> On Jun 21, 2021, at 10:35 AM, Richard Biener <rguent...@suse.de> wrote: >>> I think we can drop -fauto-var-init=pattern and just go with block >>> initializing which will cover padding as well which means we can >>> stay with the odd -ftrivial-auto-var-init name used by CLANG and >>> add no additional options. >> >> Yes, this is a good idea. >> >> block initializing will cover all paddings automatically. >> >> Shall we do block initializing for both “zero initialization” and >> “pattern initialization”? >> >> Currently, for zero initialization, I used the following: >> >>>>> + case AUTO_INIT_ZERO: >>>>> + init = build_zero_cst (TREE_TYPE (var)); >>>>> + expand_assignment (var, init, false); >>>>> + break; >> >> Looks like that the current “expand_assignment” does not initialize >> paddings with zeroes. >> Shall I also use “memset” for “zero initialization”? > > I'd say so, yes.
Okay. One more question for the current “expand_builtin_memset”: Is the current implementation of “expand_builtin_memset” automatically handle short length memset optimally? i.e, do I need to specially handle char type, short type, or other types that can fit to a register? >>> >>> There's no "safe" pattern besides all-zero for all "undefined" uses >>> (note that uses do not necessarily use declared types). Which is why >>> recommending pattern init is somewhat misguided. There's maybe >>> some useful pattern that more readily produces crashes, those that >>> produce a FP sNaN for all of the float types. >> >> So, pattern value as 0xFF might be better than 0xAA since 0xFFFFFFFF >> will be a NaN value for floating type? > > I think for debugging NaNs are quite nice, yes. For floating point, 0xFFFFFFFF is good. But for pointer type, is it good? (See my other email to Kees). >> >> Not sure whether it’s necessary to expose this to user. >> >> One question that is important to the implementation is: >> >> Shall we use “byte-repeated” or “word-repeated” pattern? >> Is “word-repeated” pattern better than “byte-repeated” pattern? >> >> For implementation, “byte-repeated” pattern will make the whole >> implementation much simpler since both “zero initialization” >> and “pattern initialization” can be implemented with “memset” with >> different “value”. >> >> So, if “word-repeated” pattern will not have too much more benefit, I >> will prefer “byte-repeated” pattern. >> >> Let me know your comments here. > > I have no strong opinion and prefer byte repetition for simplicity. But I > would document this as implementation detail that can change. Okay, if we finally decide to go with byte repetition, I will document this as implementation details that can be changed later. Qing > > Richard. > >>> >>>> >>>> >>>> As said, for example glibc allocator hardening with MALLOC_PERTURB_ >>>> uses simple byte-init. >>>> >>>> What’s the pattern glibc used? >>> >>> The value of the MALLOC_PERTURB_ environment truncated to a byte. >> >> Okay. >> >> thanks. >> >> Qing >>> >>> Richard. >>> >
