym.IRName, "");
return Error::success();
}

4
test/LTO/Resolution/X86/Inputs/mod-asm-used.ll Normal file
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@ -0,0 +1,4 @@
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
@foo = global i32 1, align 4

10
test/LTO/Resolution/X86/mod-asm-used.ll Normal file
View File

@ -0,0 +1,10 @@
; RUN: opt -module-summary -o %t.bc %s
; RUN: opt -module-summary -o %t2.bc %S/Inputs/mod-asm-used.ll
; RUN: llvm-lto2 run %t.bc -r %t.bc,foo,l %t2.bc -r %t2.bc,foo,pl -o %t3
; RUN: llvm-nm %t3.1 | FileCheck %s
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
; CHECK: D foo
module asm ".quad foo"

4
test/tools/gold/X86/asm_undefined2.ll
View File

@ -11,8 +11,8 @@
; RUN: --plugin-opt=thinlto -o %t2 %t.o
; RUN: llvm-dis < %t.o.5.precodegen.bc | FileCheck %s
; Check that foo is properly appended to llvm.compiler.used
; CHECK: @llvm.compiler.used = appending global [1 x i8*] [i8* bitcast (void ()* @foo to i8*)], section "llvm.metadata"
; Check that foo is not internalized
; CHECK: define void @foo
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"