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mold/elf/arch-riscv.cc

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// RISC-V is a clean RISC ISA. It supports PC-relative load/store for
// position-independent code. Its 32-bit and 64-bit ISAs are almost
// identical. That is, you can think RV32 as a RV64 without 64-bit
// operations. In this file, we support both RV64 and RV32.
//
// RISC-V is essentially little-endian, but the big-endian version is
// available as an extension. GCC supports `-mbig-endian` to generate
// big-endian code. Even in big-endian mode, machine instructions are
// defined to be encoded in little-endian, though. Only the behavior of
// load/store instructions are different between LE RISC-V and BE RISC-V.
//
// From the linker's point of view, the RISC-V's psABI is unique because
// sections in input object files can be shrunk while being copied to the
// output file. That is contrary to other psABIs in which sections are an
// atomic unit of copying. Let me explain it in more details.
//
// Since RISC-V instructions are 16-bit or 32-bit long, there's no way to
// embed a very large immediate into a branch instruction. In fact, JAL
// (jump and link) instruction can jump to only within PC ± 1 MiB because
// its immediate is only 21 bits long. If the destination is out of its
// reach, we need to use two instructions instead; the first instruction
// being AUIPC which sets upper 20 bits to a register and the second being
// JALR with a 12-bit immediate and the register. Combined, they specify a
// 32 bits displacement.
//
// Other RISC ISAs have the same limitation, and they solved the problem by
// letting the linker create so-called "range extension thunks". It works as
// follows: the compiler optimistically emits single jump instructions for
// function calls. If the linker finds that a branch target is out of reach,
// it emits a small piece of machine code near the branch instruction and
// redirect the branch to the linker-synthesized code. The code constructs a
// full 32-bit address in a register and jump to the destination. That
// linker-synthesized code is called "range extension thunks" or just
// "thunks".
//
// The RISC-V psABI is unique that it works the other way around. That is,
// for RISC-V, the compiler always emits two instructions (AUIPC + JAL) for
// function calls. If the linker finds the destination is reachable with a
// single instruction, it replaces the two instructions with the one and
// shrink the section size by one instruction length, instead of filling the
// gap with a nop.
//
// With the presence of this relaxation, sections can no longer be
// considered as an atomic unit. If we delete 4 bytes from the middle of a
// section, all contents after that point needs to be shifted by 4. Symbol
// values and relocation offsets have to be adjusted accordingly if they
// refer past the deleted bytes.
//
// In mold, we use `r_deltas` to memorize how many bytes have be adjusted
// for relocations. For symbols, we directly mutate their `value` member.
//
// RISC-V object files tend to have way more relocations than those for
// other targets. This is because all branches, including ones that jump
// within the same section, are explicitly expressed with relocations.
// Here is why we need them: all control-flow statements such as `if` or
// `for` are implemented using branch instructions. For other targets, the
// compiler doesn't emit relocations for such branches because they know
// at compile-time exactly how many bytes has to be skipped. That's not
// true to RISC-V because the linker may delete bytes between a branch and
// its destination. Therefore, all branches including in-section ones have
// to be explicitly expressed with relocations.
//
// Note that this mechanism only shrink sections and never enlarge, as
// the compiler always emits the longest instruction sequence. This
// makes the linker implementation a bit simpler because we don't need
// to worry about oscillation.
//
// https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc
#include "mold.h"
#include <tbb/parallel_for.h>
#include <tbb/parallel_for_each.h>
namespace mold::elf {
static u32 itype(u32 val) {
return val << 20;
}
static u32 stype(u32 val) {
return bits(val, 11, 5) << 25 | bits(val, 4, 0) << 7;
}
static u32 btype(u32 val) {
return bit(val, 12) << 31 | bits(val, 10, 5) << 25 |
bits(val, 4, 1) << 8 | bit(val, 11) << 7;
}
static u32 utype(u32 val) {
// U-type instructions are used in combination with I-type
// instructions. U-type insn sets an immediate to the upper 20-bits
// of a register. I-type insn sign-extends a 12-bits immediate and
// adds it to a register value to construct a complete value. 0x800
// is added here to compensate for the sign-extension.
return (val + 0x800) & 0xffff'f000;
}
static u32 jtype(u32 val) {
return bit(val, 20) << 31 | bits(val, 10, 1) << 21 |
bit(val, 11) << 20 | bits(val, 19, 12) << 12;
}
static u32 cbtype(u32 val) {
return bit(val, 8) << 12 | bit(val, 4) << 11 | bit(val, 3) << 10 |
bit(val, 7) << 6 | bit(val, 6) << 5 | bit(val, 2) << 4 |
bit(val, 1) << 3 | bit(val, 5) << 2;
}
static u32 cjtype(u32 val) {
return bit(val, 11) << 12 | bit(val, 4) << 11 | bit(val, 9) << 10 |
bit(val, 8) << 9 | bit(val, 10) << 8 | bit(val, 6) << 7 |
bit(val, 7) << 6 | bit(val, 3) << 5 | bit(val, 2) << 4 |
bit(val, 1) << 3 | bit(val, 5) << 2;
}
static void write_itype(u8 *loc, u32 val) {
*(ul32 *)loc &= 0b000000'00000'11111'111'11111'1111111;
*(ul32 *)loc |= itype(val);
}
static void write_stype(u8 *loc, u32 val) {
*(ul32 *)loc &= 0b000000'11111'11111'111'00000'1111111;
*(ul32 *)loc |= stype(val);
}
static void write_btype(u8 *loc, u32 val) {
*(ul32 *)loc &= 0b000000'11111'11111'111'00000'1111111;
*(ul32 *)loc |= btype(val);
}
static void write_utype(u8 *loc, u32 val) {
*(ul32 *)loc &= 0b000000'00000'00000'000'11111'1111111;
*(ul32 *)loc |= utype(val);
}
static void write_jtype(u8 *loc, u32 val) {
*(ul32 *)loc &= 0b000000'00000'00000'000'11111'1111111;
*(ul32 *)loc |= jtype(val);
}
static void write_cbtype(u8 *loc, u32 val) {
*(ul16 *)loc &= 0b111'000'111'00000'11;
*(ul16 *)loc |= cbtype(val);
}
static void write_cjtype(u8 *loc, u32 val) {
*(ul16 *)loc &= 0b111'00000000000'11;
*(ul16 *)loc |= cjtype(val);
}
// Returns the rd register of an R/I/U/J-type instruction.
static u32 get_rd(u32 val) {
return bits(val, 11, 7);
}
static void set_rs1(u8 *loc, u32 rs1) {
assert(rs1 < 32);
*(ul32 *)loc &= 0b111111'11111'00000'111'11111'1111111;
*(ul32 *)loc |= rs1 << 15;
}
template <typename E>
void write_plt_header(Context<E> &ctx, u8 *buf) {
static const ul32 insn_64[] = {
0x0000'0397, // auipc t2, %pcrel_hi(.got.plt)
0x41c3'0333, // sub t1, t1, t3 # .plt entry + hdr + 12
0x0003'be03, // ld t3, %pcrel_lo(1b)(t2) # _dl_runtime_resolve
0xfd43'0313, // addi t1, t1, -44 # .plt entry
0x0003'8293, // addi t0, t2, %pcrel_lo(1b) # &.got.plt
0x0013'5313, // srli t1, t1, 1 # .plt entry offset
0x0082'b283, // ld t0, 8(t0) # link map
0x000e'0067, // jr t3
};
static const ul32 insn_32[] = {
0x0000'0397, // auipc t2, %pcrel_hi(.got.plt)
0x41c3'0333, // sub t1, t1, t3 # .plt entry + hdr + 12
0x0003'ae03, // lw t3, %pcrel_lo(1b)(t2) # _dl_runtime_resolve
0xfd43'0313, // addi t1, t1, -44 # .plt entry
0x0003'8293, // addi t0, t2, %pcrel_lo(1b) # &.got.plt
0x0023'5313, // srli t1, t1, 2 # .plt entry offset
0x0042'a283, // lw t0, 4(t0) # link map
0x000e'0067, // jr t3
};
if constexpr (E::is_64)
memcpy(buf, insn_64, sizeof(insn_64));
else
memcpy(buf, insn_32, sizeof(insn_32));
u64 gotplt = ctx.gotplt->shdr.sh_addr;
u64 plt = ctx.plt->shdr.sh_addr;
write_utype(buf, gotplt - plt);
write_itype(buf + 8, gotplt - plt);
write_itype(buf + 16, gotplt - plt);
}
static const ul32 plt_entry_64[] = {
0x0000'0e17, // auipc t3, %pcrel_hi(function@.got.plt)
0x000e'3e03, // ld t3, %pcrel_lo(1b)(t3)
0x000e'0367, // jalr t1, t3
0x0000'0013, // nop
};
static const ul32 plt_entry_32[] = {
0x0000'0e17, // auipc t3, %pcrel_hi(function@.got.plt)
0x000e'2e03, // lw t3, %pcrel_lo(1b)(t3)
0x000e'0367, // jalr t1, t3
0x0000'0013, // nop
};
template <typename E>
void write_plt_entry(Context<E> &ctx, u8 *buf, Symbol<E> &sym) {
if constexpr (E::is_64)
memcpy(buf, plt_entry_64, sizeof(plt_entry_64));
else
memcpy(buf, plt_entry_32, sizeof(plt_entry_32));
u64 gotplt = sym.get_gotplt_addr(ctx);
u64 plt = sym.get_plt_addr(ctx);
write_utype(buf, gotplt - plt);
write_itype(buf + 4, gotplt - plt);
}
template <typename E>
void write_pltgot_entry(Context<E> &ctx, u8 *buf, Symbol<E> &sym) {
if constexpr (E::is_64)
memcpy(buf, plt_entry_64, sizeof(plt_entry_64));
else
memcpy(buf, plt_entry_32, sizeof(plt_entry_32));
u64 got = sym.get_got_addr(ctx);
u64 plt = sym.get_plt_addr(ctx);
write_utype(buf, got - plt);
write_itype(buf + 4, got - plt);
}
template <typename E>
void EhFrameSection<E>::apply_reloc(Context<E> &ctx, const ElfRel<E> &rel,
u64 offset, u64 val) {
u8 *loc = ctx.buf + this->shdr.sh_offset + offset;
switch (rel.r_type) {
case R_NONE:
break;
case R_RISCV_ADD32:
*(U32<E> *)loc += val;
break;
case R_RISCV_SUB8:
*loc -= val;
break;
case R_RISCV_SUB16:
*(U16<E> *)loc -= val;
break;
case R_RISCV_SUB32:
*(U32<E> *)loc -= val;
break;
case R_RISCV_SUB6:
*loc = (*loc & 0b1100'0000) | ((*loc - val) & 0b0011'1111);
break;
case R_RISCV_SET6:
*loc = (*loc & 0b1100'0000) | (val & 0b0011'1111);
break;
case R_RISCV_SET8:
*loc = val;
break;
case R_RISCV_SET16:
*(U16<E> *)loc = val;
break;
case R_RISCV_SET32:
*(U32<E> *)loc = val;
break;
case R_RISCV_32_PCREL:
*(U32<E> *)loc = val - this->shdr.sh_addr - offset;
break;
default:
Fatal(ctx) << "unsupported relocation in .eh_frame: " << rel;
}
}
template <typename E>
void InputSection<E>::apply_reloc_alloc(Context<E> &ctx, u8 *base) {
std::span<const ElfRel<E>> rels = get_rels(ctx);
ElfRel<E> *dynrel = nullptr;
if (ctx.reldyn)
dynrel = (ElfRel<E> *)(ctx.buf + ctx.reldyn->shdr.sh_offset +
file.reldyn_offset + this->reldyn_offset);
auto get_r_delta = [&](i64 idx) {
return extra.r_deltas.empty() ? 0 : extra.r_deltas[idx];
};
for (i64 i = 0; i < rels.size(); i++) {
const ElfRel<E> &rel = rels[i];
if (rel.r_type == R_NONE || rel.r_type == R_RISCV_RELAX)
continue;
Symbol<E> &sym = *file.symbols[rel.r_sym];
i64 r_offset = rel.r_offset - get_r_delta(i);
i64 removed_bytes = get_r_delta(i + 1) - get_r_delta(i);
u8 *loc = base + r_offset;
auto check = [&](i64 val, i64 lo, i64 hi) {
if (val < lo || hi <= val)
Error(ctx) << *this << ": relocation " << rel << " against "
<< sym << " out of range: " << val << " is not in ["
<< lo << ", " << hi << ")";
};
u64 S = sym.get_addr(ctx);
u64 A = rel.r_addend;
u64 P = get_addr() + r_offset;
u64 G = sym.get_got_idx(ctx) * sizeof(Word<E>);
u64 GOT = ctx.got->shdr.sh_addr;
switch (rel.r_type) {
case R_RISCV_32:
if constexpr (E::is_64)
*(U32<E> *)loc = S + A;
else
apply_dyn_absrel(ctx, sym, rel, loc, S, A, P, dynrel);
break;
case R_RISCV_64:
assert(E::is_64);
apply_dyn_absrel(ctx, sym, rel, loc, S, A, P, dynrel);
break;
case R_RISCV_BRANCH:
check(S + A - P, -(1 << 12), 1 << 12);
write_btype(loc, S + A - P);
break;
case R_RISCV_JAL:
check(S + A - P, -(1 << 20), 1 << 20);
write_jtype(loc, S + A - P);
break;
case R_RISCV_CALL:
case R_RISCV_CALL_PLT: {
u32 rd = get_rd(*(ul32 *)(contents.data() + rel.r_offset + 4));
if (removed_bytes == 4) {
// auipc + jalr -> jal
*(ul32 *)loc = (rd << 7) | 0b1101111;
write_jtype(loc, S + A - P);
} else if (removed_bytes == 6 && rd == 0) {
// auipc + jalr -> c.j
*(ul16 *)loc = 0b101'00000000000'01;
write_cjtype(loc, S + A - P);
} else if (removed_bytes == 6 && rd == 1) {
// auipc + jalr -> c.jal
assert(!E::is_64);
*(ul16 *)loc = 0b001'00000000000'01;
write_cjtype(loc, S + A - P);
} else {
assert(removed_bytes == 0);
u64 val = sym.esym().is_undef_weak() ? 0 : S + A - P;
check(val, -(1LL << 31), 1LL << 31);
write_utype(loc, val);
write_itype(loc + 4, val);
}
break;
}
case R_RISCV_GOT_HI20:
*(ul32 *)loc = G + GOT + A - P;
break;
case R_RISCV_TLS_GOT_HI20:
*(ul32 *)loc = sym.get_gottp_addr(ctx) + A - P;
break;
case R_RISCV_TLS_GD_HI20:
*(ul32 *)loc = sym.get_tlsgd_addr(ctx) + A - P;
break;
case R_RISCV_PCREL_HI20:
if (sym.esym().is_undef_weak()) {
// Calling an undefined weak symbol does not make sense.
// We make such call into an infinite loop. This should
// help debugging of a faulty program.
*(ul32 *)loc = 0;
} else {
*(ul32 *)loc = S + A - P;
}
break;
case R_RISCV_PCREL_LO12_I:
case R_RISCV_PCREL_LO12_S:
// These relocations are handled in the next loop.
break;
case R_RISCV_HI20:
assert(removed_bytes == 0 || removed_bytes == 4);
if (removed_bytes == 0) {
check(S + A, -(1LL << 31), 1LL << 31);
write_utype(loc, S + A);
}
break;
case R_RISCV_LO12_I:
case R_RISCV_LO12_S:
if (rel.r_type == R_RISCV_LO12_I)
write_itype(loc, S + A);
else
write_stype(loc, S + A);
// Rewrite `lw t1, 0(t0)` with `lw t1, 0(x0)` if the address is
// accessible relative to the zero register. If the upper 20 bits
// are all zero, the corresponding LUI might have been removed.
if (bits(S + A, 31, 12) == 0)
set_rs1(loc, 0);
break;
case R_RISCV_TPREL_HI20:
assert(removed_bytes == 0 || removed_bytes == 4);
if (removed_bytes == 0)
write_utype(loc, S + A - ctx.tp_addr);
break;
case R_RISCV_TPREL_ADD:
// This relocation just annotates an ADD instruction that can be
// removed when a TPREL is relaxed. No value is needed to be
// written.
assert(removed_bytes == 0 || removed_bytes == 4);
break;
case R_RISCV_TPREL_LO12_I:
case R_RISCV_TPREL_LO12_S: {
i64 val = S + A - ctx.tp_addr;
if (rel.r_type == R_RISCV_TPREL_LO12_I)
write_itype(loc, val);
else
write_stype(loc, val);
// Rewrite `lw t1, 0(t0)` with `lw t1, 0(tp)` if the address is
// directly accessible using tp. tp is x4.
if (sign_extend(val, 11) == val)
set_rs1(loc, 4);
break;
}
case R_RISCV_ADD8:
loc += S + A;
break;
case R_RISCV_ADD16:
*(U16<E> *)loc += S + A;
break;
case R_RISCV_ADD32:
*(U32<E> *)loc += S + A;
break;
case R_RISCV_ADD64:
*(U64<E> *)loc += S + A;
break;
case R_RISCV_SUB8:
loc -= S + A;
break;
case R_RISCV_SUB16:
*(U16<E> *)loc -= S + A;
break;
case R_RISCV_SUB32:
*(U32<E> *)loc -= S + A;
break;
case R_RISCV_SUB64:
*(U64<E> *)loc -= S + A;
break;
case R_RISCV_ALIGN: {
// A R_RISCV_ALIGN is followed by a NOP sequence. We need to remove
// zero or more bytes so that the instruction after R_RISCV_ALIGN is
// aligned to a given alignment boundary.
//
// We need to guarantee that the NOP sequence is valid after byte
// removal (e.g. we can't remove the first 2 bytes of a 4-byte NOP).
// For the sake of simplicity, we always rewrite the entire NOP sequence.
i64 padding_bytes = rel.r_addend - removed_bytes;
assert((padding_bytes & 1) == 0);
i64 i = 0;
for (; i <= padding_bytes - 4; i += 4)
*(ul32 *)(loc + i) = 0x0000'0013; // nop
if (i < padding_bytes)
*(ul16 *)(loc + i) = 0x0001; // c.nop
break;
}
case R_RISCV_RVC_BRANCH:
check(S + A - P, -(1 << 8), 1 << 8);
write_cbtype(loc, S + A - P);
break;
case R_RISCV_RVC_JUMP:
check(S + A - P, -(1 << 11), 1 << 11);
write_cjtype(loc, S + A - P);
break;
case R_RISCV_SUB6:
*loc = (*loc & 0b1100'0000) | ((*loc - (S + A)) & 0b0011'1111);
break;
case R_RISCV_SET6:
*loc = (*loc & 0b1100'0000) | ((S + A) & 0b0011'1111);
break;
case R_RISCV_SET8:
*loc = S + A;
break;
case R_RISCV_SET16:
*(U16<E> *)loc = S + A;
break;
case R_RISCV_SET32:
*(U32<E> *)loc = S + A;
break;
case R_RISCV_PLT32:
case R_RISCV_32_PCREL:
*(U32<E> *)loc = S + A - P;
break;
default:
unreachable();
}
}
// Handle PC-relative LO12 relocations. In the above loop, pcrel HI20
// relocations overwrote instructions with full 32-bit values to allow
// their corresponding pcrel LO12 relocations to read their values.
for (i64 i = 0; i < rels.size(); i++) {
switch (rels[i].r_type) {
case R_RISCV_PCREL_LO12_I:
case R_RISCV_PCREL_LO12_S: {
Symbol<E> &sym = *file.symbols[rels[i].r_sym];
assert(sym.get_input_section() == this);
u8 *loc = base + rels[i].r_offset - get_r_delta(i);
u32 val = *(ul32 *)(base + sym.value);
if (rels[i].r_type == R_RISCV_PCREL_LO12_I)
write_itype(loc, val);
else
write_stype(loc, val);
}
}
}
// Restore the original instructions pcrel HI20 relocations overwrote.
for (i64 i = 0; i < rels.size(); i++) {
switch (rels[i].r_type) {
case R_RISCV_GOT_HI20:
case R_RISCV_PCREL_HI20:
case R_RISCV_TLS_GOT_HI20:
case R_RISCV_TLS_GD_HI20: {
u8 *loc = base + rels[i].r_offset - get_r_delta(i);
u32 val = *(ul32 *)loc;
memcpy(loc, contents.data() + rels[i].r_offset, 4);
write_utype(loc, val);
}
}
}
}
template <typename E>
void InputSection<E>::apply_reloc_nonalloc(Context<E> &ctx, u8 *base) {
std::span<const ElfRel<E>> rels = get_rels(ctx);
for (i64 i = 0; i < rels.size(); i++) {
const ElfRel<E> &rel = rels[i];
if (rel.r_type == R_NONE || record_undef_error(ctx, rel))
continue;
Symbol<E> &sym = *file.symbols[rel.r_sym];
u8 *loc = base + rel.r_offset;
SectionFragment<E> *frag;
i64 frag_addend;
std::tie(frag, frag_addend) = get_fragment(ctx, rel);
u64 S = frag ? frag->get_addr(ctx) : sym.get_addr(ctx);
u64 A = frag ? frag_addend : (i64)rel.r_addend;
switch (rel.r_type) {
case R_RISCV_32:
*(U32<E> *)loc = S + A;
break;
case R_RISCV_64:
if (std::optional<u64> val = get_tombstone(sym, frag))
*(U64<E> *)loc = *val;
else
*(U64<E> *)loc = S + A;
break;
case R_RISCV_ADD8:
*loc += S + A;
break;
case R_RISCV_ADD16:
*(U16<E> *)loc += S + A;
break;
case R_RISCV_ADD32:
*(U32<E> *)loc += S + A;
break;
case R_RISCV_ADD64:
*(U64<E> *)loc += S + A;
break;
case R_RISCV_SUB8:
*loc -= S + A;
break;
case R_RISCV_SUB16:
*(U16<E> *)loc -= S + A;
break;
case R_RISCV_SUB32:
*(U32<E> *)loc -= S + A;
break;
case R_RISCV_SUB64:
*(U64<E> *)loc -= S + A;
break;
case R_RISCV_SUB6:
*loc = (*loc & 0b1100'0000) | ((*loc - (S + A)) & 0b0011'1111);
break;
case R_RISCV_SET6:
*loc = (*loc & 0b1100'0000) | ((S + A) & 0b0011'1111);
break;
case R_RISCV_SET8:
*loc = S + A;
break;
case R_RISCV_SET16:
*(U16<E> *)loc = S + A;
break;
case R_RISCV_SET32:
*(U32<E> *)loc = S + A;
break;
default:
Fatal(ctx) << *this << ": invalid relocation for non-allocated sections: "
<< rel;
break;
}
}
}
template <typename E>
void InputSection<E>::copy_contents_riscv(Context<E> &ctx, u8 *buf) {
// If a section is not relaxed, we can copy it as a one big chunk.
if (extra.r_deltas.empty()) {
uncompress_to(ctx, buf);
return;
}
// A relaxed section is copied piece-wise.
std::span<const ElfRel<E>> rels = get_rels(ctx);
i64 pos = 0;
for (i64 i = 0; i < rels.size(); i++) {
i64 delta = extra.r_deltas[i + 1] - extra.r_deltas[i];
if (delta == 0)
continue;
assert(delta > 0);
const ElfRel<E> &r = rels[i];
memcpy(buf, contents.data() + pos, r.r_offset - pos);
buf += r.r_offset - pos;
pos = r.r_offset + delta;
}
memcpy(buf, contents.data() + pos, contents.size() - pos);
}
template <typename E>
void InputSection<E>::scan_relocations(Context<E> &ctx) {
assert(shdr().sh_flags & SHF_ALLOC);
this->reldyn_offset = file.num_dynrel * sizeof(ElfRel<E>);
std::span<const ElfRel<E>> rels = get_rels(ctx);
// Scan relocations
for (i64 i = 0; i < rels.size(); i++) {
const ElfRel<E> &rel = rels[i];
if (rel.r_type == R_NONE || record_undef_error(ctx, rel))
continue;
Symbol<E> &sym = *file.symbols[rel.r_sym];
if (sym.is_ifunc())
sym.flags |= NEEDS_GOT | NEEDS_PLT;
switch (rel.r_type) {
case R_RISCV_32:
if constexpr (E::is_64)
scan_absrel(ctx, sym, rel);
else
scan_dyn_absrel(ctx, sym, rel);
break;
case R_RISCV_HI20:
scan_absrel(ctx, sym, rel);
break;
case R_RISCV_64:
if constexpr (!E::is_64)
Fatal(ctx) << *this << ": R_RISCV_64 cannot be used on RV32";
scan_dyn_absrel(ctx, sym, rel);
break;
case R_RISCV_CALL:
case R_RISCV_CALL_PLT:
case R_RISCV_PLT32:
if (sym.is_imported)
sym.flags |= NEEDS_PLT;
break;
case R_RISCV_GOT_HI20:
sym.flags |= NEEDS_GOT;
break;
case R_RISCV_TLS_GOT_HI20:
sym.flags |= NEEDS_GOTTP;
break;
case R_RISCV_TLS_GD_HI20:
sym.flags |= NEEDS_TLSGD;
break;
case R_RISCV_32_PCREL:
scan_pcrel(ctx, sym, rel);
break;
case R_RISCV_TPREL_HI20:
case R_RISCV_TPREL_LO12_I:
case R_RISCV_TPREL_LO12_S:
case R_RISCV_TPREL_ADD:
check_tlsle(ctx, sym, rel);
break;
case R_RISCV_BRANCH:
case R_RISCV_JAL:
case R_RISCV_PCREL_HI20:
case R_RISCV_PCREL_LO12_I:
case R_RISCV_PCREL_LO12_S:
case R_RISCV_LO12_I:
case R_RISCV_LO12_S:
case R_RISCV_ADD8:
case R_RISCV_ADD16:
case R_RISCV_ADD32:
case R_RISCV_ADD64:
case R_RISCV_SUB8:
case R_RISCV_SUB16:
case R_RISCV_SUB32:
case R_RISCV_SUB64:
case R_RISCV_ALIGN:
case R_RISCV_RVC_BRANCH:
case R_RISCV_RVC_JUMP:
case R_RISCV_RELAX:
case R_RISCV_SUB6:
case R_RISCV_SET6:
case R_RISCV_SET8:
case R_RISCV_SET16:
case R_RISCV_SET32:
break;
default:
Error(ctx) << *this << ": unknown relocation: " << rel;
}
}
}
template <typename E>
static bool is_resizable(Context<E> &ctx, InputSection<E> *isec) {
return isec && isec->is_alive && (isec->shdr().sh_flags & SHF_ALLOC) &&
(isec->shdr().sh_flags & SHF_EXECINSTR);
}
// Returns the distance between a relocated place and a symbol.
template <typename E>
static i64 compute_distance(Context<E> &ctx, Symbol<E> &sym,
InputSection<E> &isec, const ElfRel<E> &rel) {
// We handle absolute symbols as if they were infinitely far away
// because `shrink_section` may increase a distance between a branch
// instruction and an absolute symbol. Branching to an absolute
// location is extremely rare in real code, though.
if (sym.is_absolute())
return INT32_MAX;
// Likewise, relocations against weak undefined symbols won't be relaxed.
if (sym.esym().is_undef_weak())
return INT32_MAX;
// Compute a distance between the relocated place and the symbol.
i64 S = sym.get_addr(ctx);
i64 A = rel.r_addend;
i64 P = isec.get_addr() + rel.r_offset;
return S + A - P;
}
// Scan relocations to shrink sections.
template <typename E>
static void shrink_section(Context<E> &ctx, InputSection<E> &isec, bool use_rvc) {
std::span<const ElfRel<E>> rels = isec.get_rels(ctx);
isec.extra.r_deltas.resize(rels.size() + 1);
i64 delta = 0;
for (i64 i = 0; i < rels.size(); i++) {
const ElfRel<E> &r = rels[i];
Symbol<E> &sym = *isec.file.symbols[r.r_sym];
isec.extra.r_deltas[i] = delta;
// Handling R_RISCV_ALIGN is mandatory.
//
// R_RISCV_ALIGN refers NOP instructions. We need to eliminate some
// or all of the instructions so that the instruction that immediately
// follows the NOPs is aligned to a specified alignment boundary.
if (r.r_type == R_RISCV_ALIGN) {
// The total bytes of NOPs is stored to r_addend, so the next
// instruction is r_addend away.
u64 loc = isec.get_addr() + r.r_offset - delta;
u64 next_loc = loc + r.r_addend;
u64 alignment = bit_ceil(r.r_addend + 1);
assert(alignment <= (1 << isec.p2align));
delta += next_loc - align_to(loc, alignment);
continue;
}
// Handling other relocations is optional.
if (!ctx.arg.relax || i == rels.size() - 1 ||
rels[i + 1].r_type != R_RISCV_RELAX)
continue;
// Linker-synthesized symbols haven't been assigned their final
// values when we are shrinking sections because actual values can
// be computed only after we fix the file layout. Therefore, we
// assume that relocations against such symbols are always
// non-relaxable.
if (sym.file == ctx.internal_obj)
continue;
switch (r.r_type) {
case R_RISCV_CALL:
case R_RISCV_CALL_PLT: {
// These relocations refer an AUIPC + JALR instruction pair to
// allow to jump to anywhere in PC ± 2 GiB. If the jump target is
// close enough to PC, we can use C.J, C.JAL or JAL instead.
i64 dist = compute_distance(ctx, sym, isec, r);
if (dist & 1)
break;
std::string_view contents = isec.contents;
i64 rd = get_rd(*(ul32 *)(contents.data() + r.r_offset + 4));
if (rd == 0 && sign_extend(dist, 11) == dist && use_rvc) {
// If rd is x0 and the jump target is within ±2 KiB, we can use
// C.J, saving 6 bytes.
delta += 6;
} else if (rd == 1 && sign_extend(dist, 11) == dist && use_rvc && !E::is_64) {
// If rd is x1 and the jump target is within ±2 KiB, we can use
// C.JAL. This is RV32 only because C.JAL is RV32-only instruction.
delta += 6;
} else if (sign_extend(dist, 20) == dist) {
// If the jump target is within ±1 MiB, we can use JAL.
delta += 4;
}
break;
}
case R_RISCV_HI20:
// If the upper 20 bits are all zero, we can remove LUI.
// The corresponding instructions referred by LO12_I/LO12_S
// relocations will use the zero register instead.
if (bits(sym.get_addr(ctx), 31, 12) == 0)
delta += 4;
break;
case R_RISCV_TPREL_HI20:
case R_RISCV_TPREL_ADD: {
// These relocations are used to add a high 20-bit value to the
// thread pointer. The following two instructions materializes
// TP + HI20(foo) in %r5, for example.
//
// lui a5,%tprel_hi(foo) # R_RISCV_TPREL_HI20 (symbol)
// add a5,a5,tp,%tprel_add(foo) # R_RISCV_TPREL_ADD (symbol)
//
// Then thread-local variable `foo` is accessed with a low 12-bit
// offset like this:
//
// sw t0,%tprel_lo(foo)(a5) # R_RISCV_TPREL_LO12_S (symbol)
//
// However, if the variable is at TP ±2 KiB, TP + HI20(foo) is the
// same as TP, so we can instead access the thread-local variable
// directly using TP like this:
//
// sw t0,%tprel_lo(foo)(tp)
//
// Here, we remove `lui` and `add` if the offset is within ±2 KiB.
i64 val = sym.get_addr(ctx) + r.r_addend - ctx.tp_addr;
if (sign_extend(val, 11) == val)
delta += 4;
break;
}
}
}
isec.extra.r_deltas[rels.size()] = delta;
isec.sh_size -= delta;
}
// Shrink sections by interpreting relocations.
//
// This operation seems to be optional, because by default longest
// instructions are being used. However, calling this function is actually
// mandatory because of R_RISCV_ALIGN. R_RISCV_ALIGN is a directive to the
// linker to align the location referred to by the relocation to a
// specified byte boundary. We at least have to interpret them to satisfy
// the alignment constraints.
template <typename E>
i64 riscv_resize_sections(Context<E> &ctx) {
Timer t(ctx, "riscv_resize_sections");
// True if we can use the 2-byte instructions. This is usually true on
// Unix because RV64GC is generally considered the baseline hardware.
bool use_rvc = get_eflags(ctx) & EF_RISCV_RVC;
// Find all the relocations that can be relaxed.
// This step should only shrink sections.
tbb::parallel_for_each(ctx.objs, [&](ObjectFile<E> *file) {
for (std::unique_ptr<InputSection<E>> &isec : file->sections)
if (is_resizable(ctx, isec.get()))
shrink_section(ctx, *isec, use_rvc);
});
// Fix symbol values.
tbb::parallel_for_each(ctx.objs, [&](ObjectFile<E> *file) {
for (Symbol<E> *sym : file->symbols) {
if (sym->file != file)
continue;
InputSection<E> *isec = sym->get_input_section();
if (!isec || isec->extra.r_deltas.empty())
continue;
std::span<const ElfRel<E>> rels = isec->get_rels(ctx);
auto it = std::lower_bound(rels.begin(), rels.end(), sym->value,
[&](const ElfRel<E> &r, u64 val) {
return r.r_offset < val;
});
sym->value -= isec->extra.r_deltas[it - rels.begin()];
}
});
// Re-compute section offset again to finalize them.
compute_section_sizes(ctx);
return set_osec_offsets(ctx);
}
#define INSTANTIATE(E) \
template void write_plt_header(Context<E> &, u8 *); \
template void write_plt_entry(Context<E> &, u8 *, Symbol<E> &); \
template void write_pltgot_entry(Context<E> &, u8 *, Symbol<E> &); \
template void \
EhFrameSection<E>::apply_reloc(Context<E> &, const ElfRel<E> &, u64, u64); \
template void InputSection<E>::apply_reloc_alloc(Context<E> &, u8 *); \
template void InputSection<E>::apply_reloc_nonalloc(Context<E> &, u8 *); \
template void InputSection<E>::copy_contents_riscv(Context<E> &, u8 *); \
template void InputSection<E>::scan_relocations(Context<E> &); \
template i64 riscv_resize_sections(Context<E> &);
INSTANTIATE(RV64LE);
INSTANTIATE(RV64BE);
INSTANTIATE(RV32LE);
INSTANTIATE(RV32BE);
} // namespace mold::elf
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