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

[Hal Finkel via Phabricator via cfe-commits]

hfinkel added a comment.

In https://reviews.llvm.org/D28404#673260, @mehdi_amini wrote:

> Ping :)


To clarify my understanding of this thread, it seems like there are three ways forward here:

1. To have -O0 add optnone to the generated functions (enabling some degree of lack of optimization of those functions even when used with -flto)
2. To have -O0 -flto essentially turn off LTO (so that we get unoptimized objects directly for things we're debugging)
3. Add a separate flag to make optnone the default

(1) is this patch. The disadvantage of (2) is that it also precludes CFI (and other whole-program transformations). This seems highly unfortunate at best and a non-starter in the general case. The disadvantage of (3) is that it might seems confusing to users (i.e. how to explain the difference between -O0 and -foptimize-off?) and is an unnecessary exposure to users of implementation details. On this point I agree.

It is true that -O0 != optnone, in a technical sense, but in the end, both are best effort. Moreover, there is a tradeoff between disabling optimization of the functions you don't want to optimize and keeping the remainder of the code as similar as possible to how it would be if everything were being optimized. What optnone provides seems like a reasonable point in that tradeoff space. I think that we should move forward with this approach.


https://reviews.llvm.org/D28404