> On Nov 17, 2017, at 7:32 PM, Jeff Law <l...@redhat.com> wrote: > > 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.
thanks for the info. Yes, during my current implementation for B, I tried to use the interface for “tree-object-size”, i.e, “compute_builtin_object_size” to decide the maximum length of the arrays, I noticed that it did not provide the information for the simple cases. then I switched to use “get_range_strlen”. I will double check on this. > Ranges are more for integer objects. ie, i has the value [0,25] or ~[0,0]. Okay, I see. > >>> >>> 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. thanks. Qing > > jeff
