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      1 /*
      2  * Copyright (C) 2011, Google Inc. All rights reserved.
      3  *
      4  * Redistribution and use in source and binary forms, with or without
      5  * modification, are permitted provided that the following conditions
      6  * are met:
      7  * 1.  Redistributions of source code must retain the above copyright
      8  *    notice, this list of conditions and the following disclaimer.
      9  * 2.  Redistributions in binary form must reproduce the above copyright
     10  *    notice, this list of conditions and the following disclaimer in the
     11  *    documentation and/or other materials provided with the distribution.
     12  *
     13  * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS'' AND ANY
     14  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
     15  * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
     16  * DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS BE LIABLE FOR ANY
     17  * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
     18  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
     19  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
     20  * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     21  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
     22  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     23  */
     24 
     25 #ifndef DenormalDisabler_h
     26 #define DenormalDisabler_h
     27 
     28 #include "wtf/CPU.h"
     29 #include "wtf/MathExtras.h"
     30 #include <float.h>
     31 
     32 namespace WebCore {
     33 
     34 // Deal with denormals. They can very seriously impact performance on x86.
     35 
     36 // Define HAVE_DENORMAL if we support flushing denormals to zero.
     37 #if OS(WIN) && COMPILER(MSVC)
     38 #define HAVE_DENORMAL 1
     39 #endif
     40 
     41 #if COMPILER(GCC) && (CPU(X86) || CPU(X86_64))
     42 #define HAVE_DENORMAL 1
     43 #endif
     44 
     45 #if HAVE(DENORMAL)
     46 class DenormalDisabler {
     47 public:
     48     DenormalDisabler()
     49             : m_savedCSR(0)
     50     {
     51 #if OS(WIN) && COMPILER(MSVC)
     52         // Save the current state, and set mode to flush denormals.
     53         //
     54         // http://stackoverflow.com/questions/637175/possible-bug-in-controlfp-s-may-not-restore-control-word-correctly
     55         _controlfp_s(&m_savedCSR, 0, 0);
     56         unsigned int unused;
     57         _controlfp_s(&unused, _DN_FLUSH, _MCW_DN);
     58 #else
     59         m_savedCSR = getCSR();
     60         setCSR(m_savedCSR | 0x8040);
     61 #endif
     62     }
     63 
     64     ~DenormalDisabler()
     65     {
     66 #if OS(WIN) && COMPILER(MSVC)
     67         unsigned int unused;
     68         _controlfp_s(&unused, m_savedCSR, _MCW_DN);
     69 #else
     70         setCSR(m_savedCSR);
     71 #endif
     72     }
     73 
     74     // This is a nop if we can flush denormals to zero in hardware.
     75     static inline float flushDenormalFloatToZero(float f)
     76     {
     77 #if OS(WIN) && COMPILER(MSVC) && (!_M_IX86_FP)
     78         // For systems using x87 instead of sse, there's no hardware support
     79         // to flush denormals automatically. Hence, we need to flush
     80         // denormals to zero manually.
     81         return (fabs(f) < FLT_MIN) ? 0.0f : f;
     82 #else
     83         return f;
     84 #endif
     85     }
     86 private:
     87 #if COMPILER(GCC) && (CPU(X86) || CPU(X86_64))
     88     inline int getCSR()
     89     {
     90         int result;
     91         asm volatile("stmxcsr %0" : "=m" (result));
     92         return result;
     93     }
     94 
     95     inline void setCSR(int a)
     96     {
     97         int temp = a;
     98         asm volatile("ldmxcsr %0" : : "m" (temp));
     99     }
    100 
    101 #endif
    102 
    103     unsigned int m_savedCSR;
    104 };
    105 
    106 #else
    107 // FIXME: add implementations for other architectures and compilers
    108 class DenormalDisabler {
    109 public:
    110     DenormalDisabler() { }
    111 
    112     // Assume the worst case that other architectures and compilers
    113     // need to flush denormals to zero manually.
    114     static inline float flushDenormalFloatToZero(float f)
    115     {
    116         return (fabs(f) < FLT_MIN) ? 0.0f : f;
    117     }
    118 };
    119 
    120 #endif
    121 
    122 } // WebCore
    123 
    124 #undef HAVE_DENORMAL
    125 #endif // DenormalDisabler_h
    126