Chrome的V8标记函数一遍又一遍地优化直到放弃

Question

我有一个detectSingleScale JavaScript函数，V8正在试图优化它，据我所知，它无法优化它。

当使用--trace_deopt --trace_opt --trace_opt_verbose --code_comments运行Chrome时，我看到有数百行日志行，如下所示：

6087 [found optimized code for 0x1a9b67169161 <JS Function detectSingleScale (SharedFunctionInfo 0x145b5fa71bb1)> at OSR AST id 218]
6088 [didn't find optimized code in optimized code map for 0x145b5fa71bb1 <SharedFunctionInfo detectSingleScale>]
6089 [marking 0x1a9b6721ab91 <JS Function detectSingleScale (SharedFunctionInfo 0x145b5fa71bb1)> for recompilation, reason: hot and stable, ICs with typeinfo: 140/144 (97%), generic ICs: 0/144 (0%)]
6090 [found optimized code for 0x1a9b6721ab91 <JS Function detectSingleScale (SharedFunctionInfo 0x145b5fa71bb1)> at OSR AST id 218]
6091 [didn't find optimized code in optimized code map for 0x145b5fa71bb1 <SharedFunctionInfo detectSingleScale>]
6092 [marking 0x1a9b672c80f1 <JS Function detectSingleScale (SharedFunctionInfo 0x145b5fa71bb1)> for recompilation, reason: hot and stable, ICs with typeinfo: 140/144 (97%), generic ICs: 0/144 (0%)]
6093 [found optimized code for 0x1a9b672c80f1 <JS Function detectSingleScale (SharedFunctionInfo 0x145b5fa71bb1)> at OSR AST id 218]
6094 [didn't find optimized code in optimized code map for 0x145b5fa71bb1 <SharedFunctionInfo detectSingleScale>]
6095 [marking 0x1a9b67379db1 <JS Function detectSingleScale (SharedFunctionInfo 0x145b5fa71bb1)> for recompilation, reason: hot and stable, ICs with typeinfo: 140/144 (97%), generic ICs: 0/144 (0%)]

• V8标记用于优化的detectSingleScale函数。
• 然后它说它找到了优化的版本。
• 但是它说没有找到它。整个过程一次又一次地重新开始。

优化代码的地址都不同。我想知道是什么会触发这样的V8行为。在什么情况下，一个函数可以将V8置于这样的状态？

提前感谢！

Answer 1

从跟踪来看，每次detectSingleScale似乎都是一个新的闭包。每次都通过OSR (on堆栈替换)对其进行优化，因此它没有缓存的非OSR版本。

第一次创建和运行闭包时，通过OSR对其进行优化，并将其放入缓存中。

下次再次创建闭包时，首先得到didn't find optimized code消息-- Factory::NewFunctionFromSharedFunctionInfo1尝试查找非-OSR版本(OSR id设置为BailoutId::None())，但找不到，因为唯一优化的版本是OSR。

然后V8看到热循环，并决定使用OSR函数--这一次它已经在缓存中找到具有匹配OSR id的优化代码并使用它。

这里有一个例子来说明这一点

function foo() {
  print('! creating bar')
  var bar = function () {
    for (var i = 0; i < 100000; i++) {
      // OSR happens in this loop.
    }

    // Add a literal here to ensure we hit Runtime_NewClosure
    // instead of FastNewClosureStub - stub doesn't log anything.
    var a = [];
  }

  print('! running bar')
  bar();
  print('-- done')
}

foo();
foo();
foo();

当我使用d8运行这个文件时，我得到：

$ out/ia32.release/d8 --trace-opt test.js
! creating bar
! running bar
[marking 0x4301b705 <JS Function bar (SharedFunctionInfo 0x4c208765)> for recompilation, reason: small function, ICs with typeinfo: 2/2 (100%), generic ICs: 0/2 (0%)]
[compiling method 0x4301b705 <JS Function bar (SharedFunctionInfo 0x4c208765)> using Crankshaft]
[optimizing 0x4301b705 <JS Function bar (SharedFunctionInfo 0x4c208765)> - took 0.082, 0.119, 0.047 ms]
-- done
! creating bar
[didn't find optimized code in optimized code map for 0x4c208765 <SharedFunctionInfo bar>]
! running bar
[marking 0x4301bdb5 <JS Function bar (SharedFunctionInfo 0x4c208765)> for recompilation, reason: small function, ICs with typeinfo: 2/2 (100%), generic ICs: 0/2 (0%)]
[found optimized code for 0x4301bdb5 <JS Function bar (SharedFunctionInfo 0x4c208765)> at OSR AST id 10]
-- done
! creating bar
[didn't find optimized code in optimized code map for 0x4c208765 <SharedFunctionInfo bar>]
! running bar
[marking 0x4301be35 <JS Function bar (SharedFunctionInfo 0x4c208765)> for recompilation, reason: hot and stable, ICs with typeinfo: 2/2 (100%), generic ICs: 0/2 (0%)]
[found optimized code for 0x4301be35 <JS Function bar (SharedFunctionInfo 0x4c208765)> at OSR AST id 10]
-- done

1

Answer 2

V8团队目前正在优化这个案例：铬问题追踪器。

您可以使用--mark_shared_functions_for_tier_up来使用正在进行的工作。然后对detectSingleScale进行优化。

消息didn't find optimized code in optimized code map for不是很有用，最近从代码库中删除了它。