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Full Discussion: C multiplication with GCC
Top Forums Programming C multiplication with GCC Post 302544700 by pheonix on Thursday 4th of August 2011 04:07:33 PM
Old 08-04-2011
Thanx for reply, however maybe title a little misleading? I am trying to write "C"
code, usable by different processors. On an Intel processor using GCC or Clang
I wish to have:
Code:
(int)(((unsigned long long)result * (unsigned)2290649225) >> 37);

to look like:
Code:
mov $2290649225, %eax
imul (%ebp)
mov %edx, %eax
shr   %5, %eax
instead of:
imul $-2004318071, %edx, %esi
mov $-2004318071,%ecx
mov (%ebp), %eax
mull %ecx
lots more code
shr %5, %eax

if I use the second "C" language code:
Code:
(int)(((unsigned long long)result * (unsigned)1145324613) >> 36);

which is half of $2290649225 i get the first compiled result.
On i386 vs x86_64 the above codes should only differ in extension "l""d", but I believe the exact register reference with get you though both as "asm".
Again, I am trying to write "C" code, so I guess is there a way to tell compilers that (unsigned)2290649225, (uint32_t)2290649225, 2290649225U is an unsigned 32-bit number so shorter code is used?

Thanx Corona688! You got me in the direction I needed! Had me explain so understood what GCC might think? Strange that the 31st bit of a constant does this. Apparently the int result gets converted as long long then unsigned long long.
Resolved!
Code:
(int)(((unsigned long long)((unsigned)result) * (unsigned)2290649225) >> 37);

Sincere thanks! Pheonix
Last edited by pludi; 08-04-2011 at 05:16 PM..
 

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LEARN ABOUT DEBIAN

std::decimal::decimal32

std::decimal::decimal32(3cxx)											     std::decimal::decimal32(3cxx)

NAME

       std::decimal::decimal32 -

SYNOPSIS

   Public Types
       typedef float __decfloat32 __attribute__ ((mode(SD)))

   Public Member Functions
       decimal32 (decimal64 __d64)
       decimal32 (decimal128 __d128)
       decimal32 (float __r)
       decimal32 (double __r)
       decimal32 (long double __r)
       decimal32 (int __z)
       decimal32 (unsigned int __z)
       decimal32 (long __z)
       decimal32 (unsigned long __z)
       decimal32 (long long __z)
       decimal32 (unsigned long long __z)
       decimal32 (__decfloat32 __z)
       __decfloat32 __getval (void)
       void __setval (__decfloat32 __x)
       decimal32 & operator*= (decimal32 __rhs)
       decimal32 & operator*= (decimal64 __rhs)
       decimal32 & operator*= (decimal128 __rhs)
       decimal32 & operator*= (int __rhs)
       decimal32 & operator*= (unsigned int __rhs)
       decimal32 & operator*= (long __rhs)
       decimal32 & operator*= (unsigned long __rhs)
       decimal32 & operator*= (unsigned long long __rhs)
       decimal32 & operator*= (long long __rhs)
       decimal32 & operator++ ()
       decimal32 operator++ (int)
       decimal32 & operator+= (int __rhs)
       decimal32 & operator+= (decimal32 __rhs)
       decimal32 & operator+= (decimal64 __rhs)
       decimal32 & operator+= (decimal128 __rhs)
       decimal32 & operator+= (unsigned int __rhs)
       decimal32 & operator+= (long __rhs)
       decimal32 & operator+= (unsigned long __rhs)
       decimal32 & operator+= (long long __rhs)
       decimal32 & operator+= (unsigned long long __rhs)
       decimal32 & operator-- ()
       decimal32 operator-- (int)
       decimal32 & operator-= (long __rhs)
       decimal32 & operator-= (int __rhs)
       decimal32 & operator-= (long long __rhs)
       decimal32 & operator-= (unsigned long long __rhs)
       decimal32 & operator-= (unsigned long __rhs)
       decimal32 & operator-= (decimal128 __rhs)
       decimal32 & operator-= (unsigned int __rhs)
       decimal32 & operator-= (decimal32 __rhs)
       decimal32 & operator-= (decimal64 __rhs)
       decimal32 & operator/= (decimal32 __rhs)
       decimal32 & operator/= (long long __rhs)
       decimal32 & operator/= (unsigned long long __rhs)
       decimal32 & operator/= (unsigned long __rhs)
       decimal32 & operator/= (decimal64 __rhs)
       decimal32 & operator/= (unsigned int __rhs)
       decimal32 & operator/= (decimal128 __rhs)
       decimal32 & operator/= (long __rhs)
       decimal32 & operator/= (int __rhs)

Detailed Description
       3.2.2 Class decimal32.

       Definition at line 227 of file decimal.

Constructor &; Destructor Documentation
   std::decimal::decimal32::decimal32 (__decfloat32__z) [inline]
       Conforming extension: Conversion from scalar decimal type.

       Definition at line 251 of file decimal.

Author
       Generated automatically by Doxygen for libstdc++ from the source code.

libstdc++							  Tue Nov 27 2012				     std::decimal::decimal32(3cxx)
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