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[ELF2] - Optimization for R_X86_64_GOTTPOFF relocation.

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R_X86_64_GOTTPOFF is not always requires GOT entries. Some relocations can be converted to local ones.

Differential revision: http://reviews.llvm.org/D14713

llvm-svn: 253966
This commit is contained in:
George Rimar 2015-11-24 09:00:06 +00:00
parent 1938f2eb98
commit 77d1cb1ddf
4 changed files with 127 additions and 1 deletions
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6
lld/ELF/InputSection.cpp
View File

@ -134,6 +134,12 @@ void InputSectionBase<ELFT>::relocate(
continue;
}
if (Target->isTlsOptimized(Type, Body)) {
Target->relocateTlsOptimize(BufLoc, BufEnd, AddrLoc,
getSymVA<ELFT>(Body));
continue;
}
uintX_t SymVA = getSymVA<ELFT>(Body);
if (Target->relocNeedsPlt(Type, Body)) {
SymVA = Out<ELFT>::Plt->getEntryAddr(Body);

54
lld/ELF/Target.cpp
View File

@ -74,6 +74,9 @@ public:
void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t P,
uint64_t SA) const override;
bool isRelRelative(uint32_t Type) const override;
bool isTlsOptimized(unsigned Type, const SymbolBody &S) const override;
void relocateTlsOptimize(uint8_t *Loc, uint8_t *BufEnd, uint64_t P,
uint64_t SA) const override;
};
class PPC64TargetInfo final : public TargetInfo {
@ -148,6 +151,10 @@ TargetInfo *createTarget() {
TargetInfo::~TargetInfo() {}
bool TargetInfo::isTlsOptimized(unsigned Type, const SymbolBody &S) const {
return false;
}
uint64_t TargetInfo::getVAStart() const { return Config->Shared ? 0 : VAStart; }
bool TargetInfo::relocNeedsCopy(uint32_t Type, const SymbolBody &S) const {
@ -162,6 +169,9 @@ bool TargetInfo::relocPointsToGot(uint32_t Type) const { return false; }
bool TargetInfo::isRelRelative(uint32_t Type) const { return true; }
void TargetInfo::relocateTlsOptimize(uint8_t *Loc, uint8_t *BufEnd, uint64_t P,
uint64_t SA) const {}
void TargetInfo::writeGotHeaderEntries(uint8_t *Buf) const {}
void TargetInfo::writeGotPltHeaderEntries(uint8_t *Buf) const {}
@ -279,6 +289,8 @@ bool X86_64TargetInfo::relocNeedsCopy(uint32_t Type,
}
bool X86_64TargetInfo::relocNeedsGot(uint32_t Type, const SymbolBody &S) const {
if (Type == R_X86_64_GOTTPOFF)
return !isTlsOptimized(Type, S);
return Type == R_X86_64_GOTTPOFF || Type == R_X86_64_GOTPCREL ||
relocNeedsPlt(Type, S);
}
@ -344,6 +356,48 @@ bool X86_64TargetInfo::isRelRelative(uint32_t Type) const {
}
}
bool X86_64TargetInfo::isTlsOptimized(unsigned Type,
const SymbolBody &S) const {
if (Config->Shared || !S.isTLS())
return false;
return Type == R_X86_64_GOTTPOFF && !canBePreempted(&S, true);
}
// In some conditions, R_X86_64_GOTTPOFF relocation can be optimized to
// R_X86_64_TPOFF32 so that R_X86_64_TPOFF32 so that it does not use GOT.
// This function does that. Read "ELF Handling For Thread-Local Storage,
// 5.5 x86-x64 linker optimizations" (http://www.akkadia.org/drepper/tls.pdf)
// by Ulrich Drepper for details.
void X86_64TargetInfo::relocateTlsOptimize(uint8_t *Loc, uint8_t *BufEnd,
uint64_t P, uint64_t SA) const {
// Ulrich's document section 6.5 says that @gottpoff(%rip) must be
// used in MOVQ or ADDQ instructions only.
// "MOVQ foo@GOTTPOFF(%RIP), %REG" is transformed to "MOVQ $foo, %REG".
// "ADDQ foo@GOTTPOFF(%RIP), %REG" is transformed to "LEAQ foo(%REG), %REG"
// (if the register is not RSP/R12) or "ADDQ $foo, %RSP".
// Opcodes info can be found at http://ref.x86asm.net/coder64.html#x48.
uint8_t *Prefix = Loc - 3;
uint8_t *Inst = Loc - 2;
uint8_t *RegSlot = Loc - 1;
uint8_t Reg = Loc[-1] >> 3;
bool IsMov = *Inst == 0x8b;
bool RspAdd = !IsMov && Reg == 4;
// r12 and rsp registers requires special handling.
// Problem is that for other registers, for example leaq 0xXXXXXXXX(%r11),%r11
// result out is 7 bytes: 4d 8d 9b XX XX XX XX,
// but leaq 0xXXXXXXXX(%r12),%r12 is 8 bytes: 4d 8d a4 24 XX XX XX XX.
// The same true for rsp. So we convert to addq for them, saving 1 byte that
// we dont have.
if (RspAdd)
*Inst = 0x81;
else
*Inst = IsMov ? 0xc7 : 0x8d;
if (*Prefix == 0x4c)
*Prefix = (IsMov || RspAdd) ? 0x49 : 0x4d;
*RegSlot = (IsMov || RspAdd) ? (0xc0 | Reg) : (0x80 | Reg | (Reg << 3));
relocateOne(Loc, BufEnd, R_X86_64_TPOFF32, P, SA);
}
void X86_64TargetInfo::relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type,
uint64_t P, uint64_t SA) const {
switch (Type) {

4
lld/ELF/Target.h
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@ -59,7 +59,9 @@ public:
virtual bool relocNeedsPlt(uint32_t Type, const SymbolBody &S) const = 0;
virtual void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type,
uint64_t P, uint64_t SA) const = 0;
virtual bool isTlsOptimized(unsigned Type, const SymbolBody &S) const;
virtual void relocateTlsOptimize(uint8_t *Loc, uint8_t *BufEnd, uint64_t P,
uint64_t SA) const;
virtual ~TargetInfo();
protected:

64
lld/test/ELF/tls-opt.s Normal file
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@ -0,0 +1,64 @@
// RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
// RUN: ld.lld %t.o -o %t1
// RUN: llvm-readobj -r %t1 | FileCheck --check-prefix=NORELOC %s
// RUN: llvm-objdump -d %t1 | FileCheck --check-prefix=DISASM %s
// NORELOC: Relocations [
// NORELOC-NEXT: ]
// DISASM: Disassembly of section .text:
// DISASM-NEXT: _start:
// DISASM-NEXT: 11000: 48 c7 c0 f8 ff ff ff movq $-8, %rax
// DISASM-NEXT: 11007: 49 c7 c7 f8 ff ff ff movq $-8, %r15
// DISASM-NEXT: 1100e: 48 8d 80 f8 ff ff ff leaq -8(%rax), %rax
// DISASM-NEXT: 11015: 4d 8d bf f8 ff ff ff leaq -8(%r15), %r15
// DISASM-NEXT: 1101c: 48 81 c4 f8 ff ff ff addq $-8, %rsp
// DISASM-NEXT: 11023: 49 81 c4 f8 ff ff ff addq $-8, %r12
// DISASM-NEXT: 1102a: 48 c7 c0 fc ff ff ff movq $-4, %rax
// DISASM-NEXT: 11031: 49 c7 c7 fc ff ff ff movq $-4, %r15
// DISASM-NEXT: 11038: 48 8d 80 fc ff ff ff leaq -4(%rax), %rax
// DISASM-NEXT: 1103f: 4d 8d bf fc ff ff ff leaq -4(%r15), %r15
// DISASM-NEXT: 11046: 48 81 c4 fc ff ff ff addq $-4, %rsp
// DISASM-NEXT: 1104d: 49 81 c4 fc ff ff ff addq $-4, %r12
// Corrupred output:
// DISASM-NEXT: 11054: 48 8d 80 f8 ff ff ff leaq -8(%rax), %rax
// DISASM-NEXT: 1105b: 48 d1 81 c4 f8 ff ff rolq -1852(%rcx)
// DISASM-NEXT: 11062: ff 48 d1 decl -47(%rax)
// DISASM-NEXT: 11065: 81 c4 f8 ff ff ff addl $4294967288, %esp
.type tls0,@object
.section .tbss,"awT",@nobits
.globl tls0
.align 4
tls0:
.long 0
.size tls0, 4
.type tls1,@object
.globl tls1
.align 4
tls1:
.long 0
.size tls1, 4
.section .text
.globl _start
_start:
movq tls0@GOTTPOFF(%rip), %rax
movq tls0@GOTTPOFF(%rip), %r15
addq tls0@GOTTPOFF(%rip), %rax
addq tls0@GOTTPOFF(%rip), %r15
addq tls0@GOTTPOFF(%rip), %rsp
addq tls0@GOTTPOFF(%rip), %r12
movq tls1@GOTTPOFF(%rip), %rax
movq tls1@GOTTPOFF(%rip), %r15
addq tls1@GOTTPOFF(%rip), %rax
addq tls1@GOTTPOFF(%rip), %r15
addq tls1@GOTTPOFF(%rip), %rsp
addq tls1@GOTTPOFF(%rip), %r12
//Invalid input case:
xchgq tls0@gottpoff(%rip),%rax
shlq tls0@gottpoff
rolq tls0@gottpoff