[PATCH] D99160: [X86][FastISEL] Support DW_TAG_call_site_parameter with FastISEL

Mon Apr 19 10:33:28 PDT 2021

dblaikie added a comment.

In D99160#2698980 <https://reviews.llvm.org/D99160#2698980>, @alok wrote:

> In D99160#2671899 <https://reviews.llvm.org/D99160#2671899>, @dblaikie wrote:
>
>> In D99160#2670460 <https://reviews.llvm.org/D99160#2670460>, @djtodoro wrote:
>>
>>> In D99160#2669576 <https://reviews.llvm.org/D99160#2669576>, @dblaikie wrote:
>>>
>>>> In D99160#2668977 <https://reviews.llvm.org/D99160#2668977>, @djtodoro wrote:
>>>>
>>>>> I think that the Debug Entry Values feature should not be enabled by default for non optimized code, so the `TargetOptions::ShouldEmitDebugEntryValues()` should be patched with checking of optimization level (it should be > 0).
>>>>
>>>> That's currently intended to be already handled by the frontend, right? (clang only sets `EnableDebugEntryValues` (which `ShouldEmitDebugEntryValues` checks (hmm, it checks under 'or', not 'and', so I'm not sure where the "only above -O0" is implemented, but it is implemented somewhere?) if optimizations are enabled, yeah?)
>>>>
>>>> Oh, is entry_values actually not conditionalized? It's only the call_site support that's currently conditionalized on "above -O0"?
>>>
>>> Looks like there is no explicit check of optimization level (above "-O0"), neither on frontend nor backend for entry-values generation. I think it is the situation since there should not be any optimization (at least that I am aware of, in the case of C/C++) that would cause the entry-values generation...
>>>
>>>> Hmm - If that's the case, and we currently have some cases where entry_values are emitted at -O0, I'm not sure /not/ emitting those is the right call either. If we believe/have data to show that there are so few useful uses of entry_value at -O0 that it's not worth the DWARF size growth to put call_sites in at -O0, then I think it might still be worth leaving the entry_values in (unless they take up a bunch of extra space) for the cases of mixed optimization compilation (-O0 some code you're debugging, but building the rest with optimizations).
>>>
>>> Yeah... That is valuable example... I am thinking in that direction as well, and I am closer to the enabling it for -O0 case if that is useful (and there is no dramatic cost in terms of DWARF size).
>>
>> Does anyone have this example (where DW_OP_entry_value is used at -O0)? It'd be great to look at it & see if it's a case of unnecessarily losing the location, or legitimately losing it and using entry_value for best-effort recovery (& then a question of whether the loss is appropriate at -O0, or if we want to pessimize -O0 further to avoid the loss).

I think this ^ still needs understanding/investigation. Do you have an example with OP_entry_value at -O0?

>> I'd worry about turning on call_sites at -O0 - there'd be a lot more calls (especially for C++ code with lots of implicit operations), but numbers will be needed in any case, so not worth much speculation.
>
> Sorry for late response.
> I tried building  https://github.com/flang-compiler/classic-flang-llvm-project.git (branch release_11x) with compiler (current patch (https://reviews.llvm.org/D99160) and https://reviews.llvm.org/D99238) with -O0 -g . 
> Interestingly there was no difference.
> Reason: https://reviews.llvm.org/D99238 is not sufficient for clang/clang++ to enable call-site generation with FastISel though it is sufficient for Flang compiler.
> Below additional patch is needed to generate call-sites
>
>   `````````````````````````````````````
>
> diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
> index a77f52bd235b..8d4e11faa018 100644
>
> - a/clang/lib/CodeGen/CGDebugInfo.cpp
>
> +++ b/clang/lib/CodeGen/CGDebugInfo.cpp
> @@ -5149,9 +5149,9 @@ llvm::DebugLoc CGDebugInfo::SourceLocToDebugLoc(SourceLocation Loc) {
>  }
>
> llvm::DINode::DIFlags CGDebugInfo::getCallSiteRelatedAttrs() const {
>
> - // Call site-related attributes are only useful in optimized programs, and
> - // when there's a possibility of debugging backtraces.
> - if (!CGM.getLangOpts().Optimize || DebugKind == codegenoptions::NoDebugInfo ||
>
> +  // Call site-related attributes are useful when there's a possibility of
> +  // debugging backtraces.
> +  if (DebugKind == codegenoptions::NoDebugInfo ||
>
>     DebugKind == codegenoptions::LocTrackingOnly)
>   return llvm::DINode::FlagZero;
>
>   `````````````````````````````````````
>
> With this patch Clang/Clang++ turn on LLVM IR flag "DIFlagAllCallsDescribed", in absence of this LLVM does not generate call-site.
>
> With the above patch applied below is the comparison of sizes of shared libraries.
>
> Name of shared library - Size (without callsite) - Size (with callsites) - % incresase in size
>
>   ```````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````
>
> PipSqueak.so 73192 75048 2%
> SecondLib.so 73192 75048 2%
> TestPlugin.so 1694024 1700704 0%
> libLLVMDlltoolDriver.so 336568 347872 3%
> libLLVMDebugInfoPDB.so 14463832 15360784 6%
> libLLVMOrcError.so 108880 111184 2%
> libLLVMTarget.so 2645104 2677448 1%
> libLLVMFrontendOpenMP.so 2354728 2505232 6%
> libLLVMProfileData.so 7901688 8373632 5%
> libLLVMOrcJIT.so 28838432 30490640 5%
> libLLVMRemarks.so 3311680 3551856 7%
> libgtest.so 2374120 2523112 6%
> libLLVMDemangle.so 1350616 1490832 10%
> libLLVMAsmParser.so 6961216 7366040 5%
> LLVMHello.so 394624 396120 0%
> libLLVMGlobalISel.so 18886648 19698008 4%
> libLLVMDebugInfoMSF.so 1288376 1365040 5%
> libLLVMCoverage.so 4225224 4502104 6%
> libLLVMFuzzMutate.so 3859968 3973808 2%
> libRemarks.so 6696 6696 0%
> libLLVMDebugInfoDWARF.so 11914848 12750784 7%
> libLLVMMCParser.so 6419464 6873000 7%
> libLLVMTableGen.so 4855536 5180760 6%
> libLLVMDWARFLinker.so 5407528 5628576 4%
> Bye.so 1858848 1872672 0%
> libLLVMMCJIT.so 1470952 1526544 3%
> libLLVMMC.so 16931504 17741376 4%
> libLLVMipo.so 43554712 46019392 5%
> libLLVMLineEditor.so 208216 216360 3%
> libbenchmark_main.so 18904 19408 2%
> libbenchmark.so 3308304 3507632 6%
> libLTO.so 2240720 2277408 1%
> libLLVMInterpreter.so 2614696 2749128 5%
> libLLVMTransformUtils.so 47925248 50476512 5%
> libLLVMX86Desc.so 8047928 8213152 2%
> libLLVMCoroutines.so 6478080 6766880 4%
> libLLVMJITLink.so 5590936 6066736 8%
> libLLVMVectorize.so 19557808 20665544 5%
> libLLVMX86Disassembler.so 2820056 2849376 1%
> libLLVMBitReader.so 8282648 8823240 6%
> libLLVMMCA.so 3242016 3405624 5%
> libLLVMBitWriter.so 6544032 6867976 4%
> libLLVMMIRParser.so 5739688 5980104 4%
> libLLVMLTO.so 13272272 13786192 3%
> libLLVMCore.so 46109224 48840008 5%
> libLLVMBitstreamReader.so 561896 600624 6%
> libLLVMObjectYAML.so 23110160 24648160 6%
> libLLVMSupport.so 20349728 21953112 7%
> libLLVMIRReader.so 1215672 1237960 1%
> libLLVMX86Info.so 76488 76624 0%
> libLLVMSelectionDAG.so 34358968 36876128 7%
> libLLVMExecutionEngine.so 2962160 3073224 3%
> libLLVMSymbolize.so 1980760 2089728 5%
> libLLVMPasses.so 18574960 19459712 4%
> libLLVMOption.so 869784 920976 5%
> libLLVMObject.so 15138656 16383168 8%
> libLLVMTextAPI.so 3191272 3384560 6%
> libLLVMX86CodeGen.so 55202744 58068088 5%
> libLLVMAggressiveInstCombine.so 2354936 2419048 2%
> libLLVMExtensions.so 23904 23904 0%
> libLLVMWindowsManifest.so 351608 370168 5%
> libLLVMObjCARCOpts.so 4964488 5150120 3%
> libLLVMBinaryFormat.so 1325752 1431448 7%
> libLLVMDebugInfoGSYM.so 2909272 3094616 6%
> libLLVMTestingSupport.so 623608 656640 5%
> libgtest_main.so 42856 43608 1%
> libLLVMLinker.so 3070264 3210248 4%
> libLLVMCFGuard.so 1082488 1096104 1%
> libLLVMCodeGen.so 139859816 146452160 4%
> libLLVMDebugInfoCodeView.so 7742896 8497704 9%
> libLLVMX86AsmParser.so 2590816 2714008 4%
> libLLVMRuntimeDyld.so 6592016 7094048 7%
> libLLVMInstCombine.so 18194728 19705184 8%
> BugpointPasses.so 780112 788304 1%
> libLLVMScalarOpts.so 73110536 77534912 6%
> libLLVMXRay.so 3993696 4251280 6%
> libLLVMMCDisassembler.so 1292912 1313536 1%
> libLLVMFrontendOpenACC.so 96008 102656 6%
> libLLVMInstrumentation.so 21038808 22351384 6%
> libLLVMLibDriver.so 1595936 1638704 2%
> libLLVMAsmPrinter.so 25818000 26842816 3%
> libLLVMAnalysis.so 79615056 83995856 5%
>
>   `````````````````````````````````````````````````````````````````````
>
> sum 79615057 83995857 5%
>
> So the conclusion is,
>
> - We have a Flag which can help us if we want to enable callsite generation selectively (only for Flang)

It doesn't seem to me, so far, like this is a place where Flang and Clang should diverge - they're both doing the same sort of thing for the same reasons/likely with the same sort of tradeoffs of location accuracy V size cost.

> - If we are fine with 5% increase in size, we can enable call-site generation by default.

I'd actually be somewhat more worried about object size, rather than/in addition to executable size, due to the increase in relocations (especially with DWARFv5 (& especially with the `-mllvm -minimize-addr-in-v5=Ranges` which further reduces debug_addr, but can't handle labels and call sites (unlike `-minimize-addr-in-v5=Expressions` or `-minimize-addr-in-v5=Form`)), which does a lot to reduce the number of relocations by using debug_addr and address sharing on debug_rnglists/loclists/etc) which can't be compressed, etc.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D99160/new/

https://reviews.llvm.org/D99160