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

George Burgess IV via llvm-dev llvm-dev at lists.llvm.org
Fri Jun 29 15:25:01 PDT 2018

> 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.

(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
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