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Re: The bug

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Le 08/02/2018 à 22:50, jacobnavia a écrit :
> Consider this program:
>    1 #include <string.h>
>    2 #include <stdio.h>
>    3 #define MANTISSA_LENGTH 7
>    4 typedef struct tagQfloatAccum {
>    5     int sign;
>    6     unsigned int exponent;
>    7     unsigned long long mantissa[MANTISSA_LENGTH+8];
>    8 } QfloatAccum;
>    9 typedef QfloatAccum * QfloatAccump;
>   10 typedef unsigned long long QELT;
>   11 static void square(QfloatAccump a,QfloatAccump b,int prec )
>   12 {
>   13     QELT *p, *q, *r;
>   14     unsigned __int128 u, lp;
>   15     int k;
>   16
>   17     memset(b->mantissa,0,sizeof(b->mantissa));
>   18     r = &b->mantissa[prec+2];
>   19     for( k=prec+2; k>=2; k-- ) {
>   20         q = &a->mantissa[1];
>   21         p = &a->mantissa[k-1];
>   22         while( p >= q ) {
>   23             lp = ((__int128)(*p)) * (*q);
>   24             if( p != q ) {
>   25                 if( ((lp >> 127) & 1 ) )
>   26                     *(r-2) += 1;
>   27                 lp <<= 1;
>   28             }
>   29             u =  lp&0xffffffffffffffffULL;;
>   30             u += *r;
>   31             (*r) = (u&0xffffffffffffffffULL);
>   32             u = u >> 64;
>   33             u += (lp >> 64);
>   34             u += *(r-1);
>   35             *(r-1) = u;
>   36             *(r-2) += (u >> 64);
>   37             p--;
>   38             q++;
>   39         }
>   40         --r;
>   41     }
>   42 }
>   43
>   44 QfloatAccum a =
>   45 {0x0, 0x0,  {0x0, 0x5c551d94ae0bf85d, 0xdf43ff68348e9f44, 
> 0x75abbd546eb4ad2c    , 0x45928b3668d09923, 0xef0e21fbaa8bb66b,
>   46       0xd834f79a27f3d70b, 0xe29f8b325d2f09bc, 0x22342f2796cceefe, 
> 0x15136c1f    974cb8e4, 0x0, 0x0, 0x0, 0x0, 0x0}};
>   47 QfloatAccum b = {0};
>   48 QfloatAccum result =
>   49 {0x0, 0x0,  {0x0, 0x214d498f853f343e, 0xd13fda08f3fdf79d, 
> 0x6066a345e0141842    ,
>   50     0x8db2e50b963ec962, 0x703ffb7a840af142, 0xfebe1ffca06b368, 
> 0xe705569e6ab    50956, 0x8c5227bc21e99054, 0x641cbb8833747c1a,
>   51     0xd32c84d233a9e32a, 0x12d004571d8dad0a, 0x0, 0x0, 0x0}};
>   52 int main(void)
>   53 {
>   54     square(&a,&b,9);
>   55     if (memcmp(&result,&b,sizeof(QfloatAccum))) {
>   56         printf("bug!\n");
>   57     }
>   58 }
> ---------------------------------------------------------------------------
> Compiled without any optimizations, this program outputs nothing. Gcc 
> with -O2 will print "bug".
> 
> Why is this happening?
> 

I found the root cause: gcc/clang will assume that there is no "UB" i.e. 
no overflow. Then, a multiplication of two unsigned values into a signed 
result can't be negative.

Then, the compilers wrongly assume, the bit 127 (the sign bit) can never 
be set. Then, the test at line 25 can never yioeld anything other than 
zero and the code in line 27 is dead code that is eliminated.

This chain of false reasoning starts with the assumption: Overflows 
never exist.

Great.

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The bug jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-08 22:50 +0100
  Re: The bug Keith Thompson <kst-u@mib.org> - 2018-02-08 14:15 -0800
  Re: The bug jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-13 10:25 +0100
    Re: The bug David Brown <david.brown@hesbynett.no> - 2018-02-13 10:51 +0100
      Re: The bug jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-13 11:19 +0100
        Re: The bug David Brown <david.brown@hesbynett.no> - 2018-02-13 12:23 +0100
      Re: The bug Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2018-02-13 08:19 -0800
        Re: The bug David Brown <david.brown@hesbynett.no> - 2018-02-15 10:52 +0100
    Re: The bug Noob <root@127.0.0.1> - 2018-02-13 16:19 +0100
      Re: The bug Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-19 12:11 -0800
        Re: The bug supercat@casperkitty.com - 2018-02-19 16:31 -0800
          Re: The bug Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-21 23:03 -0800
            Re: The bug supercat@casperkitty.com - 2018-02-23 16:40 -0800
              Re: The bug Richard Damon <Richard@Damon-Family.org> - 2018-02-24 05:57 -0500
              Re: The bug Tim Rentsch <txr@alumni.caltech.edu> - 2018-02-24 13:11 -0800
                Re: The bug Anton Shepelev <anton.txt@gmail.com> - 2018-02-25 00:15 +0300
    Re: The bug Keith Thompson <kst-u@mib.org> - 2018-02-13 11:44 -0800
      Re: The bug asetofsymbols@gmail.com - 2018-02-13 12:13 -0800
      Re: The bug Noob <root@127.0.0.1> - 2018-02-15 09:45 +0100

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