c++ - 为什么 Webkit 运行时 pow() 计算错误？

Question

我有一个 Qt C++ 应用程序，其中有一个 GUI 线程，其中会发生一些浮点计算。QWebView它还会在带有一些视频的 Flash 播放器的位置 打开。

很明显，关闭 QWebView 会干扰新的下一个浮点操作。所以pow(double, double)返回确定但不正确的值。

在一种情况下，它返回的值1000是正确值的两倍。另一次它返回 1.#inf当与 arguments 一起使用时pow(10.0, 2.0)。

我不得不提到它是在不同的计算机上测试的，并不特定于特定的 CPU。

您对如何在 Webkit 中找到协处理器出错的地方以及如何防止它有什么建议吗？

示例（仅限 x64）

环境：Qt 4.7.4、C++、HTML 和 flowplayer

cpp

wrongpow::wrongpow(QWidget *parent, Qt::WFlags flags)
: QMainWindow(parent, flags)
{
 QVBoxLayout* layout = new QVBoxLayout(0);  
 m_view = new QWebView(this);  
 m_view->setMinimumSize(400, 400);
 m_view->settings()->setAttribute(QWebSettings::PluginsEnabled, true);
 m_view->settings()->setAttribute(QWebSettings::LocalContentCanAccessRemoteUrls, true);
 layout->addWidget(m_view);
 QDir dir(QApplication::applicationDirPath());
 dir.cd("media");
 m_view->load(QUrl(QFileInfo(dir, "index.html").absoluteFilePath()));

 QPushButton* button = new QPushButton(QLatin1String("Click on video start"), this);
 layout->addWidget(button);
 Q_ASSERT(connect(button, SIGNAL(clicked()), this, SLOT(closeView()))); 

 setLayout(layout);
 adjustSize();
}  
Q_SLOT void wrongpow::closeView()
{
 delete m_view;
 m_view = NULL;

 double wrongResult = pow(10.0, 2.0);
 Q_ASSERT(wrongResult == 100.0);
}

html

<div id='player' style='width:100%; height:100%;'>
    <object width='100%' height='100%' id='_494187117' name='_494187117' data='js/plugins/flowplayer-3.2.18.swf' type='application/x-shockwave-flash'>                      
        <param name='wmode' value='opaque'>         
        <param name='flashvars' value='config={&quot;clip&quot;:{&quot;url&quot;:&quot;mp4:vod/demo.flowplayer/buffalo_soldiers.mp4&quot;,&quot;scaling&quot;:&quot;fit&quot;,&quot;provider&quot;:&quot;hddn&quot;,&quot;live&quot;:true},&quot;plugins&quot;:{&quot;hddn&quot;:{&quot;url&quot;:&quot;js/plugins/flowplayer.rtmp-3.2.13.swf&quot;,&quot;netConnectionUrl&quot;:&quot;rtmp://r.demo.flowplayer.netdna-cdn.com/play&quot;}},&quot;canvas&quot;:{&quot;backgroundGradient&quot;:&quot;none&quot;}}'>
    </object>
</div>  

这是一个完整的程序源代码： 下载 15MB

Answer 1

结果不正确的原因pow是x87寄存器ST0-ST3变成1#SNAN了所以TAGS = 0xFF。它发生在销毁包含视频 Flash 对象的 QWebView 期间。x87 和 SSE 的控制字包含正确的值。经过测试的 Adove Flash 库：NPSWF64_14_0_0_125.dll

当WebCore::PluginView::stop方法调用 Adob​​e Flash 插件对象的析构函数时会发生这种情况。

NPError npErr = m_plugin->pluginFuncs()->destroy(m_instance, &savedData);

这是破坏寄存器的过程（NPSWF64.dll）（实际上它使用与 x87 寄存器关联的 MMX 寄存器）：

mov         qword ptr [rsp+8],rcx 
mov         qword ptr [rsp+10h],rdx 
mov         qword ptr [rsp+18h],r8 
push        rsi  
push        rdi  
push        rbx  
mov         rdi,qword ptr [rsp+20h] 
mov         rsi,qword ptr [rsp+28h] 
mov         rcx,qword ptr [rsp+30h] 
cmp         rcx,20h 
jle         000000002F9D8A2D 
sub         rcx,20h 

// writes wrong values to the registers:
movq        mm0,mmword ptr [rsi] 
movq        mm1,mmword ptr [rsi+8] 
movq        mm2,mmword ptr [rsi+10h] 
movq        mm3,mmword ptr [rsi+18h] 

add         rsi,20h 
movq        mmword ptr [rdi],mm0
movq        mmword ptr [rdi+8],mm1 
movq        mmword ptr [rdi+10h],mm2 
movq        mmword ptr [rdi+18h],mm3 
add         rdi,20h 
sub         rcx,20h 
jge         000000002F9D89F1 
add         rcx,20h 
rep movs    byte ptr [rdi],byte ptr [rsi] 
pop         rbx  
pop         rdi  
pop         rsi  
ret         

为防止此错误导致的错误计算，pow需要恢复寄存器值。我使用最简单的方法来做到这一点。当插件被销毁时，我pow用一些参数调用它并恢复寄存器。下一次调用将是正确的。
通过使用库中的方法将新值写入寄存器，有一种更复杂（但可能是正确的）方法来做同样的事情float.h。

Answer 2

稍微补充一下，我想我遇到了同样的问题（在一个调用 Math.Cos 并且恰好在 .NET Windows Forms WebBrowser 控件中显示 Flash 视频的 64 位 C# 应用程序中）。

同样，在我的情况下，64 位 Flash OCX (16.0.0.305) 中的以下代码似乎在 MM0、MM1、MM2、MM3 寄存器中留下了无效值（此代码似乎是某种快速内存复制） ：

C:\Windows\System32\Macromed\Flash\Flash64_16_0_0_305.ocx
00000000`2f9833c0 48894c2408      mov     qword ptr [rsp+8],rcx
00000000`2f9833c5 4889542410      mov     qword ptr [rsp+10h],rdx
00000000`2f9833ca 4c89442418      mov     qword ptr [rsp+18h],r8
00000000`2f9833cf 56              push    rsi
00000000`2f9833d0 57              push    rdi
00000000`2f9833d1 53              push    rbx
00000000`2f9833d2 488b7c2420      mov     rdi,qword ptr [rsp+20h]
00000000`2f9833d7 488b742428      mov     rsi,qword ptr [rsp+28h]
00000000`2f9833dc 488b4c2430      mov     rcx,qword ptr [rsp+30h]
00000000`2f9833e1 4881f920000000  cmp     rcx,20h
00000000`2f9833e8 7e43            jle     Flash64_16_0_0_305!DllUnregisterServer+0x3c2a2d (00000000`2f98342d)
00000000`2f9833ea 4881e920000000  sub     rcx,20h
00000000`2f9833f1 0f6f06          movq    mm0,mmword ptr [rsi]
00000000`2f9833f4 0f6f4e08        movq    mm1,mmword ptr [rsi+8]
00000000`2f9833f8 0f6f5610        movq    mm2,mmword ptr [rsi+10h]
00000000`2f9833fc 0f6f5e18        movq    mm3,mmword ptr [rsi+18h]
00000000`2f983400 4881c620000000  add     rsi,20h
00000000`2f983407 0f7f07          movq    mmword ptr [rdi],mm0
00000000`2f98340a 0f7f4f08        movq    mmword ptr [rdi+8],mm1
00000000`2f98340e 0f7f5710        movq    mmword ptr [rdi+10h],mm2
00000000`2f983412 0f7f5f18        movq    mmword ptr [rdi+18h],mm3
00000000`2f983416 4881c720000000  add     rdi,20h
00000000`2f98341d 4881e920000000  sub     rcx,20h
00000000`2f983424 7dcb            jge     Flash64_16_0_0_305!DllUnregisterServer+0x3c29f1 (00000000`2f9833f1)
00000000`2f983426 4881c120000000  add     rcx,20h
00000000`2f98342d f3a4            rep movs byte ptr [rdi],byte ptr [rsi]
00000000`2f98342f 5b              pop     rbx
00000000`2f983430 5f              pop     rdi
00000000`2f983431 5e              pop     rsi
00000000`2f983432 c3              ret

Flash OCX 中的上述代码在 Web 浏览器导航离开显示 Flash 视频的页面时执行。

在此代码执行之前，浮点寄存器如下（使用 WinDbg.exe 检查）：

fpcw=027f fpsw=3820 fptw=0080
st0= 6.00000000000000000000000...0e+0001 (0:4004:f000000000000000)
st1= 0.00000000000000000000000...0e+0000 (0:0000:0000000000000000)
st2= 0.00000000000000000000000...0e+0000 (0:0000:0000000000000000)
st3= 0.00000000000000000000000...0e+0000 (0:0000:0000000000000000)
st4= 0.00000000000000000000000...0e+0000 (0:0000:0000000000000000)
st5= 1.00000000000000000000000...0e+0000 (0:3fff:8000000000000000)
st6= 8.94231504669236176852000...0e-0001 (0:3ffe:e4ec5b1b9b742000)
st7= 0.00000000000000000000000...0e+0000 (0:0000:0000000000000000)
mxcsr=00001fa4

执行上述代码后，浮点寄存器如下：

fpcw=027f fpsw=0020 fptw=00ff
st0=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st1=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st2=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st3=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st4= 1.00000000000000000000000...0e+0000 (0:3fff:8000000000000000)
st5= 8.94231504669236176852000...0e-0001 (0:3ffe:e4ec5b1b9b742000)
st6= 0.00000000000000000000000...0e+0000 (0:0000:0000000000000000)
st7= 6.00000000000000000000000...0e+0001 (0:4004:f000000000000000)
mxcsr=00001fa4

此时浮点寄存器似乎处于“损坏”状态。

所以稍后，回到 C# 程序，当执行 Math.Cos 时，fld 操作码失败，因为它试图将值 2.0 推入浮点堆栈（即，当它试图准备计算 2.0 的余弦时）：

COMDouble::Cos:
000007FE`E1D01570  movsd   mmword ptr [rsp+8],xmm0
000007FE`E1D01576  fld     qword ptr [rsp+8]
000007FE`E1D0157A  fcos
000007FE`E1D0157C  fstp    qword ptr [rsp+8]
000007FE`E1D01580  movsd   xmm0,mmword ptr [rsp+8]
000007FE`E1D01586  ret

在执行 fld 操作码之前，浮点寄存器与 Flash OCX 留下的完全相同（即如上）：

fpcw=027f fpsw=0020 fptw=00ff
st0=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st1=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st2=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st3=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st4= 1.00000000000000000000000...0e+0000 (0:3fff:8000000000000000)
st5= 8.94231504669236176852000...0e-0001 (0:3ffe:e4ec5b1b9b742000)
st6= 0.00000000000000000000000...0e+0000 (0:0000:0000000000000000)
st7= 6.00000000000000000000000...0e+0001 (0:4004:f000000000000000)
mxcsr=00001fa4

在执行 fld 操作码后，浮点寄存器立即如下所示（注意 st0 是 #IND 而不是预期值 2.00000000000000000000000...0e+0000）：

fpcw=027f fpsw=3a61 fptw=00ff
st0=-1.#IND0000000000000000000...0e+0000 (1:7fff:c000000000000000)
st1=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st2=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st3=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st4=-1.#SNAN000000000000000000...0e+0000 (1:7fff:0000000000000000)
st5= 1.00000000000000000000000...0e+0000 (0:3fff:8000000000000000)
st6= 8.94231504669236176852000...0e-0001 (0:3ffe:e4ec5b1b9b742000)
st7= 0.00000000000000000000000...0e+0000 (0:0000:0000000000000000)
mxcsr=00001fa4

这意味着 fcos 操作码尝试计算 #IND 的余弦。因此，C# 中 Math.Cos 的返回值最终是 Double.NaN，而不是预期的 -0.416146836547142。

剖析上面 0x3a61 的浮点状态字 (FPSW) 值表明问题是“试图将值加载到非空闲的寄存器中”：

3a61: 0011 1010 0110 0001

    TOP (13,12,11):          111
    C3,C2,C1,C0 (14,10,9,8): 0 010  (ie. C1 is 1)  <-- loading a value into a register which is not free
    IR (7):                  0  Interrupt Request
    SF (6):                  1  Stack Fault  <-- loading a value into a register which is not free
    P (5):                   1  Precision
    U (4):                   0  Underflow
    O (3):                   0  Overflow
    Z (2):                   0  Zero Divide
    D (1):                   0  Denormalised
    I (0):                   1  Invalid Operation

请注意，该问题仅在第一次尝试 Math.Cos(2) 时发生（即在导航离开 Flash 视频页面后不久）。计算 Math.Cos(2) 的第二次和后续尝试成功。

如果在 Flash 视频播放（或暂停）时释放 WebBrowser 控件，也会出现此问题。

因此，一些潜在的解决方法是：

1. 执行一次虚拟数学计算并忽略结果。

2. 编写一个 64 位 DLL，它导出一个执行 finit 或 emms 操作码的函数（以重置浮点寄存器的状态）。从 C# 调用该函数。

3. 作为 32 位进程运行。

注意：在 C# 中抛出一个虚拟异常然后捕获该异常并没有帮助（这建议用于其他浮点损坏问题）。

下面是一些重现该问题的 C# 代码（确保它以 64 位编译和运行；在视频开始播放后单击“计算”按钮）。

using System;
using System.IO;
using System.Windows.Forms;

namespace FlashTest
{
    public partial class TestForm : Form
    {
        public TestForm()
        {
            // Windows 7 SP1 64 bit
            // Internet Explorer 11 (11.0.9600.17633)
            // Flash Player 16.0.0.305
            // Visual Studio 2013 (12.0.31101.00)
            // .NET 4.5 (4.0.30319.34209)

            InitializeComponent();
            addressTextBox.Text = "http://www.youtube.com/v/JVGdyC9CvFQ?autoplay=1";
            GoButtonClickHandler(this, EventArgs.Empty);
        }

        private void GoButtonClickHandler(object sender, EventArgs e)
        {
            string path = Path.Combine(Path.GetTempPath(), Path.GetRandomFileName() + ".html");
            File.WriteAllText(path, string.Format(@"<html><body>
                <object classid=""clsid:D27CDB6E-AE6D-11CF-96B8-444553540000"" width=""100%"" height=""100%"" id=""youtubeviewer"">
                    <param name=""movie"" value=""{0}"">
                </object></body></html>", addressTextBox.Text));
            webBrowser.Navigate(path);
        }

        private void CalculateButtonClickHandler(object sender, EventArgs e)
        {
            webBrowser.DocumentCompleted += DocumentCompletedHandler;
            webBrowser.Navigate("about:blank");
        }

        private void DocumentCompletedHandler(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            webBrowser.DocumentCompleted -= DocumentCompletedHandler;
            MessageBox.Show("Math.Cos(2) returned " + Math.Cos(2));
        }
    }
}

namespace FlashTest
{
    partial class TestForm
    {
        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows Form Designer generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.webBrowser = new System.Windows.Forms.WebBrowser();
            this.addressTextBox = new System.Windows.Forms.TextBox();
            this.goButton = new System.Windows.Forms.Button();
            this.calculateButton = new System.Windows.Forms.Button();
            this.SuspendLayout();
            // 
            // webBrowser
            // 
            this.webBrowser.Anchor = ((System.Windows.Forms.AnchorStyles)((((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Bottom) 
            | System.Windows.Forms.AnchorStyles.Left) 
            | System.Windows.Forms.AnchorStyles.Right)));
            this.webBrowser.Location = new System.Drawing.Point(12, 41);
            this.webBrowser.MinimumSize = new System.Drawing.Size(20, 20);
            this.webBrowser.Name = "webBrowser";
            this.webBrowser.Size = new System.Drawing.Size(560, 309);
            this.webBrowser.TabIndex = 3;
            // 
            // addressTextBox
            // 
            this.addressTextBox.Anchor = ((System.Windows.Forms.AnchorStyles)(((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Left) 
            | System.Windows.Forms.AnchorStyles.Right)));
            this.addressTextBox.Location = new System.Drawing.Point(12, 14);
            this.addressTextBox.Name = "addressTextBox";
            this.addressTextBox.Size = new System.Drawing.Size(398, 20);
            this.addressTextBox.TabIndex = 0;
            // 
            // goButton
            // 
            this.goButton.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Right)));
            this.goButton.Location = new System.Drawing.Point(416, 12);
            this.goButton.Name = "goButton";
            this.goButton.Size = new System.Drawing.Size(75, 23);
            this.goButton.TabIndex = 1;
            this.goButton.Text = "&Go";
            this.goButton.UseVisualStyleBackColor = true;
            this.goButton.Click += new System.EventHandler(this.GoButtonClickHandler);
            // 
            // calculateButton
            // 
            this.calculateButton.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Right)));
            this.calculateButton.Location = new System.Drawing.Point(497, 12);
            this.calculateButton.Name = "calculateButton";
            this.calculateButton.Size = new System.Drawing.Size(75, 23);
            this.calculateButton.TabIndex = 2;
            this.calculateButton.Text = "&Calculate";
            this.calculateButton.UseVisualStyleBackColor = true;
            this.calculateButton.Click += new System.EventHandler(this.CalculateButtonClickHandler);
            // 
            // TestForm
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(584, 362);
            this.Controls.Add(this.webBrowser);
            this.Controls.Add(this.goButton);
            this.Controls.Add(this.addressTextBox);
            this.Controls.Add(this.calculateButton);
            this.Name = "TestForm";
            this.Text = "Adobe Flash Test";
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion

        private System.Windows.Forms.WebBrowser webBrowser;
        private System.Windows.Forms.TextBox addressTextBox;
        private System.Windows.Forms.Button goButton;
        private System.Windows.Forms.Button calculateButton;
    }
}

Answer 3

显然，您在 Qt 的 webkit 版本中遇到了错误。我无法在 QT 5.3 中使用 MSVC 13 x64 在发布和调试中使用 QtCreator 构建进行重现。

已经报告了一些带有浮点的 QtWebKit 错误：

• https://bugreports.qt-project.org/browse/QTWEBKIT-266