46707b0a83
The start state of a new section is `EMS_None`, often leading to a $d/$x at offset 0. Introduce a MCTargetOption/cl::opt "implicit-mapsyms" to allow an alternative behavior (https://github.com/ARM-software/abi-aa/issues/274): * Set the start state to `EMS_Data` or `EMS_A64`. * For text sections, add an ending $x only if the final data is not instructions. * For non-text sections, add an ending $d only if the final data is not data commands. ``` .section .text.1,"ax" nop // emit $d .long 42 // emit $x .section .text.2,"ax" nop ``` This new behavior decreases the .symtab size significantly: ``` % bloaty a64-2/bin/clang -- a64-0/bin/clang FILE SIZE VM SIZE -------------- -------------- -5.4% -1.13Mi [ = ] 0 .strtab -50.9% -4.09Mi [ = ] 0 .symtab -4.0% -5.22Mi [ = ] 0 TOTAL ``` --- This scheme works as long as the user can rule out some error scenarios: * .text.1 assembled using the traditional behavior is combined with .text.2 using the new behavior * A linker script combining non-text sections and text sections. The lack of mapping symbols in the non-text sections could make them treated as code, unless the linker inserts extra mapping symbols. The above mix-and-match scenarios aren't an issue at all for a significant portion of users. A text section may start with data commands in rare cases (e.g. -fsanitize=function) that many users don't care about. When combing `(.text.0; .word 0)` and `(.text.1; .word 0)`, the ending $x of .text.0 and the initial $d of .text.1 may have the same address. If both sections reside in the same file, ensure the ending symbol comes before the initial $d of .text.1, so that a dumb linker respecting the symbol order will place the ending $x before the initial $d. Disassemblers using stable sort will see both symbols at the same address, and the second will win. When section ordering mechanisms (e.g. --symbol-ordering-file, --call-graph-profile-sort, `.text : { second.o(.text) first.o(.text) }`) are involved, the initial data in a text section following a text section with trailing data could be misidentified as code, but the issue is local and the risk could be acceptable. Pull Request: https://github.com/llvm/llvm-project/pull/99718
406 lines
15 KiB
C++
406 lines
15 KiB
C++
//===- lib/MC/AArch64ELFStreamer.cpp - ELF Object Output for AArch64 ------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file assembles .s files and emits AArch64 ELF .o object files. Different
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// from generic ELF streamer in emitting mapping symbols ($x and $d) to delimit
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// regions of data and code.
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//
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//===----------------------------------------------------------------------===//
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#include "AArch64ELFStreamer.h"
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#include "AArch64MCTargetDesc.h"
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#include "AArch64TargetStreamer.h"
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#include "AArch64WinCOFFStreamer.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/StringRef.h"
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#include "llvm/ADT/Twine.h"
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#include "llvm/BinaryFormat/ELF.h"
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#include "llvm/MC/MCAsmBackend.h"
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#include "llvm/MC/MCAssembler.h"
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#include "llvm/MC/MCCodeEmitter.h"
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#include "llvm/MC/MCContext.h"
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#include "llvm/MC/MCELFObjectWriter.h"
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#include "llvm/MC/MCELFStreamer.h"
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#include "llvm/MC/MCExpr.h"
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#include "llvm/MC/MCInst.h"
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#include "llvm/MC/MCSectionELF.h"
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#include "llvm/MC/MCStreamer.h"
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#include "llvm/MC/MCSubtargetInfo.h"
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#include "llvm/MC/MCSymbolELF.h"
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#include "llvm/MC/MCTargetOptions.h"
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#include "llvm/MC/MCWinCOFFStreamer.h"
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#include "llvm/Support/Casting.h"
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#include "llvm/Support/FormattedStream.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/TargetParser/Triple.h"
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using namespace llvm;
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namespace {
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class AArch64ELFStreamer;
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class AArch64TargetAsmStreamer : public AArch64TargetStreamer {
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formatted_raw_ostream &OS;
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void emitInst(uint32_t Inst) override;
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void emitDirectiveVariantPCS(MCSymbol *Symbol) override {
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OS << "\t.variant_pcs\t" << Symbol->getName() << "\n";
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}
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void emitARM64WinCFIAllocStack(unsigned Size) override {
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OS << "\t.seh_stackalloc\t" << Size << "\n";
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}
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void emitARM64WinCFISaveR19R20X(int Offset) override {
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OS << "\t.seh_save_r19r20_x\t" << Offset << "\n";
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}
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void emitARM64WinCFISaveFPLR(int Offset) override {
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OS << "\t.seh_save_fplr\t" << Offset << "\n";
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}
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void emitARM64WinCFISaveFPLRX(int Offset) override {
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OS << "\t.seh_save_fplr_x\t" << Offset << "\n";
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}
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void emitARM64WinCFISaveReg(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_reg\tx" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveRegX(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_reg_x\tx" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveRegP(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_regp\tx" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveRegPX(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_regp_x\tx" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveLRPair(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_lrpair\tx" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveFReg(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_freg\td" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveFRegX(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_freg_x\td" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveFRegP(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_fregp\td" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveFRegPX(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_fregp_x\td" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISetFP() override { OS << "\t.seh_set_fp\n"; }
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void emitARM64WinCFIAddFP(unsigned Size) override {
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OS << "\t.seh_add_fp\t" << Size << "\n";
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}
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void emitARM64WinCFINop() override { OS << "\t.seh_nop\n"; }
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void emitARM64WinCFISaveNext() override { OS << "\t.seh_save_next\n"; }
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void emitARM64WinCFIPrologEnd() override { OS << "\t.seh_endprologue\n"; }
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void emitARM64WinCFIEpilogStart() override { OS << "\t.seh_startepilogue\n"; }
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void emitARM64WinCFIEpilogEnd() override { OS << "\t.seh_endepilogue\n"; }
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void emitARM64WinCFITrapFrame() override { OS << "\t.seh_trap_frame\n"; }
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void emitARM64WinCFIMachineFrame() override { OS << "\t.seh_pushframe\n"; }
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void emitARM64WinCFIContext() override { OS << "\t.seh_context\n"; }
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void emitARM64WinCFIECContext() override { OS << "\t.seh_ec_context\n"; }
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void emitARM64WinCFIClearUnwoundToCall() override {
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OS << "\t.seh_clear_unwound_to_call\n";
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}
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void emitARM64WinCFIPACSignLR() override {
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OS << "\t.seh_pac_sign_lr\n";
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}
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void emitARM64WinCFISaveAnyRegI(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg\tx" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveAnyRegIP(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg_p\tx" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveAnyRegD(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg\td" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveAnyRegDP(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg_p\td" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveAnyRegQ(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg\tq" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveAnyRegQP(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg_p\tq" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveAnyRegIX(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg_x\tx" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveAnyRegIPX(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg_px\tx" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveAnyRegDX(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg_x\td" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveAnyRegDPX(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg_px\td" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveAnyRegQX(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg_x\tq" << Reg << ", " << Offset << "\n";
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}
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void emitARM64WinCFISaveAnyRegQPX(unsigned Reg, int Offset) override {
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OS << "\t.seh_save_any_reg_px\tq" << Reg << ", " << Offset << "\n";
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}
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public:
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AArch64TargetAsmStreamer(MCStreamer &S, formatted_raw_ostream &OS);
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};
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AArch64TargetAsmStreamer::AArch64TargetAsmStreamer(MCStreamer &S,
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formatted_raw_ostream &OS)
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: AArch64TargetStreamer(S), OS(OS) {}
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void AArch64TargetAsmStreamer::emitInst(uint32_t Inst) {
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OS << "\t.inst\t0x" << Twine::utohexstr(Inst) << "\n";
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}
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/// Extend the generic ELFStreamer class so that it can emit mapping symbols at
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/// the appropriate points in the object files. These symbols are defined in the
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/// AArch64 ELF ABI:
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/// infocenter.arm.com/help/topic/com.arm.doc.ihi0056a/IHI0056A_aaelf64.pdf
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///
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/// In brief: $x or $d should be emitted at the start of each contiguous region
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/// of A64 code or data in a section. In practice, this emission does not rely
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/// on explicit assembler directives but on inherent properties of the
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/// directives doing the emission (e.g. ".byte" is data, "add x0, x0, x0" an
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/// instruction).
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///
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/// As a result this system is orthogonal to the DataRegion infrastructure used
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/// by MachO. Beware!
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class AArch64ELFStreamer : public MCELFStreamer {
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public:
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friend AArch64TargetELFStreamer;
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AArch64ELFStreamer(MCContext &Context, std::unique_ptr<MCAsmBackend> TAB,
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std::unique_ptr<MCObjectWriter> OW,
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std::unique_ptr<MCCodeEmitter> Emitter)
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: MCELFStreamer(Context, std::move(TAB), std::move(OW),
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std::move(Emitter)),
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LastEMS(EMS_None) {
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auto *TO = getContext().getTargetOptions();
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ImplicitMapSyms = TO && TO->ImplicitMapSyms;
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}
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void changeSection(MCSection *Section, uint32_t Subsection = 0) override {
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// Save the mapping symbol state for potential reuse when revisiting the
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// section. When ImplicitMapSyms is true, the initial state is
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// EMS_A64 for text sections and EMS_Data for the others.
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LastMappingSymbols[getCurrentSection().first] = LastEMS;
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auto It = LastMappingSymbols.find(Section);
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if (It != LastMappingSymbols.end())
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LastEMS = It->second;
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else if (ImplicitMapSyms)
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LastEMS = Section->isText() ? EMS_A64 : EMS_Data;
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else
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LastEMS = EMS_None;
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MCELFStreamer::changeSection(Section, Subsection);
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}
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// Reset state between object emissions
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void reset() override {
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MCELFStreamer::reset();
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LastMappingSymbols.clear();
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LastEMS = EMS_None;
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}
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/// This function is the one used to emit instruction data into the ELF
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/// streamer. We override it to add the appropriate mapping symbol if
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/// necessary.
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void emitInstruction(const MCInst &Inst,
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const MCSubtargetInfo &STI) override {
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emitA64MappingSymbol();
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MCELFStreamer::emitInstruction(Inst, STI);
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}
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/// Emit a 32-bit value as an instruction. This is only used for the .inst
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/// directive, EmitInstruction should be used in other cases.
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void emitInst(uint32_t Inst) {
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char Buffer[4];
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// We can't just use EmitIntValue here, as that will emit a data mapping
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// symbol, and swap the endianness on big-endian systems (instructions are
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// always little-endian).
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for (char &C : Buffer) {
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C = uint8_t(Inst);
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Inst >>= 8;
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}
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emitA64MappingSymbol();
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MCELFStreamer::emitBytes(StringRef(Buffer, 4));
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}
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/// This is one of the functions used to emit data into an ELF section, so the
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/// AArch64 streamer overrides it to add the appropriate mapping symbol ($d)
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/// if necessary.
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void emitBytes(StringRef Data) override {
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emitDataMappingSymbol();
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MCELFStreamer::emitBytes(Data);
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}
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/// This is one of the functions used to emit data into an ELF section, so the
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/// AArch64 streamer overrides it to add the appropriate mapping symbol ($d)
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/// if necessary.
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void emitValueImpl(const MCExpr *Value, unsigned Size, SMLoc Loc) override {
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emitDataMappingSymbol();
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MCELFStreamer::emitValueImpl(Value, Size, Loc);
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}
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void emitFill(const MCExpr &NumBytes, uint64_t FillValue,
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SMLoc Loc) override {
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emitDataMappingSymbol();
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MCObjectStreamer::emitFill(NumBytes, FillValue, Loc);
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}
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private:
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enum ElfMappingSymbol {
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EMS_None,
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EMS_A64,
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EMS_Data
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};
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void emitDataMappingSymbol() {
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if (LastEMS == EMS_Data)
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return;
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emitMappingSymbol("$d");
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LastEMS = EMS_Data;
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}
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void emitA64MappingSymbol() {
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if (LastEMS == EMS_A64)
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return;
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emitMappingSymbol("$x");
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LastEMS = EMS_A64;
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}
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MCSymbol *emitMappingSymbol(StringRef Name) {
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auto *Symbol = cast<MCSymbolELF>(getContext().createLocalSymbol(Name));
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emitLabel(Symbol);
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return Symbol;
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}
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DenseMap<const MCSection *, ElfMappingSymbol> LastMappingSymbols;
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ElfMappingSymbol LastEMS;
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bool ImplicitMapSyms;
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};
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} // end anonymous namespace
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AArch64ELFStreamer &AArch64TargetELFStreamer::getStreamer() {
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return static_cast<AArch64ELFStreamer &>(Streamer);
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}
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void AArch64TargetELFStreamer::emitInst(uint32_t Inst) {
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getStreamer().emitInst(Inst);
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}
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void AArch64TargetELFStreamer::emitDirectiveVariantPCS(MCSymbol *Symbol) {
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getStreamer().getAssembler().registerSymbol(*Symbol);
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cast<MCSymbolELF>(Symbol)->setOther(ELF::STO_AARCH64_VARIANT_PCS);
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}
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void AArch64TargetELFStreamer::finish() {
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AArch64TargetStreamer::finish();
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AArch64ELFStreamer &S = getStreamer();
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MCContext &Ctx = S.getContext();
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auto &Asm = S.getAssembler();
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// If ImplicitMapSyms is specified, ensure that text sections end with
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// the A64 state while non-text sections end with the data state. When
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// sections are combined by the linker, the subsequent section will start with
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// the right state. The ending mapping symbol is added right after the last
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// symbol relative to the section. When a dumb linker combines (.text.0; .word
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// 0) and (.text.1; .word 0), the ending $x of .text.0 precedes the $d of
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// .text.1, even if they have the same address.
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if (S.ImplicitMapSyms) {
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auto &Syms = Asm.getSymbols();
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const size_t NumSyms = Syms.size();
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DenseMap<MCSection *, std::pair<size_t, MCSymbol *>> EndMapSym;
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for (MCSection &Sec : Asm) {
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S.switchSection(&Sec);
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if (S.LastEMS == (Sec.isText() ? AArch64ELFStreamer::EMS_Data
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: AArch64ELFStreamer::EMS_A64))
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EndMapSym.insert(
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{&Sec, {NumSyms, S.emitMappingSymbol(Sec.isText() ? "$x" : "$d")}});
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}
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if (Syms.size() != NumSyms) {
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SmallVector<const MCSymbol *, 0> NewSyms;
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DenseMap<MCSection *, size_t> Cnt;
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Syms.truncate(NumSyms);
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// Find the last symbol index for each candidate section.
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for (auto [I, Sym] : llvm::enumerate(Syms)) {
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if (!Sym->isInSection())
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continue;
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auto It = EndMapSym.find(&Sym->getSection());
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if (It != EndMapSym.end())
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It->second.first = I;
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}
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SmallVector<size_t, 0> Idx;
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for (auto [I, Sym] : llvm::enumerate(Syms)) {
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NewSyms.push_back(Sym);
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if (!Sym->isInSection())
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continue;
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auto It = EndMapSym.find(&Sym->getSection());
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// If `Sym` is the last symbol relative to the section, add the ending
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// mapping symbol after `Sym`.
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if (It != EndMapSym.end() && I == It->second.first) {
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NewSyms.push_back(It->second.second);
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Idx.push_back(I);
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}
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}
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Syms = std::move(NewSyms);
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// F.second holds the number of symbols added before the FILE symbol.
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// Take into account the inserted mapping symbols.
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for (auto &F : S.getWriter().getFileNames())
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F.second += llvm::lower_bound(Idx, F.second) - Idx.begin();
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}
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}
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MCSectionELF *MemtagSec = nullptr;
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for (const MCSymbol &Symbol : Asm.symbols()) {
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const auto &Sym = cast<MCSymbolELF>(Symbol);
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if (Sym.isMemtag()) {
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MemtagSec = Ctx.getELFSection(".memtag.globals.static",
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ELF::SHT_AARCH64_MEMTAG_GLOBALS_STATIC, 0);
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break;
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}
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}
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if (!MemtagSec)
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return;
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// switchSection registers the section symbol and invalidates symbols(). We
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// need a separate symbols() loop.
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S.switchSection(MemtagSec);
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const auto *Zero = MCConstantExpr::create(0, Ctx);
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for (const MCSymbol &Symbol : Asm.symbols()) {
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const auto &Sym = cast<MCSymbolELF>(Symbol);
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if (!Sym.isMemtag())
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continue;
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auto *SRE = MCSymbolRefExpr::create(&Sym, MCSymbolRefExpr::VK_None, Ctx);
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(void)S.emitRelocDirective(*Zero, "BFD_RELOC_NONE", SRE, SMLoc(),
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*Ctx.getSubtargetInfo());
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}
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}
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MCTargetStreamer *
|
|
llvm::createAArch64AsmTargetStreamer(MCStreamer &S, formatted_raw_ostream &OS,
|
|
MCInstPrinter *InstPrint) {
|
|
return new AArch64TargetAsmStreamer(S, OS);
|
|
}
|
|
|
|
MCELFStreamer *
|
|
llvm::createAArch64ELFStreamer(MCContext &Context,
|
|
std::unique_ptr<MCAsmBackend> TAB,
|
|
std::unique_ptr<MCObjectWriter> OW,
|
|
std::unique_ptr<MCCodeEmitter> Emitter) {
|
|
AArch64ELFStreamer *S = new AArch64ELFStreamer(
|
|
Context, std::move(TAB), std::move(OW), std::move(Emitter));
|
|
return S;
|
|
}
|