[DebugInfo@O2] CodeGenPrepare address-mode sinking limits dbg.value validity

[jmorse]

Bugzilla Link	39940
Resolution	FIXED
Resolved on	Dec 11, 2019 03:05
Version	trunk
OS	Linux
Blocks	#38116 #38102
CC	@dwblaikie,@gregbedwell,@CarlosAlbertoEnciso,@pogo59
Fixed by commit(s)	c93a9b1

Extended Description

In the code below, CodeGenPrepare duplicates the GEP calculating the %bees pointer and sinks the copy into the "next:" block. This is fine, and is designed to allow the address calculations to be folded into a machine addressing mode. The dbg.value for %bees is left in a BB where there are no other uses of %bees, and normally CodeGenPrepare's placeDbgValues would move the dbg.value into the earlier BB to avoid limitations in SelectionDAG.

However, as explored in bug 38754, this isn't a good solution, and causes the appearance of variables to the developer to be re-ordered in the debugger. When placeDbgValues is turned off, the dbg.value stays in the later BB. Then in SelectionDAG, because %bees isn't used outside of the first BB, no VReg is allocated to it, and there's no way to turn the dbg.value into a legal DBG_VALUE. The dbg.value is subsequently dropped.

I've been using the latest trunk (r348754) and have been running "llc -stop-after=codegenprepare". The fix seems obvious (update debug users in CodeGenPrepare::optimizeMemoryInst) however it's not clear whether a fix can be tested if it depends on placeDbgValues being disabled.

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

define i32 @​lala(i32 %ptr, i1 %arg) {
%bees = getelementptr i32, i32 %ptr, i32 4
%loaded = load i32, i32 *%bees
br i1 %arg, label %next, label %face

next:
call void @​llvm.dbg.value(metadata i32 *%bees, metadata !​1, metadata !DIExpression()), !dbg !​6
store i32 1, i32 *%bees
ret i32 1

face:
ret i32 0
}

!llvm.module.flags = !{#4}
!llvm.dbg.cu = !{#2}
!​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<--------

[jmorse]

assigned to @jmorse

[jmorse]

Candidate patch: https://reviews.llvm.org/D58403

[jmorse]

Fixed in rGc93a9b15ce88