Re: Looking for help with an obscure C integer problem

[Charles Mills]

Thanks all!

I decided to go with an implementation of David's algorithm below. No
external calls, and completely unit testable in Visual C++.

If I get the time I will start a new thread "why I am not reporting my
latest C compiler bug to IBM."

Charles

-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On
Behalf Of David Crayford
Sent: Monday, July 22, 2013 2:57 AM
To: IBM-MAIN@LISTSERV.UA.EDU
Subject: Re: Looking for help with an obscure C integer problem

I can see nothing obviously wrong with your code.  Because the function is
inlined there could be a subtle bug near where the function is called. Can
you reproduce the problem with a simple test driver?

FWIW, I implemented ffs64() like this.

static inline int ffs64( uint64_t word ) {
     if ( word == 0 ) return 0;
     int bit = 1;
     if ( ( word & 0xFFFFFFFF ) == 0 )
     {
         word >>= 32;
         bit += 32;
     }
     if ( ( word & 0xFFFF ) == 0 )
     {
         word >>= 16;
         bit += 16;
     }
     if ( ( word & 0xFF ) == 0 )
     {
         word >>= 8;
         bit += 8;
     }
     if ( ( word & 0xF ) == 0 )
     {
         word >>= 4;
         bit += 4;
     }
     if ( ( word & 0x3 ) == 0 )
     {
         word >>= 2;
         bit += 2;
     }
     if ( ( word & 0x1 ) == 0 )
     {
         bit += 1;
     }
     return bit;
}

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