[llvm-dev] Early CSE clobbering llvm.assume

Lawrence, Peter via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 14 19:04:16 PDT 2016

              This first part is to whoever you are quoting, I can’t tell from the email,

The more information made available to the optimizers the better the optimizations,
Asserts provide more information,
You *should* expect better code with asserts enabled.

And this is *not* a bad thing !!!

And IMHO there is no winning argument that says we should not use this information.

In other words saying “asserts aren’t for optimization” isn’t a winning argument,
Its information and it isn’t useful to throw that information away.
It’s not very far removed from saying “if, while, and for conditions” aren’t for optimization.

This second part is for your response,

Hmmm, I don’t get how you go from “assert having a call to abort()” to a bunch of talk
About “branch around unreachable” ?  the condition isn’t unreachable, the abort isn’t
Unreachable, what’s unreachable ???


From: Daniel Berlin [mailto:dberlin at dberlin.org]
Sent: Tuesday, June 14, 2016 11:23 AM
To: Hal Finkel <hfinkel at anl.gov>
Cc: Lawrence, Peter <c_plawre at qca.qualcomm.com>; llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Early CSE clobbering llvm.assume

Sanjoy’s argument is faulty, if it were true we would also find our handling of “assert” to be unacceptable
but this is not the case, no one is arguing that we need to re-design “assert”
Sure, but no one should make this argument anyway: assert is not for optimization. In fact, we don't really want it to be used for optimization, because if we do, then we might end up in a situation where the -DNDEBUG build generates worse code than the build with asserts enabled.

Also, I'll note that the reason that assume is an intrinsic, instead of a branch around unreachable, is that we aggressively remove branches around unreachable as part of our canonicalization process. We do this in order to simplify code, and this is important in order to remove abstraction penalties. Note that unreachables are also generated from undefined behavior, and one of the ways we use undefined behavior is to assume it is unreachable, enabling us to eliminate what should be dead code. This is an important technique for limiting abstraction penalties from, for example, heavily templated C++ code.

Thus, somewhat unfortunately, Sanjoy's argument is not faulty.

Asserts occur much more often than assumes, it may or may not be sensible to handle them the same way.
I would argue it is sensible, but it's also reasonable to argue it is not.
We need to be careful what we mean by "in the same way".

Yes, i simply meant "extract information from the control flow structure and comparisons they generate when they are enabled".

You are correct with all of your observations :)

We can certainly improve the representations of assumes, perhaps as Danny has suggested by converting their control dependencies into extended SSA token vales, and better capture knowledge from conditional branches, but the tradeoffs here are not trivial.

100% agreed, this is something that requires some exploration and messing around, and then a design document going through the tradeoffs of various approaches with real data.

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