https://bugs.llvm.org/show_bug.cgi?id=47927
Bug ID: 47927
Summary: Shift UB in emitLegacyZExt when "from" type is >=
64-bit
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: DebugInfo
Assignee: unassignedb...@nondot.org
Reporter: v...@apple.com
CC: jdevliegh...@apple.com, keith.wal...@arm.com,
llvm-bugs@lists.llvm.org,
paul_robin...@playstation.sony.com
Clang trips an "out of bounds shift" UBSan diagnostic when compiling
__fixunsxfti from compiler-rt.
A reduced test:
```
; RUN: llc -filetype=obj -o - %s | llvm-dwarfdump -
define void @t(i64 %x) !dbg !6 {
call void @llvm.dbg.value(metadata i64 %x, metadata !9,
metadata !DIExpression(DW_OP_LLVM_convert, 64,
DW_ATE_unsigned,
DW_OP_LLVM_convert, 128,
DW_ATE_unsigned)), !dbg !11
ret void, !dbg !12
}
declare void @llvm.dbg.value(metadata, metadata, metadata)
!llvm.dbg.cu = !{!0}
!llvm.debugify = !{!3, !4}
!llvm.module.flags = !{!5}
!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer:
"debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug,
enums: !2)
!1 = !DIFile(filename: "legacy-zext.ll", directory: "/")
!2 = !{}
!3 = !{i64 2}
!4 = !{i64 1}
!5 = !{i64 2, !"Debug Info Version", i32 3}
!6 = distinct !DISubprogram(name: "t", linkageName: "t", scope: null, file: !1,
line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition |
DISPFlagOptimized, unit: !0, retainedNodes: !8)
!7 = !DISubroutineType(types: !2)
!8 = !{!9}
!9 = !DILocalVariable(name: "1", scope: !6, file: !1, line: 1, type: !10)
!10 = !DIBasicType(name: "ty64", size: 32, encoding: DW_ATE_unsigned)
!11 = !DILocation(line: 1, column: 1, scope: !6)
!12 = !DILocation(line: 2, column: 1, scope: !6)
```
Here's the code that trips the UBSan diagnostic:
```
void DwarfExpression::emitLegacyZExt(unsigned FromBits) {
// (X & (1 << FromBits - 1))
emitOp(dwarf::DW_OP_constu);
emitUnsigned((1ULL << FromBits) - 1); //<<< HERE (FromBits >= 64)
emitOp(dwarf::DW_OP_and);
}
```
This legacy zext operation is a fallback DW_OP_convert implementation when
targeting Dwarf 4.
One way to address the diagnostic might be to plumb APInt through
DwarfExpression, MCStreamer, MCAsmStreamer, ByteStreamer, DIEInteger and so on.
I'm not sure whether that's a direction others in the community want to go. The
complexity of the change seems somewhat out of proportion to the problem it's
trying to fix.
Is there some other way to implement the legacy zext? Or should we try not to
emit it if we can't emit the mask?
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