Re: Efficient output for integer types

From: Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>
To: David Fetter <david(at)fetter(dot)org>
Cc: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Efficient output for integer types
Date: 2019-09-23 12:16:36
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>>>>> "David" == David Fetter <david(at)fetter(dot)org> writes:

David> + return pg_ltostr_zeropad(str, (uint32)0 - (uint32)value, minwidth - 1);

No, this is just reintroducing the undefined behavior again. Once the
value has been converted to unsigned you can't cast it back to signed or
pass it to a function expecting a signed value, since it will overflow
in the INT_MIN case. (and in this example would probably output '-'
signs until it ran off the end of memory).

Here's how I would do it:

char *
pg_ltostr_zeropad(char *str, int32 value, int32 minwidth)
int32 len;
uint32 uvalue = value;

Assert(minwidth > 0);

if (value >= 0)
if (value < 100 && minwidth == 2) /* Short cut for common case */
memcpy(str, DIGIT_TABLE + value*2, 2);
return str + 2;
*str++ = '-';
minwidth -= 1;
uvalue = (uint32)0 - uvalue;

len = pg_ultoa_n(uvalue, str);
if (len >= minwidth)
return str + len;

memmove(str + minwidth - len, str, len);
memset(str, '0', minwidth - len);
return str + minwidth;

David> pg_ltostr(char *str, int32 value)
David> + int32 len = pg_ultoa_n(value, str);
David> + return str + len;

This seems to have lost its handling of negative numbers entirely (which
doesn't say much for the regression test coverage)

Andrew (irc:RhodiumToad)

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