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[X86] Add REX prefix for GOTTPOFF/TLSDESC relocs in x32 mode 

The REX prefix is needed to allow linker relaxations: even if the
instruction we emit may not need it, the linker may change it to a
different instruction which does need it.
This commit is contained in:
Harald van Dijk 2020-12-15 23:07:34 +00:00
parent e9cda7c5a0
commit 2aae2136d5
No known key found for this signature in database
GPG Key ID: 7D71BB318A5BD56D
2 changed files with 46 additions and 11 deletions
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37
llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp
View File

@ -93,7 +93,8 @@ private:
bool emitOpcodePrefix(int MemOperand, const MCInst &MI,
const MCSubtargetInfo &STI, raw_ostream &OS) const;
bool emitREXPrefix(int MemOperand, const MCInst &MI, raw_ostream &OS) const;
bool emitREXPrefix(int MemOperand, const MCInst &MI,
const MCSubtargetInfo &STI, raw_ostream &OS) const;
};
} // end anonymous namespace
@ -1201,6 +1202,7 @@ void X86MCCodeEmitter::emitVEXOpcodePrefix(int MemOperand, const MCInst &MI,
///
/// \returns true if REX prefix is used, otherwise returns false.
bool X86MCCodeEmitter::emitREXPrefix(int MemOperand, const MCInst &MI,
const MCSubtargetInfo &STI,
raw_ostream &OS) const {
uint8_t REX = [&, MemOperand]() {
uint8_t REX = 0;
@ -1221,15 +1223,28 @@ bool X86MCCodeEmitter::emitREXPrefix(int MemOperand, const MCInst &MI,
// If it accesses SPL, BPL, SIL, or DIL, then it requires a 0x40 REX prefix.
for (unsigned i = CurOp; i != NumOps; ++i) {
const MCOperand &MO = MI.getOperand(i);
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (Reg == X86::AH || Reg == X86::BH || Reg == X86::CH || Reg == X86::DH)
UsesHighByteReg = true;
if (X86II::isX86_64NonExtLowByteReg(Reg))
// FIXME: The caller of determineREXPrefix slaps this prefix onto
// anything that returns non-zero.
REX |= 0x40; // REX fixed encoding prefix
if (MO.isReg()) {
unsigned Reg = MO.getReg();
if (Reg == X86::AH || Reg == X86::BH || Reg == X86::CH ||
Reg == X86::DH)
UsesHighByteReg = true;
if (X86II::isX86_64NonExtLowByteReg(Reg))
// FIXME: The caller of determineREXPrefix slaps this prefix onto
// anything that returns non-zero.
REX |= 0x40; // REX fixed encoding prefix
} else if (MO.isExpr() &&
STI.getTargetTriple().getEnvironment() == Triple::GNUX32) {
// GOTTPOFF and TLSDESC relocations require a REX prefix to allow
// linker optimizations: even if the instructions we see may not require
// any prefix, they may be replaced by instructions that do. This is
// handled as a special case here so that it also works for hand-written
// assembly without the user needing to write REX, as with GNU as.
const auto *Ref = dyn_cast<MCSymbolRefExpr>(MO.getExpr());
if (Ref && (Ref->getKind() == MCSymbolRefExpr::VK_GOTTPOFF ||
Ref->getKind() == MCSymbolRefExpr::VK_TLSDESC)) {
REX |= 0x40; // REX fixed encoding prefix
}
}
}
switch (TSFlags & X86II::FormMask) {
@ -1352,7 +1367,7 @@ bool X86MCCodeEmitter::emitOpcodePrefix(int MemOperand, const MCInst &MI,
assert((STI.hasFeature(X86::Mode64Bit) || !(TSFlags & X86II::REX_W)) &&
"REX.W requires 64bit mode.");
bool HasREX = STI.hasFeature(X86::Mode64Bit)
? emitREXPrefix(MemOperand, MI, OS)
? emitREXPrefix(MemOperand, MI, STI, OS)
: false;
// 0x0F escape code must be emitted just before the opcode.

20
llvm/test/MC/X86/tlsdesc-x32.s Normal file
View File

@ -0,0 +1,20 @@
# RUN: llvm-mc -triple x86_64-pc-linux-gnux32 %s | FileCheck --check-prefix=PRINT %s
# RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnux32 %s -o %t
# RUN: llvm-readelf -s %t | FileCheck --check-prefix=SYM %s
# RUN: llvm-objdump -d -r %t | FileCheck --match-full-lines %s
# PRINT: leal a@tlsdesc(%rip), %eax
# PRINT-NEXT: callq *a@tlscall(%eax)
# SYM: TLS GLOBAL DEFAULT UND a
# CHECK: 0: 40 8d 05 00 00 00 00 leal (%rip), %eax # 7 <{{.*}}>
# CHECK-NEXT: 00000003: R_X86_64_GOTPC32_TLSDESC a-0x4
# CHECK-NEXT: 7: 67 ff 10 callq *(%eax)
# CHECK-NEXT: 00000007: R_X86_64_TLSDESC_CALL a
# CHECK-NEXT: a: 8d 05 34 12 00 00 leal 4660(%rip), %eax # {{.*}}
lea a@tlsdesc(%rip), %eax
call *a@tlscall(%eax)
lea 0x1234(%rip), %eax