srctree

Robin Linden parent 204b3971 96bcd61b
azm: Generate better machine code for short jumps

inlinesplit
azm/amd64/assembler.h added: 40, removed: 26, total 14
@@ -1,4 +1,4 @@
// SPDX-FileCopyrightText: 2023 Robin Lindén <dev@robinlinden.eu>
// SPDX-FileCopyrightText: 2023-2024 Robin Lindén <dev@robinlinden.eu>
//
// SPDX-License-Identifier: BSD-2-Clause
 
@@ -96,9 +96,17 @@ public:
// JMP rel32
if (std::holds_alternative<Label::Linked>(label.v)) {
auto const &linked = std::get<Label::Linked>(label.v);
static constexpr int kInstructionSize = 4;
auto const jmp_dst = static_cast<std::ptrdiff_t>(linked.offset - assembled_.size());
static constexpr int kShortInstructionSize = 2;
if (jmp_dst >= (-128 + kShortInstructionSize) && jmp_dst <= 0) {
emit(0xeb);
emit(static_cast<std::uint8_t>(jmp_dst) - kShortInstructionSize);
return;
}
 
static constexpr int kNearInstructionSize = 5;
emit(0xe9);
emit(Imm32{static_cast<std::uint32_t>(linked.offset - assembled_.size() - kInstructionSize)});
emit(Imm32{static_cast<std::uint32_t>(jmp_dst - kNearInstructionSize)});
return;
}
 
 
azm/amd64/assembler_test.cpp added: 40, removed: 26, total 14
@@ -1,4 +1,4 @@
// SPDX-FileCopyrightText: 2023 Robin Lindén <dev@robinlinden.eu>
// SPDX-FileCopyrightText: 2023-2024 Robin Lindén <dev@robinlinden.eu>
//
// SPDX-License-Identifier: BSD-2-Clause
 
@@ -7,6 +7,7 @@
#include "etest/etest2.h"
 
#include <cstdint>
#include <iterator>
#include <optional>
#include <type_traits>
#include <vector>
@@ -53,23 +54,14 @@ int main() {
 
a.expect_eq(assembler.take_assembled(),
CodeVec{
0xe9, // jmp rel32
0xfb, // -5
0xff,
0xff,
0xff,
0xeb, // jmp rel32
0xfe, // -2
0x0f, // ud2
0x0b,
0xe9, // jmp rel32
0xf4, // -12
0xff,
0xff,
0xff,
0xe9, // jmp rel32
0xfb, // -5
0xff,
0xff,
0xff,
0xeb, // jmp rel32
0xfa, // -6
0xeb, // jmp rel32
0xfe, // -2
});
});
 
@@ -97,14 +89,28 @@ int main() {
0x00,
0x00,
0x00,
0xe9, // jmp rel32
0xfb, // -5
0xff,
0xff,
0xff,
0xeb, // jmp rel32
0xfe, // -2
});
});
 
s.add_test("JMP, short backwards", [](etest::IActions &a) {
Assembler assembler;
 
auto slot1 = assembler.label();
// Pad w/ a 1-byte instruction to force the maximum-length short backwards jmp.
for (std::uint8_t i = 0; i < 0x7e; ++i) {
assembler.ret();
}
 
assembler.jmp(slot1);
assembler.jmp(slot1);
// Remove the padding we don't care about.
auto assembled = assembler.take_assembled();
assembled.erase(assembled.begin(), std::next(assembled.begin(), 0x7e));
a.expect_eq(assembled, CodeVec{0xeb, 0x80, 0xe9, 0x7b, 0xff, 0xff, 0xff});
});
 
s.add_test("MOV r32, imm32", [](etest::IActions &a) {
Assembler assembler;