FPTruncateMode.h

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/* Copyright 2001-4 tim goetze <tim@quitte.de> -- see 'COPYING'. */
 
/* Sets the FP rounding mode to 'truncate' in the constructor
 * and loads the previous FP conrol word in the destructor. 
 *
 * By directly using the machine instruction to convert float to int
 * we avoid the performance hit that loading the control word twice for
 * every (int) cast causes on i386.
 *
 * On other architectures this is a no-op.
 */
 
#ifndef _DSP_FP_TRUNCATE_MODE_H_
#define _DSP_FP_TRUNCATE_MODE_H_
 
#ifdef __i386__
        #define fstcw(i) \
                __asm__ __volatile__ ("fstcw %0" : "=m" (i))
 
        #define fldcw(i) \
                __asm__ __volatile__ ("fldcw %0" : : "m" (i))
 
        /* gcc chokes on __volatile__ sometimes. */
        #define fistp(f,i) \
                __asm__ ("fistpl %0" : "=m" (i) : "t" (f) : "st")
#else /* ! __i386__ */
        #include <cstdint>
 
        inline void fistp(float f, int32_t& i) { i = static_cast<int32_t>(f); }
#endif
 
namespace DSP {
        
class FPTruncateMode
{
        public:
#ifdef __i386__
                int cw0; /* fp control word */
 
                FPTruncateMode()
                        {
                                fstcw (cw0);
                                const int cw1 = cw0 | 0xC00;
                                fldcw (cw1);
                        }
 
                ~FPTruncateMode()
                        {
                                fldcw (cw0);
                        }
#else
        // Avoid warnings about unused variables
        FPTruncateMode() { (void)0; }
        ~FPTruncateMode() { (void)0; }
#endif
};
 
} /* namespace DSP */
 
#endif /* _DSP_FP_TRUNCATE_MODE_H_ */