[DebugInfo@O2] LiveDebugVariables can drop DBG_VALUEs through misinterpreting fragments

[jmorse]

Bugzilla Link	41992
Resolution	FIXED
Resolved on	Feb 13, 2020 07:33
Version	trunk
OS	Linux
Blocks	#38116
CC	@adrian-prantl,@dstenb,@gregbedwell,@jdm,@OCHyams,@pogo59,@Melamoto
Fixed by commit(s)	1e40799

Extended Description

With LLVM/clang 361045, it seems LiveDebugVariables can drop DBG_VALUE instructions in the presence of complex expressions, as demonstrated in the code below, when fed into "llc -stop-after=virtregrewriter -o -". The code is adapted from the dbg-addr-dse.ll test, and after virtregrewriter the third DBG_VALUE instruction is not re-inserted, which makes an assignment disappear.

The cause seems to be this check [0] in LiveDebugVariables: it intends to ensure that fragments are recorded as different "UserValue"s. However the fact it's only comparing Expression pointers means that any complex expression at all will be considered a different fragment of the variable. Thus the DW_OP_deref's in the code below effectively make the dbg.value's refer to different fragments, according to LiveDebugVariables.

In the example code, after calling collectDebugValues, LiveDebugVariables::print represents the program state thus:

!"x,3" [32r;32d):0 [128r;128d):0 Loc0=%stack.0
!"x,3" [112r;112d):0 Loc0=1

Here the first and last DBG_VALUE insts are recorded in the first line as being in location 0 (%stack.0) at slot indexes 32 and 128. The second line represents the middle DBG_VALUE, as a different fragment, having location const-1 at slot index 112. (The DIExpressions aren't printed).

After calling computeIntervals the LiveDebugVariables state becomes:

!"x,3" [48B;240B):0 Loc0=%stack.0
!"x,3" [112r;240B):0 Loc0=1

The first line coalesces its locations to the end of the block (240B) because that "fragment" always appears to be in %stack.0. Then when virtregrewriter re-inserts DBG_VALUEs, it doesn't see any reason to replace what was the third DBG_VALUE in the original code: as far as it sees it, there are two different fragments that get their locations defined once each, and that's it. Thus, the third DBG_VALUE goes missing.

I anticipate LiveDebugVariables could be convinced to insert fresh DBG_VALUEs where it shouldn't in a similar way. UserValue::extendDef can be convinced to extend the lifetime of a fragment to a copy of a register location, if the original register is killed. That would then lead to it inserting a fresh DBG_VALUE, possibly terminating an earlier DBG_VALUE for the same (but unrecognised) fragment.

(I was going to just submit a patch for this, but I think some non-trivial datastructure changes are required, and it's not something I'm immediately working on).

[0]

llvm-project/llvm/lib/CodeGen/LiveDebugVariables.cpp

Line 207 in e85bbf5

// FIXME: The fragment should be part of the equivalence class, but not

--------8<--------
target triple = "x86_64-unknown-linux"

declare void @​llvm.dbg.addr(metadata, metadata, metadata)
declare void @​llvm.dbg.value(metadata, metadata, metadata)
declare void @​escape(i32*)

@​global = external global i32, align 4

define void @​f(i32 %x) !dbg !​8 {
entry:
%x.addr = alloca i32, align 4
store i32 %x, i32* %x.addr, align 4
call void @​llvm.dbg.value(metadata i32* %x.addr, metadata !​13, metadata !DIExpression(DW_OP_deref)), !dbg !​18
call void @​escape(i32* %x.addr), !dbg !​18
call void @​llvm.dbg.value(metadata i32 1, metadata !​13, metadata !DIExpression()), !dbg !​18
store i32 1, i32* @​global, align 4, !dbg !​18
call void @​llvm.dbg.value(metadata i32* %x.addr, metadata !​13, metadata !DIExpression(DW_OP_deref)), !dbg !​18
store i32 2, i32* %x.addr, align 4, !dbg !​18
call void @​escape(i32* %x.addr), !dbg !​18
ret void, !dbg !​18
}

!llvm.dbg.cu = !{#0}
!llvm.module.flags = !{#3, !​4}
!llvm.ident = !{#7}

!​0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !​1, producer: "clang version 6.0.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !​2)
!​1 = !DIFile(filename: "dse.c", directory: "C:\5Csrc\5Cllvm-project\5Cbuild")
!​2 = !{}
!​3 = !{i32 2, !"Dwarf Version", i32 4}
!​4 = !{i32 2, !"Debug Info Version", i32 3}
!​7 = !{!"clang version 6.0.0 "}
!​8 = distinct !DISubprogram(name: "f", scope: !​1, file: !​1, line: 3, type: !​9, isLocal: false, isDefinition: true, scopeLine: 3, flags: DIFlagPrototyped, isOptimized: true, unit: !​0, retainedNodes: !​12)
!​9 = !DISubroutineType(types: !​10)
!​10 = !{null, !​11}
!​11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!​12 = !{#13}
!​13 = !DILocalVariable(name: "x", arg: 1, scope: !​8, file: !​1, line: 3, type: !​11)
!​18 = !DILocation(line: 3, column: 12, scope: !​8)
-------->8--------

[jmorse]

assigned to @OCHyams

[OCHyams]

WIP patch here https://reviews.llvm.org/D70121

[OCHyams]

Adding this to the DebugInfo@O2 umbrella bug 38768.

[OCHyams]

Fixed with D74053 (1e40799).