Fix UB in DIExpression::appendOffset()

[aprantl]

Summary

The absolute value of 0x8000000000000000 does not fit into an int64_t and UBSan tells us that by crashing.

rdar://98799670

[dblaikie]

SGTM

[JDevlieghere]

llvm/test/DebugInfo/salvage-overflow.ll

4

[djtodoro]

llvm/lib/IR/DebugInfoMetadata.cpp

1388

was else dropped intentionally?

[Orlando]

llvm/lib/IR/DebugInfoMetadata.cpp

1388

nit: the else appears to have gone missing

[aprantl]

llvm/lib/IR/DebugInfoMetadata.cpp

1388

whoops, thanks!

[dblaikie]

llvm/lib/IR/DebugInfoMetadata.cpp

1388

ooh, that might be worth implementing a warning for that at some point, like misleading indentation, etc.

llvm/lib/IR/DebugInfoMetadata.cpp

DIExpression::getFragmentInfo(expr_op_iterator Start, expr_op_iterator End) {
   return None;
 }
 
 void DIExpression::appendOffset(SmallVectorImpl<uint64_t> &Ops,
                                 int64_t Offset) {
   if (Offset > 0) {
     Ops.push_back(dwarf::DW_OP_plus_uconst);
     Ops.push_back(Offset);
   } else if (Offset < 0) {
     Ops.push_back(dwarf::DW_OP_constu);
-    Ops.push_back(-Offset);
+    // Avoid UB when encountering LLONG_MIN, because in 2's complement
+    // abs(LLONG_MIN) is LLONG_MAX+1.
+    uint64_t AbsMinusOne = -(Offset+1);
+    Ops.push_back(AbsMinusOne + 1);
     Ops.push_back(dwarf::DW_OP_minus);
   }
 }
 
 bool DIExpression::extractIfOffset(int64_t &Offset) const {
   if (getNumElements() == 0) {
     Offset = 0;
     return true;

llvm/test/DebugInfo/salvage-overflow.ll

+; RUN: opt %s -sroa -early-cse -S | FileCheck %s
+; CHECK: DIExpression(DW_OP_constu, 9223372036854775808, DW_OP_minus, DW_OP_stack_value)
+; Created from the following C input (and then delta-reduced the IR):
+;
+; extern unsigned long long use(unsigned long long);
+; void f(unsigned long long x) {
+;   for (; x > 0; x --) {
+;     unsigned long long y = x + 0x8000000000000000;
+;     use(x);
+;   }
+; }
+
+define void @f(i64 noundef %x) #0 !dbg !9 {
+entry:
+  %x.addr = alloca i64, align 8
+  %y = alloca i64, align 8
+  br label %for.cond
+for.cond:                                         ; preds = %for.inc, %entry
+  %0 = load i64, i64* %x.addr, align 8
+  %1 = bitcast i64* %y to i8*
+  call void @llvm.dbg.declare(metadata i64* %y, metadata !15, metadata !DIExpression())
+, !dbg !29
+  %2 = load i64, i64* %x.addr, align 8
+  %add = add i64 %2, -9223372036854775808
+  store i64 %add, i64* %y, align 8
+  br label %for.cond
+}
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+
+!llvm.module.flags = !{!3,!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None, sysroot: "/")
+!1 = !DIFile(filename: "t.c", directory: "/")
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!7 = !{i32 7, !"frame-pointer", i32 2}
+!9 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 2, type: !10, scopeLine: 2, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !13)
+!10 = !DISubroutineType(types: !11)
+!11 = !{null, !12}
+!12 = !DIBasicType(name: "unsigned long long", size: 64, encoding: DW_ATE_unsigned)
+!13 = !{}
+!15 = !DILocalVariable(name: "y", scope: !16, file: !1, line: 4, type: !12)
+!16 = distinct !DILexicalBlock(scope: !17, file: !1, line: 3, column: 23)
+!17 = distinct !DILexicalBlock(scope: !18, file: !1, line: 3, column: 3)
+!18 = distinct !DILexicalBlock(scope: !9, file: !1, line: 3, column: 3)
+!29 = !DILocation(line: 4, column: 24, scope: !16)