Hi all
Our implementation of << is a direct wrapper around the C operator. It
does not check the right-hand side's value.
Datum int8shl(PG_FUNCTION_ARGS) { 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;?column?
---------- 4
(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.
Thoughts?
-- Craig Ringer http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services
