On Wed, Jan 23, 2019 at 12:33 PM Jakub Jelinek <ja...@redhat.com> wrote: > > On Wed, Jan 23, 2019 at 10:40:43AM +0000, Jonathan Wakely wrote: > > There's a patch to add __builtin_dynamic_object_size to clang: > > https://reviews.llvm.org/D56760 > > > > It was suggested that this could be done via a new flag bit for > > __builtin_object_size, but only if GCC would support that too > > (otherwise it would be done as a separate builtin). > > > > Is there any interest in adding that as an option to __builtin_object_size? > > > > I know Jakub is concerned about arbitrarily complex expressions, when > > __builtin_object_size is supposed to always be efficient and always > > evaluate at compile time (which would imply the dynamic behaviour > > should be a separate builtin, if it exists at all). > > The current modes (0-3) certainly must not be changed and must return a > constant, that is what huge amounts of code in the wild relies on.
I wouldn't overload _bos but only use a new builtin. > The reason to choose constants only was the desire to make _FORTIFY_SOURCE > cheap at runtime. For the dynamically computed expressions, the question > is how far it should go, how complex expressions it wants to build and how > much code and runtime can be spent on computing that. > > The rationale for __builtin_dynamic_object_size lists only very simple > cases, where the builtin is just called on result of malloc, so that is > indeed easy, the argument is already evaluated before the malloc call, so > you can just save it in a temporary and use later. Slightly more complex > is calloc, where you need to multiply two numbers (do you handle overflow > somehow, or not?). But in real world, it can be arbitrarily more complex, > there can be pointer arithmetics with constant or variable offsets, > there can be conditional adjustments of pointers or PHIs with multiple > "dynamic" expressions for the sizes (shall the dynamic expression evaluate > as max over expressions for different phi arguments (that is essentially > what is done for the constant __builtin_object_size, but for dynamic > expressions max needs to be computed at runtime, or shall it reconstruct > the conditional or remember it and compute whatever ? val1 : val2), > loops which adjust pointers, etc. and all that can be done many times in > between where the objects are allocated and where the builtin is used. Which means I'd like to see a thorough specification of the builtin. If it is allowed to return "failure" in any event then of what use is the builtin in practice? Richard. > Jakub
