Hi all

Our implementation of << is a direct wrapper around the C operator. It
does not check the right-hand side's value.

          int64           arg1 = PG_GETARG_INT64(0);
          int32           arg2 = PG_GETARG_INT32(1);

          PG_RETURN_INT64(arg1 << arg2);

This means that an operation like:

     1::bigint << 65

directly relies on the compiler and platforms' handling of the
undefined shift. On x64 intel gcc linux it does a rotation but that's
not AFAIK guaranteed by anything, and we should probably not be
relying on this or exposing it at the user level.

Pg returns:

test=> SELECT BIGINT '1' << 66;
(1 row)

A test program:

#include "stdio.h"
int main(int argc, char * argv[])
    printf("Result is %ld", 1l << 66);
    return 0;

returns zero when the compiler constant-folds, but when done at runtime:

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char * argv[])
    const char * num = "66";
    printf("Result is %ld", 1l << atoi(num));
    return 0;

IMO we should specify the behaviour in this case. Then issue a WARNING
that gets promoted to an ERROR in a few versions.

Consideration of << with a negative right-operand, and of
out-of-bounds >>, is probably also needed.


 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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