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

Mon Apr 19 21:25:15 PDT 2021

alok added a comment.

In D99160#2699038 <https://reviews.llvm.org/D99160#2699038>, @dblaikie wrote:

> 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.

Below is the comparison of total size of object files (*.o) for same build (default (O0) + -g).

sum 3535044960 3680496672 4%

I shall come back with numbers for options O0 -gdwarf-5 -mllvm -minimize-addr-in-v5=Ranges .

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

https://reviews.llvm.org/D99160