Please review writeup for fixing PR 78809 (inline strcmp for small constant strings)

[Richard Biener]

On November 17, 2017 11:20:45 PM GMT+01:00, Qing Zhao <qing.zhao@oracle.com> wrote:
>
>> On Nov 16, 2017, at 6:24 PM, Martin Sebor <msebor@gmail.com> wrote:
>>> 
>>> In my current local implementation, I used the following routine to
>get the range info:  (and use the MINMAXLEN[1]+1 for the length of the
>non-constant string)
>>> 
>>> /* Determine the minimum and maximum value or string length that ARG
>>>   refers to and store each in the first two elements of MINMAXLEN.
>>>   For expressions that point to strings of unknown lengths that are
>>>   character arrays, use the upper bound of the array as the maximum
>>>   length.  For example, given an expression like 'x ? array : "xyz"'
>>>   and array declared as 'char array[8]', MINMAXLEN[0] will be set
>>>   to 3 and MINMAXLEN[1] to 7, the longest string that could be
>>>   stored in array.
>>>   Return true if the range of the string lengths has been obtained
>>>   from the upper bound of an array at the end of a struct.  Such
>>>   an array may hold a string that's longer than its upper bound
>>>   due to it being used as a poor-man's flexible array member.  */
>>> 
>>> bool
>>> get_range_strlen (tree arg, tree minmaxlen[2])
>>> {
>>> }
>>> 
>>> However, this routine currently miss a very obvious case as the
>following:
>>> 
>>> char s[100] = {'a','b','c','d’};
>>> 
>>> __builtin_strcmp(s, "abc") != 0
>>> 
>>> So, I have to change this routine to include such common case.
>> 
>> There was a discussion some time ago about converting CONSTRUCTOR
>> trees emitted for array initializers like the above to STRING_CST
>> (see bug 71625 for some background).  I think that would still be
>> the ideal solution.  Then you wouldn't have to change
>> get_range_strlen.

IIRC Martin had a patch for this. 

Richard. 

>thanks for the info, Martin.
>
>In my case, it’s the size of “100” cannot be collected in the
>MINMAXLEN[1] for the string “s”. 
>
>I need to make sure that the size of variable string s is larger than
>the size of constant string “abc” to guarantee the safety of the
>transformation.
>
>currently, “get_range_strlen” cannot identify the simple VAR_DECL with
>array_type to determine the maximum size of the string. 
>
>Qing
>> 
>> Martin