George Burgess IV via llvm-dev
2018-Jun-29 22:25 UTC
[llvm-dev] Cleaning up ‘br i1 false’ cases in CodeGenPrepare
> we lower llvm.objectsize later than we shouldIs 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 > Foundation Collaborative Project > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180629/0284ede1/attachment.html>
Friedman, Eli via llvm-dev
2018-Jun-29 22:59 UTC
[llvm-dev] Cleaning up ‘br i1 false’ cases in CodeGenPrepare
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 <mailto: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 > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-- Employee of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180629/845f27f6/attachment.html>
George Burgess IV via llvm-dev
2018-Jul-09 22:06 UTC
[llvm-dev] Cleaning up ‘br i1 false’ cases in CodeGenPrepare
> 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 >> >> _______________________________________________ >> LLVM Developers mailing list >> llvm-dev at lists.llvm.org >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >> > > -- > Employee of Qualcomm Innovation Center, Inc. > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180709/9be1b2da/attachment.html>