Skip to content

Commit e29e142

Browse files
pccpcc
committedMay 5, 2016
ELF: Do not use -1 to mark pieces of merge sections as being tail merged.
We were previously using an output offset of -1 for both GC'd and tail merged pieces. We need to distinguish these two cases in order to filter GC'd symbols from the symbol table -- we were previously asserting when we asked for the VA of a symbol pointing into a dead piece, which would end up asking the tail merging string table for an offset even though we hadn't initialized it properly. This patch fixes the bug by using an offset of -1 to exclusively mean GC'd pieces, using 0 for tail merges, and distinguishing the tail merge case from an offset of 0 by asking the output section whether it is tail merge. Differential Revision: http://reviews.llvm.org/D19953 llvm-svn: 268604
·
llvmorg-21-initllvmorg-3.9.0-rc1
1 parent 2cb6f0c commit e29e142

7 files changed

+85
-11
lines changed
 

‎lld/ELF/InputSection.cpp

+5-4
Original file line numberDiff line numberDiff line change
@@ -419,7 +419,8 @@ MergeInputSection<ELFT>::MergeInputSection(elf::ObjectFile<ELFT> *F,
419419
StringRef Data((const char *)D.data(), D.size());
420420
std::vector<std::pair<uintX_t, uintX_t>> &Offsets = this->Offsets;
421421

422-
uintX_t V = Config->GcSections ? -1 : 0;
422+
uintX_t V = Config->GcSections ? MergeInputSection<ELFT>::PieceDead
423+
: MergeInputSection<ELFT>::PieceLive;
423424
if (Header->sh_flags & SHF_STRINGS) {
424425
uintX_t Offset = 0;
425426
while (!Data.empty()) {
@@ -478,15 +479,15 @@ typename ELFT::uint MergeInputSection<ELFT>::getOffset(uintX_t Offset) {
478479
// Compute the Addend and if the Base is cached, return.
479480
uintX_t Addend = Offset - Start;
480481
uintX_t &Base = I->second;
481-
if (Base != uintX_t(-1))
482+
auto *MOS = static_cast<MergeOutputSection<ELFT> *>(this->OutSec);
483+
if (!MOS->shouldTailMerge())
482484
return Base + Addend;
483485

484486
// Map the base to the offset in the output section and cache it.
485487
ArrayRef<uint8_t> D = this->getSectionData();
486488
StringRef Data((const char *)D.data(), D.size());
487489
StringRef Entry = Data.substr(Start, End - Start);
488-
Base =
489-
static_cast<MergeOutputSection<ELFT> *>(this->OutSec)->getOffset(Entry);
490+
Base = MOS->getOffset(Entry);
490491
return Base + Addend;
491492
}
492493

‎lld/ELF/InputSection.h

+14-1
Original file line numberDiff line numberDiff line change
@@ -144,9 +144,22 @@ template <class ELFT> class SplitInputSection : public InputSectionBase<ELFT> {
144144
typename InputSectionBase<ELFT>::Kind SectionKind);
145145

146146
// For each piece of data, we maintain the offsets in the input section and
147-
// in the output section. The latter may be -1 if it is not assigned yet.
147+
// in the output section.
148148
std::vector<std::pair<uintX_t, uintX_t>> Offsets;
149149

150+
// Merge input sections may use the following special values as the output
151+
// section offset:
152+
enum {
153+
// The piece is dead.
154+
PieceDead = uintX_t(-1),
155+
// The piece is live, and an offset has not yet been assigned. After offsets
156+
// have been assigned, if the output section uses tail merging, the field
157+
// will still have this value and the output section offset is available
158+
// from MergeOutputSection<ELFT>::getOffset(). Otherwise, this value has no
159+
// special significance, it just means that the offset is 0.
160+
PieceLive = uintX_t(0),
161+
};
162+
150163
std::pair<std::pair<uintX_t, uintX_t> *, uintX_t>
151164
getRangeAndSize(uintX_t Offset);
152165
};

‎lld/ELF/MarkLive.cpp

+1-1
Original file line numberDiff line numberDiff line change
@@ -143,7 +143,7 @@ template <class ELFT> void elf::markLive() {
143143
if (auto *MS = dyn_cast<MergeInputSection<ELFT>>(R.Sec)) {
144144
std::pair<std::pair<uintX_t, uintX_t> *, uintX_t> T =
145145
MS->getRangeAndSize(R.Offset);
146-
T.first->second = 0;
146+
T.first->second = MergeInputSection<ELFT>::PieceLive;
147147
}
148148
if (R.Sec->Live)
149149
return;

‎lld/ELF/OutputSections.cpp

+2-2
Original file line numberDiff line numberDiff line change
@@ -1271,15 +1271,15 @@ void MergeOutputSection<ELFT>::addSection(InputSectionBase<ELFT> *C) {
12711271
if (this->Header.sh_flags & SHF_STRINGS) {
12721272
for (unsigned I = 0, N = Offsets.size(); I != N; ++I) {
12731273
auto &P = Offsets[I];
1274-
if (P.second == (uintX_t)-1)
1274+
if (P.second == MergeInputSection<ELFT>::PieceDead)
12751275
continue;
12761276

12771277
uintX_t Start = P.first;
12781278
uintX_t End = (I == N - 1) ? Data.size() : Offsets[I + 1].first;
12791279
StringRef Entry = Data.substr(Start, End - Start);
12801280
uintX_t OutputOffset = Builder.add(Entry);
12811281
if (shouldTailMerge())
1282-
OutputOffset = -1;
1282+
OutputOffset = MergeInputSection<ELFT>::PieceLive;
12831283
P.second = OutputOffset;
12841284
}
12851285
return;

‎lld/ELF/OutputSections.h

+1-2
Original file line numberDiff line numberDiff line change
@@ -310,15 +310,14 @@ template <class ELFT>
310310
class MergeOutputSection final : public OutputSectionBase<ELFT> {
311311
typedef typename ELFT::uint uintX_t;
312312

313-
bool shouldTailMerge() const;
314-
315313
public:
316314
MergeOutputSection(StringRef Name, uint32_t Type, uintX_t Flags,
317315
uintX_t Alignment);
318316
void addSection(InputSectionBase<ELFT> *S) override;
319317
void writeTo(uint8_t *Buf) override;
320318
unsigned getOffset(StringRef Val);
321319
void finalize() override;
320+
bool shouldTailMerge() const;
322321

323322
private:
324323
llvm::StringTableBuilder Builder;

‎lld/ELF/Writer.cpp

+8-1
Original file line numberDiff line numberDiff line change
@@ -1118,9 +1118,16 @@ template <class ELFT> static bool includeInSymtab(const SymbolBody &B) {
11181118
return false;
11191119

11201120
if (auto *D = dyn_cast<DefinedRegular<ELFT>>(&B)) {
1121+
// Always include absolute symbols.
1122+
if (!D->Section)
1123+
return true;
11211124
// Exclude symbols pointing to garbage-collected sections.
1122-
if (D->Section && !D->Section->Live)
1125+
if (!D->Section->Live)
11231126
return false;
1127+
if (auto *S = dyn_cast<MergeInputSection<ELFT>>(D->Section))
1128+
if (S->getRangeAndSize(D->Value).first->second ==
1129+
MergeInputSection<ELFT>::PieceDead)
1130+
return false;
11241131
}
11251132
return true;
11261133
}

‎lld/test/ELF/string-gc.s

+54
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,54 @@
1+
// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
2+
// RUN: ld.lld %t.o -o %t --gc-sections
3+
// RUN: llvm-readobj -symbols %t | FileCheck %s
4+
5+
// CHECK: Symbols [
6+
// CHECK-NEXT: Symbol {
7+
// CHECK-NEXT: Name: (0)
8+
// CHECK-NEXT: Value: 0x0
9+
// CHECK-NEXT: Size: 0
10+
// CHECK-NEXT: Binding: Local (0x0)
11+
// CHECK-NEXT: Type: None (0x0)
12+
// CHECK-NEXT: Other: 0
13+
// CHECK-NEXT: Section: Undefined (0x0)
14+
// CHECK-NEXT: }
15+
// CHECK-NEXT: Symbol {
16+
// CHECK-NEXT: Name: s1 (8)
17+
// CHECK-NEXT: Value: 0x10120
18+
// CHECK-NEXT: Size: 0
19+
// CHECK-NEXT: Binding: Local (0x0)
20+
// CHECK-NEXT: Type: Object (0x1)
21+
// CHECK-NEXT: Other [ (0x2)
22+
// CHECK-NEXT: STV_HIDDEN (0x2)
23+
// CHECK-NEXT: ]
24+
// CHECK-NEXT: Section: .rodata (0x1)
25+
// CHECK-NEXT: }
26+
// CHECK-NEXT: Symbol {
27+
// CHECK-NEXT: Name: _start (1)
28+
// CHECK-NEXT: Value: 0x11000
29+
// CHECK-NEXT: Size: 0
30+
// CHECK-NEXT: Binding: Global (0x1)
31+
// CHECK-NEXT: Type: Function (0x2)
32+
// CHECK-NEXT: Other: 0
33+
// CHECK-NEXT: Section: .text (0x2)
34+
// CHECK-NEXT: }
35+
// CHECK-NEXT: ]
36+
37+
.text
38+
.globl _start
39+
.type _start,@function
40+
_start:
41+
movl $s1, %eax
42+
43+
.hidden s1
44+
.type s1,@object
45+
.section .rodata.str1.1,"aMS",@progbits,1
46+
.globl s1
47+
s1:
48+
.asciz "abcd"
49+
50+
.hidden s2
51+
.type s2,@object
52+
.globl s2
53+
s2:
54+
.asciz "efgh"

0 commit comments

Comments
 (0)
Please sign in to comment.