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     Post subject:Data types: 64-bits or 64-bit?
    PostPosted:Tue Aug 18, 2009 7:40 pm 
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    Joined:Sun Feb 12, 2006 8:56 pm
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    Ever since I checked out the wiki page for data types I've wondered about the 64-bits row.
    Does it mean for use only on a 64-bit architecture, or that it uses 64-bits of data (and is usable on a 32-bit architecture)?

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     Post subject:Re: Data types: 64-bits or 64-bit?
    PostPosted:Tue Aug 18, 2009 10:30 pm 
     

    Joined:Thu Apr 28, 2005 3:57 pm
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    It uses 64 bits. The thing about a 64 bit architecture is that pointers are longs instead of ints. However, longs can be used on a 32 bit architecture to store a number, but not a pointer.

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     Post subject:Re: Data types: 64-bits or 64-bit?
    PostPosted:Thu Aug 20, 2009 4:53 pm 
    Just so you know, you brought this upon yourself. Dancefight.
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    Joined:Sat Apr 02, 2005 3:31 pm
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    On GCC, a 64-bit integer can be explicitly used with 'long long', and on MSVC++, you can use '__int64'. Note that 64-bit math in 32-bit code is very slow. For example, let's say you compile this for x86 and x86_64:
    Code:
    #ifdef _MSC_VER typedef __int64 int64_t; #else typedef long long int64_t; #endif int64_t add64(int64_t x, int64_t y) { return x + y; }
    x86_64, with GCC 4.1.2 (-O2 -fno-strict-aliasing -masm=intel -S):
    Code:
    add64: lea %rax, [%rdi+%rsi] ret
    x86, with GCC 4.1.2 (-m32 -O2 -fno-strict-aliasing -masm=intel -S):
    Code:
    add64: push %ebp mov %ebp, %esp mov %eax, DWORD PTR [%ebp+8] add %eax, DWORD PTR [%ebp+16] mov %edx, DWORD PTR [%ebp+12] adc %edx, DWORD PTR [%ebp+20] leave ret
    So on 64-bit, it's literally one add. With 32-bit, it's 3 moves, an add, and an add with carry. Much more work.

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     Post subject:Re: Data types: 64-bits or 64-bit?
    PostPosted:Sat Aug 22, 2009 7:15 am 
     

    Joined:Sun Jan 18, 2009 4:20 am
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    So, looking at it from the surface - all x64 code is compatible with x86 architectures, just slower?


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     Post subject:Re: Data types: 64-bits or 64-bit?
    PostPosted:Sat Aug 22, 2009 8:32 am 
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    Joined:Sun Feb 12, 2006 8:56 pm
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    Quote:
    So, looking at it from the surface - all x64 code is compatible with x86 architectures, just slower?
    I wish that were the other way around, all the stinking assembly tutorials and informational sites deal with x86, and half of the code doesn't even work on x64 for some reason.

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     Post subject:Re: Data types: 64-bits or 64-bit?
    PostPosted:Sat Aug 22, 2009 3:54 pm 
    Just so you know, you brought this upon yourself. Dancefight.
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    Joined:Sat Apr 02, 2005 3:31 pm
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    No, don't confuse 64-bit integers with 64-bit code. x86_64 (also called x64) uses a different instruction set, adds and removes certain features, etc.

    The thing to realize is that you _can_ use 64-bit math in 32-bit code, it just won't perform as well as it would in native 64-bit code.

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