[avr-libc-dev] Plans to update scanf().

[Dmitry K.]

Hi.

In the near future I am going to update vfscanf() function
- a core of all scanf family (first only the HEAD branch).

========================================================================

Function vfscanf() is rewriten. The float point version is optimized
considerably: the expense of a stack was cut by half, and the size of
function has decreased approximately on 1KB. Opportunities of standard
and minimal versions are expanded, and their size has remained at a
former level.

Briefly:
. Restrictions of a float point format are eliminated.
. Conversion '%[' is present in standart version now.
. The minimal version differs from standard only absence of '%[' conversion.
. The 'hh' type qualifier is added ('char *', C99).
. The maximal field width is increased up to 255 (65535 in float version).
. All known bugs are fixed.

New vfscanf() concedes old in:
. The size of the minimal and standard versions has increased for old
branches of compiler and ancient chips (without MOVW).
. The maximum quantity of conversions now is limited (255).

More in detail:

. The old vfscanf() used the intermediate buffer for float point numbers.
The size of this buffer made 40 bytes. Though for practice it is too much,
all range has not been blocked (1.4e-45 ... 3.4e+38). In the new vfscanf()
decoding goes on the fly, and for the size of number there are no
restrictions. Also the new version understands abreviatury for NaN and Inf.

. The '%[' is realized as separate function, therefore a stack spent only
in case of real usage.

. Earlier the minimal version considerably differed from standard: there
was no comparison of symbols, there was no miss of storing, there was no
width of a field. Actually, it was absolutely other function. As a rule,
the program written counting upon standard scanf() could not be translated
without serious consequences on the minimal version.

. The size of the minimal and standard versions has increased for old
branches of compiler and ancient chips (without MOVW). For modern chips
and compilers of 4.1 - 4.3 branches, it is possible to tell, that the size
about the same. Besides now the minimal version covers all functions old
standard.

. Restriction of conversion quantity has arisen because of reduction of
the size of the counter up to one byte. It has allowed to reduce
considerably the size connected with shortage of registers. Let's notice,
that the call scanf() with 255 conversions will demand more than 0.5KB
a stack only on transfer of addresses.

. In the source the manual adjustment of parameters (the size of width of
a field, presence of '%[' and a float conversions) is stipulated.


Benchmarks of old sscanf():
~~~~~~~~~~~~~~~~~~~~~~~~~~
AVR:   at90s8515__________________________  atmega8____________________________ 
GCC:   3.3.6 3.4.6 4.0.4 4.1.2 4.2.2 4.3.X  3.3.6 3.4.6 4.0.4 4.1.2 4.2.2 4.3.X 
-------------------------------------------------------------------------------
sscanf_min("12345","%d",&x)
Flash:  1356  1354  1374  1372  1386  1392   1222  1222  1238  1240  1250  1254 
Stack:    49    49    53    53    59    55     49    49    53    53    59    55 
 Time:  1681  1672  1640  1638  1643  1685   1371  1367  1359  1357  1356  1392 
-------------------------------------------------------------------------------
sscanf_std("12345","%d",&x)
Flash:  1680  1672  1796  1762  1736  1764   1512  1504  1588  1562  1552  1558 
Stack:    50    50    54    54    61    56     50    50    54    54    61    56 
 Time:  1715  1736  1714  1694  1749  1751   1413  1434  1432  1417  1461  1463 
-------------------------------------------------------------------------------
sscanf_flt("1.2345","%e",&x)
Flash:  4022  3982  4078  4002  4108  4360   3676  3636  3682  3646  3746  3976 
Stack:   124   124   126   128   140   132    124   124   126   128   140   132 
 Time:  3026  3025  3023  3043  3104  3165   2651  2651  2652  2673  2729  2778 
===============================================================================


Benchmarks of new sscanf():
~~~~~~~~~~~~~~~~~~~~~~~~~~
AVR:   at90s8515__________________________  atmega8____________________________ 
GCC:   3.3.6 3.4.6 4.0.4 4.1.2 4.2.2 4.3.X  3.3.6 3.4.6 4.0.4 4.1.2 4.2.2 4.3.X 
-------------------------------------------------------------------------------
sscanf_min("12345","%d",&x)
Flash:  1522  1482  1442  1368  1428  1388   1312  1278  1260  1186  1244  1186 
Stack:    52    52    54    53    55    52     52    52    54    53    55    52 
 Time:  1476  1477  1358  1360  1339  1500   1359  1359  1260  1261  1240  1361 
-------------------------------------------------------------------------------
sscanf_std("12345","%d",&x)
Flash:  1926  1894  1926  1836  1838  1802   1660  1632  1686  1596  1594  1540 
Stack:    52    52    53    51    53    52     52    52    53    51    53    52 
 Time:  1480  1481  1357  1355  1334  1503   1363  1363  1259  1256  1235  1364 
-------------------------------------------------------------------------------
sscanf_flt("1.2345","%e",&x)
Flash:  3506  3454  3304  3162  3186  3232   3064  3020  2880  2788  2808  2834 
Stack:    60    60    69    61    63    60     60    60    67    61    63    60 
 Time:  2337  2351  2353  2179  2187  2355   1961  1967  1924  1826  1833  1962 
===============================================================================

Dmitry.