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azm: Implement JMP rel32 #785

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Dec 6, 2023
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azm: Implement JMP rel32 #785

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2179c64
azm: Implement JMP rel32
robinlinden Dec 5, 2023
97c48f3
azm: Support JMP to a later offset as well
robinlinden Dec 5, 2023
29805c1
azm: Simplify label usage
robinlinden Dec 5, 2023
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54 changes: 54 additions & 0 deletions azm/amd64/assembler.h
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Original file line number Diff line number Diff line change
Expand Up @@ -5,10 +5,13 @@
#ifndef AZM_AMD64_ASSEMBLER_H_
#define AZM_AMD64_ASSEMBLER_H_

#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iostream>
#include <optional>
#include <utility>
#include <variant>
#include <vector>

namespace azm::amd64 {
Expand Down Expand Up @@ -38,11 +41,46 @@ constexpr std::optional<std::uint8_t> register_index(Reg32 reg) {
return std::nullopt;
}

struct Label {
struct Linked {
std::size_t offset{};
};
struct Unlinked {
std::vector<std::size_t> patch_offsets{};
};

static Label linked(std::size_t jmp_target_offset) { return {Linked{jmp_target_offset}}; }
static Label unlinked() { return {Unlinked{}}; }

std::variant<Linked, Unlinked> v;
};

// https://www.felixcloutier.com/x86/
class Assembler {
public:
[[nodiscard]] std::vector<std::uint8_t> take_assembled() { return std::exchange(assembled_, {}); }

Label label() const { return Label::linked(assembled_.size()); }
Label unlinked_label() const { return Label::unlinked(); }

void link(Label &label) {
assert(std::holds_alternative<Label::Unlinked>(label.v));
static constexpr int kInstructionSize = 4;
std::size_t const jmp_target_offset = assembled_.size();

auto const &unlinked = std::get<Label::Unlinked>(label.v);
for (std::size_t patch_offset : unlinked.patch_offsets) {
auto const rel32 = static_cast<std::uint32_t>(jmp_target_offset - patch_offset - kInstructionSize);
assembled_[patch_offset + 0] = rel32 & 0xff;
assembled_[patch_offset + 1] = (rel32 >> 8) & 0xff;
assembled_[patch_offset + 2] = (rel32 >> 16) & 0xff;
assembled_[patch_offset + 3] = (rel32 >> 24) & 0xff;
}

label = Label::linked(jmp_target_offset);
}

// Instructions
void add(Reg32 dst, Imm32 imm32) {
if (dst != Reg32::Eax) {
std::cerr << "add: Unhandled dst " << static_cast<int>(dst) << '\n';
Expand All @@ -54,6 +92,22 @@ class Assembler {
emit(imm32);
}

void jmp(Label &label) {
// JMP rel32
if (std::holds_alternative<Label::Linked>(label.v)) {
auto const &linked = std::get<Label::Linked>(label.v);
static constexpr int kInstructionSize = 4;
emit(0xe9);
emit(Imm32{static_cast<std::uint32_t>(linked.offset - assembled_.size() - kInstructionSize)});
return;
}

auto &unlinked = std::get<Label::Unlinked>(label.v);
emit(0xe9);
unlinked.patch_offsets.push_back(assembled_.size());
emit(Imm32{0xdeadbeef});
}

void mov(Reg32 dst, Imm32 imm32) {
emit(0xb8 + register_index(dst).value());
emit(imm32);
Expand Down
64 changes: 64 additions & 0 deletions azm/amd64/assembler_test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -41,6 +41,70 @@ int main() {
a.expect_eq(unsupported_add_code, assembler.take_assembled());
});

s.add_test("JMP, backwards", [](etest::IActions &a) {
Assembler assembler;

auto slot1 = assembler.label();
assembler.jmp(slot1);
assembler.ud2();
assembler.jmp(slot1);
auto slot2 = assembler.label();
assembler.jmp(slot2);

a.expect_eq(assembler.take_assembled(),
CodeVec{
0xe9, // jmp rel32
0xfb, // -5
0xff,
0xff,
0xff,
0x0f, // ud2
0x0b,
0xe9, // jmp rel32
0xf4, // -12
0xff,
0xff,
0xff,
0xe9, // jmp rel32
0xfb, // -5
0xff,
0xff,
0xff,
});
});

s.add_test("JMP, forwards", [](etest::IActions &a) {
Assembler assembler;

auto slot1 = assembler.unlinked_label();
assembler.jmp(slot1);
assembler.ud2();
assembler.jmp(slot1);
assembler.link(slot1);
assembler.jmp(slot1);

a.expect_eq(assembler.take_assembled(),
CodeVec{
0xe9, // jmp rel32
0x07, // 7
0x00,
0x00,
0x00,
0x0f, // ud2
0x0b,
0xe9, // jmp rel32
0x00, // 0
0x00,
0x00,
0x00,
0xe9, // jmp rel32
0xfb, // -5
0xff,
0xff,
0xff,
});
});

s.add_test("MOV r32, imm32", [](etest::IActions &a) {
Assembler assembler;

Expand Down
11 changes: 10 additions & 1 deletion azm/azm_example.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -11,13 +11,22 @@
int main() {
using namespace azm::amd64;
Assembler assembler;
auto forward = assembler.unlinked_label();
assembler.jmp(forward);
assembler.mov(Reg32::Eax, Imm32{3});
assembler.add(Reg32::Eax, Imm32{39});
assembler.mov(Reg32::Ecx, Imm32{0x4321});

auto end = assembler.label();
assembler.ret();

assembler.link(forward);
assembler.mov(Reg32::Edx, Imm32{0x12345678});
assembler.mov(Reg32::Ebx, Imm32{0x1234});
assembler.ret();
assembler.jmp(end);

assembler.ud2();

auto code = assembler.take_assembled();
// Print the machine code in a format usable for something like
// `objdump -D -b binary -mi386:x86-64 -Mintel <file>`.
Expand Down