[ELF] Implement infrastructure for thunk code creation
Some targets might require creation of thunks. For example, MIPS targets
require stubs to call PIC code from non-PIC one. The patch implements
infrastructure for thunk code creation and provides support for MIPS
LA25 stubs. Any MIPS PIC code function is invoked with its address
in register $t9. So if we have a branch instruction from non-PIC code
to the PIC one we cannot make the jump directly and need to create a small
stub to save the target function address.
See page 3-38 ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
- In relocation scanning phase we ask target about thunk creation necessity
by calling `TagetInfo::needsThunk` method. The `InputSection` class
maintains list of Symbols requires thunk creation.
- Reassigning offsets performed for each input sections after relocation
scanning complete because position of each section might change due
thunk creation.
- The patch introduces new dedicated value for DefinedSynthetic symbols
DefinedSynthetic::SectionEnd. Synthetic symbol with that value always
points to the end of the corresponding output section. That allows to
escape updating synthetic symbols if output sections sizes changes after
relocation scanning due thunk creation.
- In the `InputSection::writeTo` method we write thunks after corresponding
input section. Each thunk is written by calling `TargetInfo::writeThunk` method.
- The patch supports the only type of thunk code for each target. For now,
it is enough.
Differential Revision: http://reviews.llvm.org/D17934
llvm-svn: 265059
diff --git a/lld/ELF/Target.cpp b/lld/ELF/Target.cpp
index d56984b..0e945bc 100644
--- a/lld/ELF/Target.cpp
+++ b/lld/ELF/Target.cpp
@@ -17,6 +17,7 @@
#include "Target.h"
#include "Error.h"
+#include "InputFiles.h"
#include "OutputSections.h"
#include "Symbols.h"
@@ -198,9 +199,12 @@
void writePlt(uint8_t *Buf, uint64_t GotEntryAddr, uint64_t PltEntryAddr,
int32_t Index, unsigned RelOff) const override;
void writeGotHeader(uint8_t *Buf) const override;
+ void writeThunk(uint8_t *Buf, uint64_t S) const override;
bool needsCopyRelImpl(uint32_t Type) const override;
bool needsGot(uint32_t Type, const SymbolBody &S) const override;
bool needsPltImpl(uint32_t Type) const override;
+ bool needsThunk(uint32_t Type, const InputFile &File,
+ const SymbolBody &S) const override;
void relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type, uint64_t P,
uint64_t SA) const override;
bool isHintRel(uint32_t Type) const override;
@@ -331,6 +335,11 @@
return Plt_No;
}
+bool TargetInfo::needsThunk(uint32_t Type, const InputFile &File,
+ const SymbolBody &S) const {
+ return false;
+}
+
bool TargetInfo::isTlsInitialExecRel(uint32_t Type) const { return false; }
bool TargetInfo::pointsToLocalDynamicGotEntry(uint32_t Type) const {
@@ -1581,6 +1590,7 @@
PageSize = 65536;
PltEntrySize = 16;
PltZeroSize = 32;
+ ThunkSize = 16;
UseLazyBinding = true;
CopyRel = R_MIPS_COPY;
PltRel = R_MIPS_JUMP_SLOT;
@@ -1695,6 +1705,20 @@
}
template <class ELFT>
+void MipsTargetInfo<ELFT>::writeThunk(uint8_t *Buf, uint64_t S) const {
+ // Write MIPS LA25 thunk code to call PIC function from the non-PIC one.
+ // See MipsTargetInfo::writeThunk for details.
+ const endianness E = ELFT::TargetEndianness;
+ write32<E>(Buf, 0x3c190000); // lui $25, %hi(func)
+ write32<E>(Buf + 4, 0x08000000); // j func
+ write32<E>(Buf + 8, 0x27390000); // addiu $25, $25, %lo(func)
+ write32<E>(Buf + 12, 0x00000000); // nop
+ writeMipsHi16<E>(Buf, S);
+ write32<E>(Buf + 4, 0x08000000 | (S >> 2));
+ writeMipsLo16<E>(Buf + 8, S);
+}
+
+template <class ELFT>
bool MipsTargetInfo<ELFT>::needsCopyRelImpl(uint32_t Type) const {
return !isRelRelative(Type);
}
@@ -1715,6 +1739,31 @@
}
template <class ELFT>
+bool MipsTargetInfo<ELFT>::needsThunk(uint32_t Type, const InputFile &File,
+ const SymbolBody &S) const {
+ // Any MIPS PIC code function is invoked with its address in register $t9.
+ // So if we have a branch instruction from non-PIC code to the PIC one
+ // we cannot make the jump directly and need to create a small stubs
+ // to save the target function address.
+ // See page 3-38 ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
+ if (Type != R_MIPS_26)
+ return false;
+ auto *F = dyn_cast<ELFFileBase<ELFT>>(&File);
+ if (!F)
+ return false;
+ // If current file has PIC code, LA25 stub is not required.
+ if (F->getObj().getHeader()->e_flags & EF_MIPS_PIC)
+ return false;
+ auto *D = dyn_cast<DefinedRegular<ELFT>>(&S);
+ if (!D || !D->Section)
+ return false;
+ // LA25 is required if target file has PIC code
+ // or target symbol is a PIC symbol.
+ return (D->Section->getFile()->getObj().getHeader()->e_flags & EF_MIPS_PIC) ||
+ (D->Sym.st_other & STO_MIPS_MIPS16) == STO_MIPS_PIC;
+}
+
+template <class ELFT>
uint64_t MipsTargetInfo<ELFT>::getImplicitAddend(uint8_t *Buf,
uint32_t Type) const {
const endianness E = ELFT::TargetEndianness;