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

[Qing Zhao <qing dot zhao at oracle dot com>]

> 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.

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