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Re: [RFC][PATCH] Extend DCE to remove unnecessary new/delete-pairs


On 11/28/2017 09:11 PM, Martin Sebor wrote:
> On 11/27/2017 02:22 AM, Dominik Inführ wrote:
>> Thanks for all the reviews! I’ve revised the patch, the
>> operator_delete_flag is now stored in tree_decl_with_vis (there
>> already seem to be some FUNCTION_DECL-flags in there). I’ve also added
>> the option -fallocation-dce to disable this optimization. It
>> bootstraps and no regressions on aarch64 and x86_64.
>>
> It's great to be able to eliminate pairs of these calls.  For
> unpaired calls, though, I think it would be even more useful to
> also issue a warning.  Otherwise the elimination will mask bugs
> that might only show up without optimization, or with other
> compilers (such as older versions of GCC).  I realize GCC doesn't
> warn for these bugs involving malloc, but I think it should for
> the same reason.
Aldy and I looked at this stuff a while ago -- to do anything reasonable
you have to build a much stronger static analysis engine than we've got
in GCC right now.  THe false positive rate will be way too high to be
useful on real world code.

I don't see how the optimizatino changes anything WRT warnings though.
If you don't have a clear pair where all the proper preconditions are
met, then you don't do anything.  Those preconditions would include that
you actually have a pair as opposed to just the allocation point.


Jeff


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