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

[Jeff Law]

On 11/17/2017 03:45 PM, Qing Zhao wrote:
>>> do you think using this routine is good? or do you have other
>>> suggestions (since I am still not very familiar with the internals of
>>> GCC, might not find the best available one now…)
>> The range information attached to an SSA_NAME is global data.  ie, it
>> must hold at all locations where the object in question might be
>> referenced.  This implies that it will sometimes (often?) be less
>> precise than you might like.
> 
> do you mean the “value_range” attached to SSA_NAME?
> 
> For my purpose, I’d like to get the maximum length of char array s[100] is 100, which is larger than the size of constant string “abc”, then
> I can safely apply the transformation to memcmp. 
> 
> can “value_range” info serve this purpose?
No it can't.  Sorry for leading you the wrong direction.  What you're
looking for is the object size interfaces.

See tree-object-size.[ch]

That's a pass that tries to compute the sizes of various objects
referenced by the IL.

Note that the object size is different than say the length of a string
stored in an object for which you'll probably be looking at
tree-ssa-strlen's interfaces.

Ranges are more for integer objects.  ie, i has the value [0,25] or ~[0,0].





> 
>>
>> I am currently working towards an embeddable context sensitive range
>> analyzer that in theory could be used within tree-ssa-strlen pass to
>> give more precise range information.  I'm hoping to wrap that work up in
>> the next day or so so that folks can use it in gcc-8.
> 
> such context sensitive range info should be useful when we relax the constant “N” to be an expression whose Min value is larger than the length
> of constant string, with it, we can catch more opportunities. 
> let me know when this info is available.
Hoping to have the basics into the trunk within the next few days as
reviews flow in.

jeff