[Lldb-commits] [PATCH] D80519: [lldb/DWARF] Add support for pre-standard GNU call site attributes

Vedant Kumar via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Mon Jun 1 10:12:24 PDT 2020

vsk accepted this revision.
vsk added a comment.
This revision is now accepted and ready to land.

Thanks, lgtm!

Comment at: lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp:3740
+      if (tail_call)
+        call_inst_pc = low_pc;
+      else
labath wrote:
> vsk wrote:
> > labath wrote:
> > > vsk wrote:
> > > > I think this needs to be `call_inst_pc = low_pc - 1`, see `DwarfCompileUnit::constructCallSiteEntryDIE` for the rationale, and `StackFrameList::SynthesizeTailCallFrames` for where we use this information. The relevant part of the comment from SynthesizeTailCallFrames is:
> > > > 
> > > > "We do not want to subtract 1 from this PC, as it's the actual address of the tail-calling branch instruction. This address is provided by the compiler via DW_AT_call_pc."
> > > > 
> > > > In GNU+Dwarf4 mode, that's no longer true, the DW_AT_low_pc is a fake "return address" for the tail call (really: the address of the instruction after the tail-calling jump).
> > > > 
> > > > On x86_64, this test doesn't seem to stress this case, but the test breaks on Darwin/arm64 without the adjustment.
> > > Heh, you're right. I should've looked at what the code does instead of just trying to reverse engineer the logic from the output. I've now added the -1.
> > > After looking that this code some more, I've come to realize that it's usage of the lack of DW_AT_call_return_pc to indicate a tail call is not correct -- I don't see anything preventing a producer from generating this attribute even for tail calls. I'm going to try refactoring this in another patch to store the tail-call-ness more explicitly. That should also make this part slightly cleaner.
> > The parsing code uses DW_AT_call_tail_call to determine whether or not there's a tail call, which is already explicit. However, the CallEdge representation does take an invalid return_pc address to mean there's a tail call. My $0.02 is that that's legit, and a producer that emits DW_AT_call_return_pc at a tail call site is behaving badly. If we care to support that, we could do it by changing the condition on line 3710 to `attr == DW_AT_call_return_pc && !tail_call`.
> You're right again. I confused a tail call with a (regular) call to a noreturn function.
> The real reason why I wanted to do that change was to avoid the -1 thingy here. If we carried the information about a tail call and the information whether the address points before/after the call instruction explicitly,  I believe that the -1 here wouldn't be necessary, and the code in `SynthesizeTailCallFrames` could just set `behaves_like_frame_zero` depending on what kind of address it gets. I have a feeling that would be cleaner, but I have to try it out...
I think you could do this by adding a flag to the CallEdge class, like:

bool have_one_after_call_inst_pc = ...;
union {
  addr_t call_inst_pc;
  addr_t one_after_call_inst_pc;

and then set `behaves_like_frame_zero = !call_edge.have_one_after_call_inst_pc`. But I'm not convinced this successfully separates the concerns re: parsing DWARF vs. adjusting PC values.

Comment at: lldb/test/API/functionalities/tail_call_frames/disambiguate_paths_to_common_sink/main.cpp:8
+  // FROM-FUNC1-NEXT: func1
+  // FROM-FUNC1-SAME: [artificial]
+  // FROM-FUNC1-NEXT: main
labath wrote:
> vsk wrote:
> > Are these test updates necessary because lldb doesn't print '[opt]' and '[artificial]' next to frame descriptions in a consistent way across platforms? Or is it just that you don't think matching '[opt]' is relevant to the test?
> Right, I wanted to mention that as it's not very obvious, but I forgot...
> The `[opt]` thingy is not printed at all with -ggdb because the attribute we get this information from -- DW_AT_APPLE_optimized -- is only emitted for -glldb. The optimization flag did not seem very relevant for these tests (I mean, technically the compiler could emit call site attributes even in non-optimized mode) so instead of forking the expectations I chose to simply remove it.
Sounds good.

  rG LLVM Github Monorepo



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