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[ELF][MIPS] Put local GOT entries accessed via a 16-bit index first 

Some MIPS relocations used to access GOT entries are able to manipulate
16-bit index. The other ones like R_MIPS_CALL_HI16/LO16 can handle
32-bit indexes. 16-bit relocations are generated by default. The 32-bit
relocations are generated by -mxgot flag passed to compiler. Usually
these relocation are not mixed in the same code but files like crt*.o
contain 16-bit relocations so even if all "user's" code compiled with
-mxgot flag a few 16-bit relocations might come to the linking phase.

Now LLD does not differentiate local GOT entries accessed via a 16-bit
and 32-bit indexes. That might lead to relocation's overflow if 16-bit
entries are allocated to far from the beginning of the GOT.

The patch introduces new "part" of MIPS GOT dedicated to the local GOT
entries accessed by 32-bit relocations. That allows to put local GOT
entries accessed via a 16-bit index first and escape relocation's overflow.

Differential revision: https://reviews.llvm.org/D25833

llvm-svn: 284809
This commit is contained in:
Simon Atanasyan 2016-10-21 07:22:30 +00:00
parent c1db0db864
commit bed04bf1df
9 changed files with 85 additions and 19 deletions
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1
lld/ELF/InputSection.cpp
View File

@ -325,6 +325,7 @@ static typename ELFT::uint getSymVA(uint32_t Type, typename ELFT::uint A,
// of sum the symbol's value and the addend.
return Out<ELFT>::Got->getMipsLocalPageOffset(Body.getVA<ELFT>(A));
case R_MIPS_GOT_OFF:
case R_MIPS_GOT_OFF32:
// In case of MIPS if a GOT relocation has non-zero addend this addend
// should be applied to the GOT entry content not to the GOT entry offset.
// That is why we use separate expression type.

14
lld/ELF/OutputSections.cpp
View File

@ -202,8 +202,15 @@ void GotSection<ELFT>::addMipsEntry(SymbolBody &Sym, uintX_t Addend,
// Ignore addends for preemptible symbols. They got single GOT entry anyway.
AddEntry(Sym, 0, MipsGlobal);
Sym.IsInGlobalMipsGot = true;
} else
} else if (Expr == R_MIPS_GOT_OFF32) {
AddEntry(Sym, Addend, MipsLocal32);
Sym.Is32BitMipsGot = true;
} else {
// Hold local GOT entries accessed via a 16-bit index separately.
// That allows to write them in the beginning of the GOT and keep
// their indexes as less as possible to escape relocation's overflow.
AddEntry(Sym, Addend, MipsLocal);
}
}
template <class ELFT> bool GotSection<ELFT>::addDynTlsEntry(SymbolBody &Sym) {
@ -250,6 +257,8 @@ GotSection<ELFT>::getMipsGotOffset(const SymbolBody &B, uintX_t Addend) const {
GotBlockOff = getMipsTlsOffset();
else if (B.IsInGlobalMipsGot)
GotBlockOff = getMipsLocalEntriesNum() * sizeof(uintX_t);
else if (B.Is32BitMipsGot)
GotBlockOff = (MipsPageEntries + MipsLocal.size()) * sizeof(uintX_t);
else
GotBlockOff = MipsPageEntries * sizeof(uintX_t);
// Calculate index of the GOT entry in the block.
@ -288,7 +297,7 @@ const SymbolBody *GotSection<ELFT>::getMipsFirstGlobalEntry() const {
template <class ELFT>
unsigned GotSection<ELFT>::getMipsLocalEntriesNum() const {
return MipsPageEntries + MipsLocal.size();
return MipsPageEntries + MipsLocal.size() + MipsLocal32.size();
}
template <class ELFT> void GotSection<ELFT>::finalize() {
@ -347,6 +356,7 @@ template <class ELFT> void GotSection<ELFT>::writeMipsGot(uint8_t *Buf) {
writeUint<ELFT>(Entry, VA);
};
std::for_each(std::begin(MipsLocal), std::end(MipsLocal), AddEntry);
std::for_each(std::begin(MipsLocal32), std::end(MipsLocal32), AddEntry);
std::for_each(std::begin(MipsGlobal), std::end(MipsGlobal), AddEntry);
// Initialize TLS-related GOT entries. If the entry has a corresponding
// dynamic relocations, leave it initialized by zero. Write down adjusted

1
lld/ELF/OutputSections.h
View File

@ -217,6 +217,7 @@ private:
typedef std::vector<MipsGotEntry> MipsGotEntries;
llvm::DenseMap<MipsGotEntry, size_t> MipsGotMap;
MipsGotEntries MipsLocal;
MipsGotEntries MipsLocal32;
MipsGotEntries MipsGlobal;
// Write MIPS-specific parts of the GOT.

19
lld/ELF/Relocations.cpp
View File

@ -62,10 +62,11 @@ namespace elf {
static bool refersToGotEntry(RelExpr Expr) {
return Expr == R_GOT || Expr == R_GOT_OFF || Expr == R_MIPS_GOT_LOCAL_PAGE ||
Expr == R_MIPS_GOT_OFF || Expr == R_MIPS_TLSGD ||
Expr == R_MIPS_TLSLD || Expr == R_GOT_PAGE_PC || Expr == R_GOT_PC ||
Expr == R_GOT_FROM_END || Expr == R_TLSGD || Expr == R_TLSGD_PC ||
Expr == R_TLSDESC || Expr == R_TLSDESC_PAGE;
Expr == R_MIPS_GOT_OFF || Expr == R_MIPS_GOT_OFF32 ||
Expr == R_MIPS_TLSGD || Expr == R_MIPS_TLSLD ||
Expr == R_GOT_PAGE_PC || Expr == R_GOT_PC || Expr == R_GOT_FROM_END ||
Expr == R_TLSGD || Expr == R_TLSGD_PC || Expr == R_TLSDESC ||
Expr == R_TLSDESC_PAGE;
}
static bool isPreemptible(const SymbolBody &Body, uint32_t Type) {
@ -303,11 +304,11 @@ static bool isStaticLinkTimeConstant(RelExpr E, uint32_t Type,
const SymbolBody &Body) {
// These expressions always compute a constant
if (E == R_SIZE || E == R_GOT_FROM_END || E == R_GOT_OFF ||
E == R_MIPS_GOT_LOCAL_PAGE || E == R_MIPS_GOT_OFF || E == R_MIPS_TLSGD ||
E == R_GOT_PAGE_PC || E == R_GOT_PC || E == R_PLT_PC ||
E == R_TLSGD_PC || E == R_TLSGD || E == R_PPC_PLT_OPD ||
E == R_TLSDESC_CALL || E == R_TLSDESC_PAGE || E == R_HINT ||
E == R_THUNK_PC || E == R_THUNK_PLT_PC)
E == R_MIPS_GOT_LOCAL_PAGE || E == R_MIPS_GOT_OFF ||
E == R_MIPS_GOT_OFF32 || E == R_MIPS_TLSGD || E == R_GOT_PAGE_PC ||
E == R_GOT_PC || E == R_PLT_PC || E == R_TLSGD_PC || E == R_TLSGD ||
E == R_PPC_PLT_OPD || E == R_TLSDESC_CALL || E == R_TLSDESC_PAGE ||
E == R_HINT || E == R_THUNK_PC || E == R_THUNK_PLT_PC)
return true;
// These never do, except if the entire file is position dependent or if

1
lld/ELF/Relocations.h
View File

@ -36,6 +36,7 @@ enum RelExpr {
R_HINT,
R_MIPS_GOT_LOCAL_PAGE,
R_MIPS_GOT_OFF,
R_MIPS_GOT_OFF32,
R_MIPS_TLSGD,
R_MIPS_TLSLD,
R_NEG_TLS,

8
lld/ELF/Symbols.cpp
View File

@ -93,13 +93,13 @@ static typename ELFT::uint getSymVA(const SymbolBody &Body,
SymbolBody::SymbolBody(Kind K, uint32_t NameOffset, uint8_t StOther,
uint8_t Type)
: SymbolKind(K), NeedsCopyOrPltAddr(false), IsLocal(true),
IsInGlobalMipsGot(false), Type(Type), StOther(StOther),
NameOffset(NameOffset) {}
IsInGlobalMipsGot(false), Is32BitMipsGot(false), Type(Type),
StOther(StOther), NameOffset(NameOffset) {}
SymbolBody::SymbolBody(Kind K, StringRef Name, uint8_t StOther, uint8_t Type)
: SymbolKind(K), NeedsCopyOrPltAddr(false), IsLocal(false),
IsInGlobalMipsGot(false), Type(Type), StOther(StOther),
Name({Name.data(), Name.size()}) {}
IsInGlobalMipsGot(false), Is32BitMipsGot(false), Type(Type),
StOther(StOther), Name({Name.data(), Name.size()}) {}
StringRef SymbolBody::getName() const {
assert(!isLocal());

3
lld/ELF/Symbols.h
View File

@ -120,6 +120,9 @@ public:
// True if this symbol has an entry in the global part of MIPS GOT.
unsigned IsInGlobalMipsGot : 1;
// True if this symbol is referenced by 32-bit GOT relocations.
unsigned Is32BitMipsGot : 1;
// The following fields have the same meaning as the ELF symbol attributes.
uint8_t Type; // symbol type
uint8_t StOther; // st_other field value

9
lld/ELF/Target.cpp
View File

@ -1940,13 +1940,14 @@ RelExpr MipsTargetInfo<ELFT>::getRelExpr(uint32_t Type,
return R_MIPS_GOT_LOCAL_PAGE;
// fallthrough
case R_MIPS_CALL16:
case R_MIPS_CALL_HI16:
case R_MIPS_CALL_LO16:
case R_MIPS_GOT_DISP:
case R_MIPS_GOT_HI16:
case R_MIPS_GOT_LO16:
case R_MIPS_TLS_GOTTPREL:
return R_MIPS_GOT_OFF;
case R_MIPS_CALL_HI16:
case R_MIPS_CALL_LO16:
case R_MIPS_GOT_HI16:
case R_MIPS_GOT_LO16:
return R_MIPS_GOT_OFF32;
case R_MIPS_GOT_PAGE:
return R_MIPS_GOT_LOCAL_PAGE;
case R_MIPS_TLS_GD:

48
lld/test/ELF/mips-xgot-order.s Normal file
View File

@ -0,0 +1,48 @@
# Check that GOT entries accessed via 16-bit indexing are allocated
# in the beginning of the GOT.
# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
# RUN: ld.lld %t.o -o %t.exe
# RUN: llvm-objdump -d -s -t %t.exe | FileCheck %s
# REQUIRES: mips
# CHECK: Disassembly of section .text:
# CHECK-NEXT: __start:
# CHECK-NEXT: 20000: 3c 02 00 00 lui $2, 0
# CHECK-NEXT: 20004: 8c 42 80 20 lw $2, -32736($2)
# CHECK-NEXT: 20008: 3c 02 00 00 lui $2, 0
# CHECK-NEXT: 2000c: 8c 42 80 24 lw $2, -32732($2)
#
# CHECK: bar:
# CHECK-NEXT: 20010: 8c 42 80 1c lw $2, -32740($2)
# CHECK-NEXT: 20014: 8c 42 80 18 lw $2, -32744($2)
# CHECK-NEXT: 20018: 20 42 00 00 addi $2, $2, 0
# CHECK: Contents of section .got:
# CHECK-NEXT: 30000 00000000 80000000 00040000 00020010
# ^ %hi(loc)
# ^ %got(bar)
# CHECK-NEXT: 30010 00020000 00040000
# ^ %got_hi/lo(start)
# ^ %got_hi/lo(loc)
# CHECK: 00040000 .data 00000000 loc
# CHECK: 00020000 .text 00000000 __start
# CHECK: 00020010 .text 00000000 bar
.text
.global __start, bar
__start:
lui $2, %got_hi(__start)
lw $2, %got_lo(__start)($2)
lui $2, %got_hi(loc)
lw $2, %got_lo(loc)($2)
bar:
lw $2, %got(bar)($2)
lw $2, %got(loc)($2)
addi $2, $2, %lo(loc)
.data
loc:
.word 0