[PATCH] D28404: IRGen: Add optnone attribute on function during O0

Paul Robinson via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Mon Jan 9 16:03:56 PST 2017

probinson added a comment.

In https://reviews.llvm.org/D28404#640588, @mehdi_amini wrote:

> Actually, as mentioned before, I could be fine with making `O0` incompatible with LTO, however security features like CFI (or other sort of whole-program analyses/instrumentations) requires LTO.

Well, "requires LTO" is overstating the case, AFAICT from the link you gave me.  Doesn't depend on //optimization// at all.  It depends on some interprocedural analyses given some particular scope/visibility boundary, which it is convenient to define as a set of linked bitcode modules, that by some happy chance is the same set of linked bitcode modules that LTO will operate on.

If it's important to support combining a bitcode version of my-application with your-bitcode-library for this CFI or whatever, and you also want to let me have my-application be unoptimized while your-bitcode-library gets optimized, NOW we have a use-case.  (Maybe that's what you had in mind earlier, but for some reason I wasn't able to extract that out of any prior comments.  No matter.)

I'm now thinking along the lines of a `-foptimize-off` flag (bikesheds welcome) which would set the default for the pragma to 'off'.  How is that different than what you wanted for `-O0`?  It is defined in terms of an existing pragma, which is WAY easier to explain and WAY easier to implement.  And, it still lets us say that `-c -O0 -flto` is a mistake, if that seems like a useful thing to say.

Does that seem reasonable?  Fit your understanding of the needs?


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