背景:我为各种编程语言编写了一个愚蠢的代码生成程序，目的是比较各种编译器编译由简单函数组成的荒谬长的 100K+ LOC 程序需要多长时间。但是在编译时，当优化标志被赋予 rustc 时，程序的 Rust 版本从未完成编译。 .
我发现制作一个非常小的 Rust 程序(下面的示例)很容易，它需要(根据我的感受)太长而无法使用 -C opt-level=2 进行编译。或 -C opt-level=3 rustc 的标志.我在 Linux 上尝试了 1.16 stable、1.30 stable、1.32.0-nightly 和 macOS 和 Windows 上的 1.30 stable - 所有这些都让我觉得编译的时间太长了。我这样说是因为相比之下，大致等效的 C++ 用 clang++ -O3 编译需要不到一秒钟的时间。 .这让我想到几个问题:
rustc正在(缓慢地)处理这个 clang 的代码不是吗？ rustc ?看看 opt-level=2 中是否有特定的优化会很酷是罪魁祸首。 rustc guide告诉如何打开调试日志。使用它得到:
INFO 2018-12-09T19:37:54Z: rustc_codegen_ssa::base: codegen_instance(std::rt::lang_start::<()>)
INFO 2018-12-09T19:37:54Z: rustc_codegen_ssa::base: codegen_instance(std::rt::lang_start::{{closure}}::<(), i8, extern "rust-call" fn(()) -> i32, fn()>)
INFO 2018-12-09T19:37:54Z: rustc_codegen_ssa::base: codegen_instance(std::sys::unix::process::process_common::ExitCode::as_i32)
INFO 2018-12-09T19:37:54Z: rustc_codegen_ssa::base: codegen_instance(<[closure@DefId(1/1:1916 ~ std[424f]::rt[0]::lang_start[0]::{{closure}}[0]) 0:fn()] as std::ops::FnOnce<()>>::call_once - shim)
INFO 2018-12-09T19:37:54Z: rustc_codegen_ssa::base: codegen_instance(<[closure@DefId(1/1:1916 ~ std[424f]::rt[0]::lang_start[0]::{{closure}}[0]) 0:fn()] as std::ops::FnOnce<()>>::call_once - shim(vtable))
INFO 2018-12-09T19:37:54Z: rustc_codegen_ssa::base: codegen_instance(std::ptr::real_drop_in_place::<[closure@DefId(1/1:1916 ~ std[424f]::rt[0]::lang_start[0]::{{closure}}[0]) 0:fn()]> - shim(None))
INFO 2018-12-09T19:37:54Z: rustc_codegen_ssa::base: codegen_instance(<() as std::process::Termination>::report)
INFO 2018-12-09T19:37:54Z: rustc_codegen_ssa::base: codegen_instance(<std::process::ExitCode as std::process::Termination>::report)
INFO 2018-12-09T19:37:54Z: rustc_codegen_ssa::base: codegen_instance(std::fmt::ArgumentV1::new::<i32>)
INFO 2018-12-09T19:39:12Z: rustc_codegen_llvm::back::lto: 5 symbols to preserve in this crate
INFO 2018-12-09T19:39:12Z: rustc_codegen_llvm::back::lto: going for that thin, thin LTO
rustc_codegen_ssa::base 之间的微小差距和 rustc_codegen_llvm::back::lto - 我应该如何解释这个？Rust gist和 C++ gist
//$ rustc -C opt-level=2 test_20.rs
// takes over a minute to compile, rustc 1.32.0-nightly
// see https://gist.github.com/ajdust/5e92cab52ffab5ea2a52edbd47aa348a
#![allow(unused_parens)]
fn f0(p: i32) -> i32 {
let x1: i32 = (p - ((((21 | 1) | p) ^ 84) & ((48 ^ (52 | (p & (2 ^ 61)))) - 67)));
let x2: i32 = x1;
let mut x3: i32 = 54;
let x4: i32 = 75;
let x5: i32 = (77 & 39);
let x6: i32 = (x2 * x5);
let x7: i32 = (88 * (8 + x1));
x3 = (x3 + 60);
((((((((32 * p) & x1) ^ x2) - x3) ^ x4) & x5) | x6) | x7)
}
fn f1(p: i32) -> i32 {
let mut x1: i32 = f0(78);
x1 = (x1 ^ p);
let mut x2: i32 = f0(x1);
x2 = (x2 * 3);
let x3: i32 = f0(x1);
let x4: i32 = ((21 & (x3 - ((93 * (x3 - (f0(x3) - (x2 - (f0(x1) | 43))))) | (f0(p) - f0(x1))))) * 41);
((((((f0(p) | x2) ^ p) & x1) ^ x2) | x3) - x4)
}
fn f2(p: i32) -> i32 {
let mut x1: i32 = f1(50);
x1 = (x1 * p);
x1 = (x1 | f0(p));
let mut x2: i32 = f1(x1);
x2 = (x2 | f1(x2));
let mut x3: i32 = (24 * f0(x1));
x2 = (x2 & f0(p));
x3 = (x3 ^ x1);
let x4: i32 = x1;
(((((x4 ^ p) | x1) * x2) + x3) | x4)
}
fn f3(p: i32) -> i32 {
let mut x1: i32 = f2(75);
let x2: i32 = x1;
x1 = (x1 & x2);
let x3: i32 = f0(p);
let x4: i32 = ((f1(x3) ^ f1(x2)) + 92);
x1 = (x1 | (x2 ^ 94));
x1 = (x1 * x2);
let x5: i32 = (f0(x1) & (3 ^ (f0(x1) * f2(x4))));
x1 = (x1 + x2);
(((((((x1 * x5) * p) - x1) | x2) * x3) - x4) - x5)
}
fn f4(p: i32) -> i32 {
let mut x1: i32 = f3(14);
x1 = (x1 + f0(p));
let mut x2: i32 = f1(x1);
x1 = (x1 - 41);
x2 = (x2 ^ 61);
let x3: i32 = f2(p);
x2 = (x2 ^ p);
let x4: i32 = x2;
x1 = (x1 - p);
x1 = (x1 * x4);
((((((88 & 11) & p) - x1) * x2) ^ x3) | x4)
}
fn f5(p: i32) -> i32 {
let mut x1: i32 = f4(50);
x1 = (x1 ^ 13);
((35 + p) | x1)
}
fn f6(p: i32) -> i32 {
let mut x1: i32 = f5(51);
x1 = (x1 + 27);
let x2: i32 = (p + (p | f1(x1)));
x1 = (x1 + f0(x2));
let x3: i32 = f0(x1);
let mut x4: i32 = 48;
x1 = (x1 ^ f0(p));
x1 = (x1 & 26);
x4 = (x4 * f1(x4));
(((((99 - p) * x1) ^ x2) & x3) + x4)
}
fn f7(p: i32) -> i32 {
let mut x1: i32 = f6(71);
x1 = (x1 & 66);
x1 = (x1 & p);
let x2: i32 = 57;
x1 = (x1 * 26);
let x3: i32 = (21 & p);
let x4: i32 = (f0(x1) & (f3(p) * f2(p)));
let x5: i32 = f6(x3);
((((((x5 + p) | x1) + x2) - x3) & x4) * x5)
}
fn f8(p: i32) -> i32 {
let mut x1: i32 = f7(57);
x1 = (x1 & f5(p));
x1 = (x1 ^ (x1 & f1(p)));
let x2: i32 = 25;
let x3: i32 = f5(x1);
((((x1 - p) * x1) & x2) ^ x3)
}
fn f9(p: i32) -> i32 {
let mut x1: i32 = f8(23);
x1 = (x1 | (((26 | f4(x1)) - f0(p)) | f8(p)));
let x2: i32 = x1;
let mut x3: i32 = 58;
x3 = (x3 - p);
let x4: i32 = f7(x1);
let x5: i32 = f7(x2);
let x6: i32 = (f7(x1) & 79);
(((((((33 | p) - x1) + x2) + x3) * x4) ^ x5) + x6)
}
fn f10(p: i32) -> i32 {
let mut x1: i32 = f9(75);
x1 = (x1 | 37);
(((f8(x1) + f3(x1)) | p) * x1)
}
fn f11(p: i32) -> i32 {
let mut x1: i32 = f10(8);
x1 = (x1 ^ f6(x1));
let mut x2: i32 = p;
x2 = (x2 ^ 84);
let x3: i32 = (f5(p) ^ f5(p));
x1 = (x1 * f5(p));
x1 = (x1 | f1(x2));
x1 = (x1 * f8(p));
((((((f0(x3) | f9(p)) - f4(x1)) + p) & x1) & x2) - x3)
}
fn f12(p: i32) -> i32 {
let mut x1: i32 = f11(33);
x1 = (x1 * 84);
let mut x2: i32 = (67 - f0(p));
x2 = (x2 | x1);
x1 = (x1 - 67);
x2 = (x2 - f6(p));
(((p - p) * x1) | x2)
}
fn f13(p: i32) -> i32 {
let mut x1: i32 = f12(90);
x1 = (x1 + (f6(x1) - f4(p)));
x1 = (x1 - 19);
let x2: i32 = 92;
let mut x3: i32 = f9(x1);
let mut x4: i32 = x3;
x4 = (x4 - (87 | f5(x3)));
x3 = (x3 | 49);
let x5: i32 = 25;
let x6: i32 = x3;
(((((((2 & p) - x1) ^ x2) ^ x3) ^ x4) | x5) | x6)
}
fn f14(p: i32) -> i32 {
let mut x1: i32 = f13(66);
let x2: i32 = f2(p);
x1 = (x1 - 11);
let mut x3: i32 = 69;
x3 = (x3 * x2);
let x4: i32 = 91;
(((((19 * p) + x1) | x2) ^ x3) & x4)
}
fn f15(p: i32) -> i32 {
let mut x1: i32 = f14(79);
x1 = (x1 + (f8(p) & p));
let x2: i32 = p;
x1 = (x1 | ((f5(p) & x2) ^ x2));
let mut x3: i32 = x1;
x1 = (x1 - p);
x3 = (x3 * p);
((((40 * p) ^ x1) + x2) + x3)
}
fn f16(p: i32) -> i32 {
let x1: i32 = f15(77);
let mut x2: i32 = 5;
let mut x3: i32 = x1;
let x4: i32 = p;
x2 = (x2 + p);
let x5: i32 = x4;
x3 = (x3 | f9(x4));
let x6: i32 = (68 ^ (61 ^ (24 * f14(x4))));
(((((((88 + p) - x1) & x2) | x3) & x4) ^ x5) | x6)
}
fn f17(p: i32) -> i32 {
let mut x1: i32 = f16(41);
x1 = (x1 | 4);
let mut x2: i32 = x1;
x1 = (x1 | 52);
x1 = (x1 & 49);
x2 = (x2 & (f8(x2) ^ p));
let mut x3: i32 = x2;
x3 = (x3 ^ ((x1 ^ x2) + f15(x2)));
let mut x4: i32 = (f13(x2) ^ 73);
x4 = (x4 - f12(x1));
(((((x3 - p) + x1) ^ x2) + x3) | x4)
}
fn f18(p: i32) -> i32 {
let mut x1: i32 = f17(3);
x1 = (x1 & (p - ((33 * (95 | 87)) | (9 - f1(x1)))));
x1 = (x1 & (80 - f16(x1)));
x1 = (x1 & p);
x1 = (x1 + p);
x1 = (x1 | (82 - ((81 ^ p) - 97)));
((20 - p) * x1)
}
fn f19(p: i32) -> i32 {
let x1: i32 = f18(24);
let x2: i32 = (p & p);
let mut x3: i32 = 82;
let x4: i32 = (4 + x1);
x3 = (x3 | ((f10(p) + (f16(x3) - 34)) - f10(x1)));
let x5: i32 = (x4 | (x1 * (((f16(x1) + f4(x4)) - 43) & f7(x3))));
(((((((f14(x3) | f9(x5)) - p) & x1) * x2) & x3) * x4) + x5)
}
fn f20(p: i32) -> i32 {
let x1: i32 = f19(78);
let x2: i32 = 81;
let x3: i32 = (x2 + (59 & x1));
(((((f9(x3) ^ f11(x3)) * p) * x1) - x2) ^ x3)
}
fn main() {
let mut x0: i32 = f20(65);
x0 = (x0 * (53 + 37));
let mut x1: i32 = (x0 - ((41 | ((f20(x0) * f9(x0)) + ((((f20(x0) + (77 + (f14(x0) ^ 60))) * 27) & 62) + x0))) & f20(x0)));
let x2: i32 = f15(x1);
x1 = (x1 | (x0 * (4 ^ 37)));
let m: i32 = (((x2 | x0) | x1) | x2);
println!("{}", m);
}
最佳答案
这在评论中得到了有效回答 - 谢谢@bluss。路过-Cinline-threshold=1000至 rustc将编译速度降低到不到一秒左右的预期速度。
由于这仍然可以在 2.5 年后复制，因此我最终遵循了在 rust-lang 存储库中创建错误报告问题的建议，以查看是否有可以改进 Rust 的方法。您可以在此处查看错误报告问题:https://github.com/rust-lang/rust/issues/86870
感谢所有评论的人。
关于rust - 为什么 rustc 编译这个 250 行的程序需要超过一分钟的时间，而 clang 需要不到一秒钟的时间？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/53696057/