[DebugInfo@O2] icmp-eq-chain to switch optimisation drops dbg.values

[jmorse]

Bugzilla Link	39866
Version	6.0
OS	Linux
Blocks	#38102
CC	@dwblaikie,@gregbedwell,@CarlosAlbertoEnciso,@pogo59

Extended Description

The dbg.value in the code below is dropped when run through simplifycfg:

opt-6.0 < test.ll -simplifycfg -S

in any recent version of LLVM. It's relatively easy to understand: chains of equality comparisons are converted into being a switch statement. However, any debug metadata on the path from the first comparison to any of the leafs gets dropped, because the switch statement point at the leafs and nothing is salvaged from the intermediate blocks. Thus, if you put that dbg.value in any block other than 'cont1' below, it survives to output.

The example is contrived (and the DebugInfo cooked), but IMHO we can easily do better by replicating intervening dbg.value's into the leaf blocks.

Context: this is another bug seen when investigating bug 38754. Conditional code like this [0] (and line 9688) gets converted to being a switch, and the dbg.value's of the casted expression usually get shifted to an earlier block by placeDbgValues. When left in the correct place (in the 'if' condition) a condition similar to the example below occur.

-------->8--------
declare void @​llvm.dbg.value(metadata, metadata, metadata)

define i32 @​lala(i32 *%ptr, i32 *%otherptr) {
%val = load i32, i32 *%ptr
%isone = icmp eq i32 %val, 1
br i1 %isone, label %isoneb, label %cont1

isoneb:
store i32 1, i32 *%otherptr
ret i32 1

cont1:
call void @​llvm.dbg.value(metadata i32 %val, metadata !​1, metadata !DIExpression()), !dbg !​6
%istwo = icmp eq i32 %val, 2
br i1 %istwo, label %istwob, label %cont2

istwob:
store i32 2, i32 *%otherptr
ret i32 2

cont2:
ret i32 3
}

!llvm.module.flags = !{#4}
!​1 = !DILocalVariable(name: "bees", scope: !​5, type: null)
!​2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !​3, producer: "beards", isOptimized: true, runtimeVersion: 4, emissionKind: FullDebug)
!​3 = !DIFile(filename: "bees.cpp", directory: "")
!​4 = !{i32 2, !"Debug Info Version", i32 3}
!​5 = distinct !DISubprogram(name: "nope", scope: !​2, file: !​3, line: 1, unit: !​2)
!​6 = !DILocation(line: 0, scope: !​5)
--------8<--------

[0] https://github.com/llvm-mirror/clang/blob/aa528ab4a083268edddd038d0f251afe792cfa36/lib/Sema/SemaOpenMP.cpp#L9694