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From: "marc.glisse at normalesup dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/53100] New: Optimize __int128 with range information
Date: Tue, 24 Apr 2012 07:08:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53100

             Bug #: 53100
           Summary: Optimize __int128 with range information
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: marc.glisse@normalesup.org


(not sure about the "component" field)

In the following program, on x86_64, the first version generates two imulq,
while the second generates 4 imulq and 2 mulq. It would be convenient if I
could just write the whole code with __int128 and let the compiler do the
optimization by tracking the range of numbers.

int f(int a,int b,int c,int d,int e,int f){
#if 0
  long x=a;
  long y=b;
  long z=c;
  long t=d;
  long u=e;
  long v=f;
  return (z-x)*(__int128)(v-y) < (u-x)*(__int128)(t-y);
#else
  __int128 x=a;
  __int128 y=b;
  __int128 z=c;
  __int128 t=d;
  __int128 u=e;
  __int128 v=f;
  return (z-x)*(v-y) < (u-x)*(t-y);
#endif
}