[llvm-dev] Please dogfood LLD

Thu Mar 16 14:41:30 PDT 2017

To be specific - you can see a few of the later frames (endFunction,
runOnMachineFunction, runOnFunction) doesn't have source information.

On Thu, Mar 16, 2017 at 2:40 PM David Blaikie <dblaikie at gmail.com> wrote:

> On Thu, Mar 16, 2017 at 2:36 PM Rui Ueyama <ruiu at google.com> wrote:
>
> What program did you use to test the feature, and what was missing
> information? I'd like to file that as a bug so that we can fix this later.
>
>
> I was using GDB 7.9 (a google internal build, not sure how much that
> matters - there might be some things that didn't make it upstream) to debug
> LLVM's 'llc' binary on an assert-failure.
>
> The backtrace was missing source information from some frames:
>
> Not the best representation of it, but here's a chunk of my debugging
> session:
>
> #9  0x00000000037d9278 in llvm::DwarfCompileUnit::constructVariableDIEImpl
> (this=0x540a390, DV=..., Abstract=false) at /usr/local/google/hom
> e/blaikie/dev/llvm/src/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp:543
> (gdb)
> #10 0x00000000037d8c6c in llvm::DwarfCompileUnit::constructVariableDIE
> (this=0x540a390, DV=..., Abstract=false) at /usr/local/google/home/bl
> aikie/dev/llvm/src/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp:479
> (gdb)
> #11 0x00000000037d97b6 in llvm::DwarfCompileUnit::constructVariableDIE
> (this=0x540a390, DV=..., Scope=..., ObjectPointer=@0x7fffffffb488: 0x
> 0) at
> /usr/local/google/home/blaikie/dev/llvm/src/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp:560
> (gdb)
> #12 0x00000000037d8231 in llvm::DwarfCompileUnit::createScopeChildrenDIE
> (this=0x540a390, Scope=0x5482df0, Children=llvm::SmallVector of len
> gth 0, capacity 8, ChildScopeCount=0x0) at
> /usr/local/google/home/blaikie/dev/llvm/src/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp:572
> (gdb)
> #13 0x00000000037d99b7 in
> llvm::DwarfCompileUnit::createAndAddScopeChildren (this=0x540a390,
> Scope=0x5482df0, ScopeDIE=...) at /usr/local/go
>
> ogle/home/blaikie/dev/llvm/src/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp:614
> (gdb)
> #14 0x00000000037d98a4 in
> llvm::DwarfCompileUnit::constructSubprogramScopeDIE (this=0x540a390,
> Sub=0x53cd568, Scope=0x5482df0) at /usr/local
>
> /google/home/blaikie/dev/llvm/src/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp:594
> (gdb)
> #15 0x000000000376a087 in llvm::DwarfDebug::endFunctionImpl
> (this=0x54082c0, MF=0x5439b60) at
> /usr/local/google/home/blaikie/dev/llvm/src/li
> b/CodeGen/AsmPrinter/DwarfDebug.cpp:1240
> (gdb)
> #16 0x00000000037c93c9 in
> llvm::DebugHandlerBase::endFunction(llvm::MachineFunction const*) ()
> (gdb)
> #17 0x0000000003733d6a in llvm::AsmPrinter::EmitFunctionBody
> (this=0x53fa9a0) at
> /usr/local/google/home/blaikie/dev/llvm/src/lib/CodeGen/Asm
> Printer/AsmPrinter.cpp:1083
> (gdb)
> #18 0x0000000003107e61 in
> llvm::X86AsmPrinter::runOnMachineFunction(llvm::MachineFunction&) ()
> (gdb)
> #19 0x0000000003a263b1 in
> llvm::MachineFunctionPass::runOnFunction(llvm::Function&) ()
>
>
>
> On Thu, Mar 16, 2017 at 2:34 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
> FWIW - selfhosting I did find that GDB wasn't able to find the source code
> for some functions when using LLD's gdb_index, so I've switched back to
> gold+gdb_index for now (given the performance problems you mentioned,
> sounds like I wasn't gaining much/anything in terms of link time by using
> lld anyway).
>
>
> On Thu, Mar 16, 2017 at 11:17 AM Rui Ueyama <ruiu at google.com> wrote:
>
> I personally haven't tried gdb_index, and I don't know the quality of the
> produced index. Most of the code was written by George.
>
> One thing I noticed about the feature (and filed as
> http://bugs.llvm.org/show_bug.cgi?id=32228) is that our gdb_index feature
> is much slower than the gold. Apparently there's room for improvement.
>
> On Thu, Mar 16, 2017 at 8:35 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
> Any idea of the quality/completeness of the gdb_index support? I've
> started using gdb_index recently (reduces GDB startup time for Clang from
> 50 seconds to 3 seconds - though I haven't measure how much it increases
> link time*) & it's really handy. Would love to have some assurance it's
> expected to work for LLD.
>
> Simple experiments seem to indicate that it's OK/good. Though a quick
> llvm-dwarfdump of gdb_index between the two linkers shows about a 10%
> disparity in size (LLD's is smaller) given the same inputs. (219MB v 196MB
> of text dumped - I didn't compare the actual section sizes)
>
> *though honestly I'd be willing to trade startup time for link time 1:1
> even, because builds/links are already long enough that I go off and do
> something else - whereas getting GDB start down to 3 seconds, I can start
> writing my breakpoint command, etc, in those 3 seconds and be all but
> unaffected by it - at 50 seconds I basically have to go off and do
> something else/context switch/etc.
>
> On Tue, Mar 14, 2017 at 11:40 AM Rui Ueyama via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi all,
>
> LLVM 4.0.0 is out, and I can say that LLD/ELF is now ready for production
> use at least for x86-64 (and probably for AArch64 and MIPS). I believe
> you've heard a few good news about the linker -- it just works
> <http://lld.llvm.org/#features> and is very fast
> <http://lld.llvm.org/#performance>, clean, compact and supported by the
> active community. I don't think I need to reiterate why having a good
> linker is important for us as the LLVM community.
>
> There's one thing you can help us without writing even a line of code.
> Please use it!
>
> Using LLD to link LLVM/clang/etc is easy. You need to check out the LLD
> repository just like you probably already did for clang, build it, and then
> install it. LLD will be installed as ld.lld (and it won't be used by
> default.) After that, re-run cmake with -DLLVM_ENABLE_LLD=On along with
> your usual options so that LLD will be used to build LLVM.
>
> For the details of the build process, please see
> http://lld.llvm.org/#build.
>
> Thanks,
> Rui
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>
>
>
>
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