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[Debuginfo][Salvaging] (5/7) Support for DW_OP_implicit_pointer for named and unnamed variables (second strategy).
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Authored by alok on Jul 18 2020, 5:09 PM.

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Summary
This patch stems from D84112.
This patch includes improvement in debuginfo salvaging.

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Event Timeline

alok created this revision.Jul 18 2020, 5:09 PM
Herald added a project: Restricted Project. · View Herald TranscriptJul 18 2020, 5:09 PM
jmorse added a subscriber: jmorse.Jul 30 2020, 5:58 AM

Could you elaborate on why module-level information needs to be preserved? As mentioned on llvm-dev, I think you might have missed a place where promotion from stack to SSA/values happens -- IMHO we shouldn't add module-level debuginfo-tracking unless it's absolutely necessary.

llvm/include/llvm/IR/Module.h
205

You'll need to use a value handle (WeakVH perhaps) to store Value references, to avoid storing pointers that become dangling pointers later. The comment should indicate what's being mapped -- I believe it's old alloca to the DILocalVariable, because insertVarAddressMap inserts the second operand of a dbg.declare?

In addition, it looks like salvageDebugInfoForDbgValues never dereferences the mapped value, so could this instead be a set instead of a map?

llvm/lib/IR/Instruction.cpp
81–82

Does this make the debug-info metadata code in the Instruction destructor redundant?

800–803

The rest of Instruction.cpp is very detail-agnostic, and it seems out of place to check the DWARF version here (and presumably meaningless for CodeView). Could this be turned into a more generic compile option, or target option, so that it's less DWARF specific?

(It might be that LLVM doesn't support such a form of option).

812–813

Quite a lot of this function is now dedicated to "if we're an alloca being deleted, that had associated dbg.declares". IMO, it's clearer to put that code into a separate function.

llvm/test/DebugInfo/X86/implicit_pointer_temp_dyn_alloc.cc
2–3

We can't call clang from within llvm as it's not guaranteed to be built. Getting the unoptimised IR with -O0 -gdwarf-5 -emit-llvm -S -Xclang -disable-O0-optnone, and writing a normal lit test for that, would be best.

You might also want to put an end-to-end Dexter test into the debuginfo-tests.

llvm/unittests/IR/DebugInfoTest.cpp
186

This should be a positive test that the dbg.value points at a correct thing, not that it doesn't point at an undef. There are a variety of other Values it could point at by mistake.

alok updated this revision to Diff 304753.Nov 12 2020, 1:53 AM
alok retitled this revision from [Debuginfo] (6/N) Support for DW_OP_implicit_pointer for named and unnamed variables (second strategy). to [Debuginfo][Salvaging] (5/7) Support for DW_OP_implicit_pointer for named and unnamed variables (second strategy)..

Updated to re-base.

djtodoro added inline comments.
llvm/include/llvm/Transforms/Utils/Local.h
312

Can we leave this here in Local ?

llvm/lib/IR/Instruction.cpp
797

nit: Preserve

822

why only for DWARF 5?
There is dw_op_gnu_implicit_pointer for the later version ?

822

Please add a comment describing what we are doing here.

825

Please use early continue here.

djtodoro added inline comments.Nov 12 2020, 2:43 AM
llvm/lib/IR/Instruction.cpp
822

'later version' --> I meant earlier