[llvm-dev] Cleaning up ‘br i1 false’ cases in CodeGenPrepare

[George Burgess IV via llvm-dev]

> But someone should measure before we move it.

I ran numbers on a large, varied codebase with an up-to-date clang-based
FORTIFY implementation. With the current forced lowering in CGP, we lowered
64,662 calls to @llvm.objectsize to non-failure values and lowered 111,224
to failure values. Making the instcombine iteration after
EP_VectorizerStart require that all objectsize intrinsics are lowered, we
found successful values for 64,552 llvm.objectsize intrinsics, and returned
failure values for 120,616.

Taken literally, we fail to lower 63.2% of calls with "successful" values
today, and this change makes us fail to lower 65.1%. However, given that
the earlier lowering makes us lower a little over 9,000 additional
intrinsics, I'd imagine that most of these 'new' failures got DCE'd away
before hitting CGP in the past.

In any case, I'd like to note that these numbers don't include calls to
__builtin_object_size that clang is able to lower itself, so from a clang
user's perspective, any degradation mentioned here is likely an
overstatement.

Given the above, I've no issues with forcing @llvm.objectsize lowering to
earlier in the pipeline. I have a patch <https://reviews.llvm.org/D49103>
to do this as part of InstCombine. Happy to make a
LowerBestEffortPostOptimizationIntrinsicsPass (or whatever) specifically
for this, if that would be preferable. Also happy to dig into where some of
those additional objectsizes appear from if people really want.

On Fri, Jun 29, 2018 at 3:59 PM Friedman, Eli <efriedma at codeaurora.org>
wrote:

> On 6/29/2018 3:25 PM, George Burgess IV wrote:
>
> > we lower llvm.objectsize later than we should
>
> Is there a well-accepted best (or even just better) place to lower
> objectsize? I ask because I sorta fear that these kinds of problems will
> become more pronounced as llvm.is.constant, which is also lowered in CGP,
> gains popularity.
>
>
> After the "simplification" part of the optimization pipeline (after we've
> finished inlining and the associated function simplification passes have
> run), we're unlikely to find new information that would help simplify an
> llvm.objectsize or llvm.is.constant call.  So roughly around
> EP_VectorizerStart is probably appropriate.  But someone should measure
> before we move it.
>
> -Eli
>
>
> (To be clear, I think it totally makes sense to lower is.constant and
> objectsize in the same place. I'm just saying that if the ideal piece of
> code to do that isn't CGP, ...)
>
> On Fri, Jun 29, 2018 at 12:21 PM Friedman, Eli via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On 6/28/2018 9:44 PM, Bharathi Seshadri via llvm-dev wrote:
>> > Hi,
>> >
>> > I have come across a couple of cases where the code generated after
>> > CodeGenPrepare pass has "br i1 false .." with both true and false
>> > conditions preserved and this propagates further and remains the same
>> > in the final assembly code/executable.
>> >
>> > In CodeGenPrepare::runOnFunction, ConstantFoldTerminator (which
>> > handles the br i1 false condition) is called only once and if after
>> > the transformation of code by ConstantFoldTerminator() and
>> > DeleteDeadBlock() we end up with code like "br i1 false", there is no
>> > further opportunity to clean them up. So calling this code under
>> > (!DisableBranchOpts) in a loop until no more transformations are made
>> > fixes this issue. Is this reasonable ?
>>
>> I would expect the precise case you're running into is rare: the second
>> iteration of the loop does nothing useful unless the IR specifically has
>> an i1 phi node in a block whose predecessors were erased.  And the
>> default optimization pipeline runs SimplifyCFG at the very end, which is
>> close to CodeGenPrepare, so the CFG simplification will usually be a
>> no-op anyway.
>>
>> We really shouldn't be doing this sort of folding in CodeGenPrepare in
>> the first place.  It looks like it was added to work around the fact
>> that we we lower llvm.objectsize later than we should.
>>
>> -Eli
>>
>> --
>> Employee of Qualcomm Innovation Center, Inc.
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a
>> Linux Foundation Collaborative Project
>>
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>
> --
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>
>
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