[PATCH] D29369: [ubsan] Omit superflous overflow checks for promoted arithmetic (PR20193)

Thu Feb 9 18:35:40 PST 2017

vsk added a comment.

In https://reviews.llvm.org/D29369#672166, @dtzWill wrote:

> After some thought, can we discuss why this is a good idea?

The goal is to lower ubsan's compile-time + instrumentation overhead at -O0, since this reduces the friction of debugging a ubsan-instrumented project.

> This increases the cyclomatic complexity of code that already is difficult to reason about, and seems like it's both brittle and out-of-place in CGExprScalar.

Are there cleanups or ways to reorganize the code that would make this sort of change less complex / brittle? I'm open to taking that on.

> It really seems it would be better to let InstCombine or some other analysis/transform deal with proving checks redundant instead of attempting to do so on-the-fly during CodeGen.

-O1/-O2 do get rid of a lot of checks, but they also degrade the debugging experience, so it's not really a solution for this use case.

> Can you better motivate why this is worth these costs, or explain your use case a bit more?

I have some numbers from LNT. I did a pre-patch and post-patch run at -O0 + -fsanitize=signed-integer-overflow,unsigned-integer-overflow. There were 4,672 object files produced in each run. This patch brings the average object size down from 36,472.0 to 36,378.3 bytes (a 0.26% improvement), and the average number of overflow checks per object down from 66.8 to 66.2 (a 0.81% improvement).

I don't have reliable compile-time numbers, but not emitting IR really seems like a straightforward improvement over emitting/analyzing/removing it.

So, those are the benefits. IMO getting close to 1% better at reducing instrumentation overhead is worth the complexity cost here.

https://reviews.llvm.org/D29369