Jmp
C++20 Static Branch library
Install / Use
/learn @qlibs/JmpREADME
// <!-- // The MIT License (MIT) // // Copyright (c) 2024 Kris Jusiak <kris@jusiak.net> // // Permission is hereby granted, free of charge, to any person obtaining a copy // of this software and associated documentation files (the "Software"), to deal // in the Software without restriction, including without limitation the rights // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell // copies of the Software, and to permit persons to whom the Software is // furnished to do so, subject to the following conditions: // // The above copyright notice and this permission notice shall be included in all // copies or substantial portions of the Software. // // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE // SOFTWARE. // #if 0 // --> Overview / Examples / API / FAQ / Resources
jmp: Static Branch library
https://en.wikipedia.org/wiki/Branch_(computer_science)
Use cases (Performance)
- Branch is relatively stable through its life cycle
and/or - Branch is expensive to compute / require memory access
and/or - Branch is hard to learn by the branch predictor
Examples: logging, tracing, dispatching, fast path, devirtualization, ...
Features
- Single header (https://raw.githubusercontent.com/qlibs/jmp/main/jmp) / C++20 module (https://raw.githubusercontent.com/qlibs/jmp/main/jmp.cppm)
- Minimal API
- Verifies itself upon include (can be disabled with
-DNTEST- see FAQ)
Requirements
- C++20 (clang++10+, g++10+) / x86-64 / Linux
Overview
static_branch<bool>(https://godbolt.org/z/v8W3Pzbxd)
/**
* constexpr minimal overhead static branch changed at run-time via code patching
*/
constexpr jmp::static_branch<bool> static_bool = false;
/**
* Note: `fn` can be inline/noinline/constexpr/etc.
*/
void fn() {
if (static_bool) { // Note: [[likely]], [[unlikely]] has no impact
std::puts("taken");
} else {
std::puts("not taken");
}
}
int main() {
if (not jmp::init()) { // enables run-time code patching
return errno;
}
fn(); // not taken
static_bool = true;
fn(); // taken
}
main: // $CXX -O3
# init
...
# fn(); // not taken
nop
.Ltmp0:
mov edi, OFFSET FLAT:.LC0
jmp puts
ret
.Ltmp1:
mov edi, OFFSET FLAT:.LC1
jmp puts
ret
# static_bool = true;
call static_bool.operator=(true)
# fn(); // taken
jmp .Ltmp1 // code patching (nop->jmp .Ltmp1)
.Ltmp0:
mov edi, OFFSET FLAT:.LC0
jmp puts
ret
.Ltmp1:
mov edi, OFFSET FLAT:.LC1
jmp puts
ret
.LC0: .asciz "not taken"
.LC1: .asciz "taken"
static_branch<bool> vs bool(https://godbolt.org/z/jvKGdPMWK)
constexpr jmp::static_branch<bool> static_bool = false;
void fn() {
if (static_bool) {
throw;
} else {
std::puts("else");
}
}
fn():
// static_bool = false;
[1] [2] [3] [4] [5] [6] Instructions:
1 0 0.25 nop
1 1 0.33 lea rdi, [rip + .L.str]
1 1 0.50 jmp puts
1 1 1.00 * .LBB0_1: push rax
1 1 0.50 call __cxa_rethrow@PLT
// static_bool = true;
[1] [2] [3] [4] [5] [6] Instructions:
1 1 0.50 jmp .LBB0_1
1 1 0.50 lea rdi, [rip + .L.str]
1 1 0.50 jmp puts
3 2 1.00 * .LBB0_1: push rax
4 3 1.00 call __cxa_rethrow@PLT
[1]: #uOps [2]: Latency [3]: RThroughput
[4]: MayLoad [5]: MayStore [6]: HasSideEffects (U)
bool dynamic_bool = false;
void fn() {
if (dynamic_bool) {
throw;
} else {
std::puts("else");
}
}
fn():
// dynamic_bool = false;
[1] [2] [3] [4] [5] [6] Instructions:
2 5 0.50 * cmp byte ptr [rip + dynamic_bool], 1
1 1 0.25 je .LBB0_1
1 1 0.25 lea rdi, [rip + .L.str]
1 1 0.25 jmp puts
1 1 0.50 * .LBB0_1: push rax
1 1 0.25 call __cxa_rethrow@PLT
// dynamic_bool = true;
[1] [2] [3] [4] [5] [6] Instructions:
2 5 0.50 * cmp byte ptr [rip + dynamic_bool], 1
1 1 0.25 je .LBB0_1
1 1 0.25 lea rdi, [rip + .L.str]
1 1 0.25 jmp puts
1 1 0.50 * .LBB0_1: push rax
1 1 0.25 call __cxa_rethrow@PLT
[1]: #uOps [2]: Latency [3]: RThroughput
[4]: MayLoad [5]: MayStore [6]: HasSideEffects (U)
static_branch<T, T Min, T Max>(https://godbolt.org/z/Tz4ox7ncv)
constexpr jmp::static_branch<int, 0, 2> static_int = 0; // range: <0, 2>
void fn() {
switch (static_int) {
default: std::unreachable();
case 0: std::puts("0"); return;
case 1: std::puts("1"); return;
case 2: std::puts("2"); return;
}
}
int main() {
if (not jmp::init()) { // enables run-time code patching
return errno;
}
fn(); // 0
static_int = 1;
fn(); // 1
static_int = 2;
fn(); // 2
}
fn: // $CXX -O3 -fno-inline
nop # code patching (nop->jmp .Ltmp1|.Ltmp2)
.Ltmp0:
mov edi, OFFSET FLAT:.LC0
jmp puts
ret
.Ltmp1:
mov edi, OFFSET FLAT:.LC1
jmp puts
ret
.Ltmp2:
mov edi, OFFSET FLAT:.LC2
jmp puts
ret
main:
// ... init
fn() // 0
call static_int.operator=(1)
fn() // 1
call static_int.operator=(2)
fn() // 2
.LC0: .asciz "0"
.LC1: .asciz "1"
.LC2: .asciz "2"
Examples
variant(https://godbolt.org/z/TKPdYPv3P) | (https://wg21.link/P2996)
template<class... Ts>
class variant {
static constexpr jmp::static_branch<std::size_t, 0, sizeof...(Ts)> index_ = 0u;
public:
constexpr variant() = default;
template<class T> requires (not std::is_base_of_v<variant, std::remove_cvref_t<T>>)
constexpr explicit(false) variant(T&& t) {
constexpr auto index = [] {
std::array match{std::is_same_v<Ts, std::remove_cvref_t<T>>...};
return std::ranges::find(match, true) - match.begin();
}();
index_ = index;
std::construct_at(&storage_.[:
nonstatic_data_members_of(^storage)[index + 1u]
:], std::forward<T>(t));
}
constexpr ~variant()
requires (std::is_trivially_destructible_v<Ts> and ...) = default;
template<class Fn>
constexpr auto visit(Fn&& fn) const -> decltype(auto) {
return [&]<auto I = 0u>(this auto&& self) {
if constexpr (I == sizeof...(Ts)) {
std::unreachable();
} else {
switch (index_) {
default: return self.template operator()<I + 1u>();
case I: return std::invoke(std::forward<Fn>(fn), storage_.[:
nonstatic_data_members_of(^storage)[I + 1u]
:]);
}
}
}();
}
private:
union storage;
struct empty{ };
static_assert(is_type(define_class(^storage, {
std::meta::data_member_spec(^empty, {.name = "empty"}),
std::meta::data_member_spec(^Ts)...
})));
storage storage_{.empty={}};
};
void usage(const variant<bool, int, float>& v) {
v.visit(overload{
[](bool) { std::puts("bool"); },
[](int) { std::puts("int"); },
[](float) { std::puts("float"); },
});
}
int main() {
if (not jmp::init()) { // enables run-time code patching
return errno;
}
variant<bool, int, float> v{};
v = true;
usage(v);
v = 42;
usage(v);
v = 42.f;
usage(v);
}
usage(variant<bool, int, float> const&):
nop # code patching (nop->jmp .Ltmp1|.Ltmp2)
.Ltmp0:
mov edi, OFFSET FLAT:.LC0
jmp puts
ret
.Ltmp1:
mov edi, OFFSET FLAT:.LC1
jmp puts
ret
.Ltmp2:
mov edi, OFFSET FLAT:.LC2
jmp puts
ret
.LC0: .asciz "bool"
.LC1: .asciz "int"
.LC2: .asciz "float"
Dispatching techniques (https://godbolt.org/z/cfKP9E8W9)
auto f1() -> int { return 42; }
auto f2() -> int { return 77; }
auto f3() -> int { return 99; }
auto if_else(bool b) -> int { # if_else(bool):
if (b) { # testl %edi, %edi
return f1(); # movl $42, %ecx
} else { # movl $77, %eax
return f2(); # cmovnel %ecx, %eax # cmove or cmp
} # retq
} #
auto if_else_likely(bool b) -> int { # if_else_likely(bool):
if (b) [[likely]] { # movl $42, %eax # likely
return f1();
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