[llvm-dev] Inferring nsw/nuw flags for increment/decrement based on relational comparisons

Thu Sep 29 10:20:51 PDT 2016

On 28 Sep 2016, at 17:50, Philip Reames <listmail at philipreames.com<mailto:listmail at philipreames.com>> wrote:

On 09/27/2016 09:55 AM, Matti Niemenmaa wrote:
On 2016-09-27 02:28, Philip Reames wrote:
On 09/20/2016 12:05 PM, Matti Niemenmaa via llvm-dev wrote:
I posted some questions related to implementing inference of nsw/nuw
flags based on known icmp results to Bug 30428 (
https://llvm.org/bugs/show_bug.cgi?id=30428 ) and it was recommended
that I engage a wider audience by coming here. The minimal context is
the following, please see the bug report for more detail:

> 1. If (X s< Y), then both X + 1 and Y - 1 are nsw.
> 2. If (X u< Y), then both X + 1 and Y - 1 are nuw.
If this is the only case you want to support, this sounds like a fairly
straight forward extension to the LazyValueInfo analysis.  In
particular, take a look at getValueFromICmpCondition.  I'd be happy to
help review a patch here once you've got something working.

The basic idea would be that (X s<Y ) implies X s< INT_MAX since Y must
be INT_MAX or smaller and X is less than that.  We can tell this without
needing to know anything about Y.

Looks like a good idea, but I'm not sure how LazyValueInfo's interface would support this case. Did you mean synthesizing the INT_MAX constant and then checking specifically for "X s< INT_MAX" using LazyValueInfo::getPredicateAt? At a glance that seems like it would work, but it feels like an odd way of doing it.

Initially I was looking at LVI::getConstantRange but its "at the end of the specified block" interface seems too restrictive. The block containing the comparison may end in a conditional br and so surely LVI can't prove anything there. And the block containing the increment/decrement instruction may contain some later information that LVI can prove at the end of the block, but is not true at the instruction?

CorrelatedValuePropagation and JumpThreading appear to be the only transformation passes making use of LVI at the moment, and that's probably something we don't want to change. This kind of nsw/nuw flag inference doesn't really fit in either, but CVP is definitely the closer match and it should be possible to shoehorn it in there.
CVP actually already supports this case in ToT; it's just hidden behind an option while we address the regression previously mentioned.  See -cvp-dont-process-adds and the relevant guarded code.

Your point about the end of block bit can be split into two pieces:
1) Asking a question about an add guarded by a condition in a *previous* block.  This case LVI handles elegantly.  It's pretty much it's reason for existence.
2) An add which is guarded by an assume or guard in the *same* block does look concerning.  From a quick glance at the (off by default) functionality, it looks like there might be a bug here?  (Artur, please write a test case to check and submit it.)
CVP doesn’t look at instructions which operands are in the same basic block at the instruction. The motivation is not to spend time for cases which can be handled without expensive LVI machinery.

This test checks that LVI uses information from guard intrinsics:
https://github.com/llvm-mirror/llvm/blob/master/test/Transforms/CorrelatedValuePropagation/guards.ll

Artur

Fair warning, we're actively working through issues related to nsw/nuw
inference causing overall regressions.  I think we've got the key one
identified and a patch is under review, but I suspect you'll stumble
across the same thing.

Interesting, so adding nsw/nuw flags is pessimizing the generated code? Can you provide any links?

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