Fix an offset underflow bug in DwarfExpression when describing small values with subregisters

DwarfExpression::addMachineReg() knows how to build a larger register
that isn't expressible in DWARF by combining multiple
subregisters. However, if the entire value fits into just one
subregister, it would still emit the other subregisters, leading to
all sorts of inconsistencies down the line.

This patch fixes that by moving an already existing(!) check whether
the subregister's offset is before the end of the value to the right
place.

rdar://problem/57294211

Differential Revision: https://reviews.llvm.org/D70508

llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp

@@ -155,20 +155,18 @@ bool DwarfExpression::addMachineReg(const TargetRegisterInfo &TRI,
     CurSubReg.set(Offset, Offset + Size);
 
     // If this sub-register has a DWARF number and we haven't covered
-    // its range, emit a DWARF piece for it.
-    if (CurSubReg.test(Coverage)) {
+    // its range, and its range covers the value, emit a DWARF piece for it.
+    if (Offset < MaxSize && CurSubReg.test(Coverage)) {
       // Emit a piece for any gap in the coverage.
       if (Offset > CurPos)
-        DwarfRegs.push_back({-1, Offset - CurPos, "no DWARF register encoding"});
+        DwarfRegs.push_back(
+            {-1, Offset - CurPos, "no DWARF register encoding"});
       DwarfRegs.push_back(
           {Reg, std::min<unsigned>(Size, MaxSize - Offset), "sub-register"});
-      if (Offset >= MaxSize)
-        break;
-
-      // Mark it as emitted.
-      Coverage.set(Offset, Offset + Size);
-      CurPos = Offset + Size;
     }
+    // Mark it as emitted.
+    Coverage.set(Offset, Offset + Size);
+    CurPos = Offset + Size;
   }
   // Failed to find any DWARF encoding.
   if (CurPos == 0)
@@ -391,6 +389,7 @@ void DwarfExpression::addExpression(DIExpressionCursor &&ExprCursor,
       // empty DW_OP_piece / DW_OP_bit_piece before we emitted the base
       // location.
       assert(OffsetInBits >= FragmentOffset && "fragment offset not added?");
+      assert(SizeInBits >= OffsetInBits - FragmentOffset && "size underflow");
 
       // If addMachineReg already emitted DW_OP_piece operations to represent
       // a super-register by splicing together sub-registers, subtract the size

llvm/test/DebugInfo/MIR/ARM/larger-subregister.mir

@@ -0,0 +1,36 @@
+# RUN: llc -start-before=livedebugvalues -filetype=obj -o - %s | \
+# RUN:     llvm-dwarfdump -debug-info - | FileCheck %s
+#
+# In this test the first DBG_VALUE is 32 bits large and sits in 128-bit register Q8,
+# which has to be decomposed into two 64-bit register D16, D17.
+# This test ensures that the unused D17 is suppressed in the output.
+#
+# CHECK: .debug_info contents:
+# CHECK: DW_TAG_formal_parameter
+# CHECK-NEXT: DW_AT_location
+# CHECK-NEXT: DW_OP_regx D16, DW_OP_piece 0x4, DW_OP_regx D15, DW_OP_bit_piece 0x20 0x20
+# CHECK-NEXT: DW_AT_name {{.*}}"boundingRect"
+--- |
+  target triple = "thumbv7k-apple-watchos"
+  define swiftcc void @f() !dbg !5 {
+       ret void
+  }
+  !llvm.module.flags = !{!0}
+  !llvm.dbg.cu = !{!2}
+  !0 = !{i32 2, !"Debug Info Version", i32 3}
+  !2 = distinct !DICompileUnit(language: DW_LANG_Swift, file: !3,
+  isOptimized: true, runtimeVersion: 5, emissionKind: FullDebug, enums: !4, globals: !4, imports: !4)
+  !3 = !DIFile(filename: "t.swift", directory: "/")
+  !4 = !{}
+  !5 = distinct !DISubprogram(name: "f", scope: !3, file: !3, line: 388, type: !9, scopeLine: 388, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !4)
+  !9 = !DISubroutineType(types: !4)
+  !10 = !DILocation(line: 0, scope: !5)
+  !12 = !DILocalVariable(name: "boundingRect", arg: 1, scope: !5, file: !3, line: 388, type: !13)
+  !13 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !14)
+  !14 = !DICompositeType(tag: DW_TAG_structure_type, name: "CGRect", scope: !5, file: !3, line: 46, size: 128, elements: !4, runtimeLang: DW_LANG_Swift, identifier: "$sSo6CGRectVD")
+name:            'f'
+body:             |
+  bb.0:
+    DBG_VALUE $s31, $noreg, !12, !DIExpression(DW_OP_LLVM_fragment, 32, 32), debug-location !10
+    DBG_VALUE $q8, $noreg, !12, !DIExpression(DW_OP_LLVM_fragment, 0, 32), debug-location !10
+    renamable $r0 = t2ADDri $r6, 144, 14, $noreg, $noreg, debug-location !10

llvm/test/MC/X86/dwarf-size-field-overflow.test

@@ -1,7 +1,7 @@
 # This test generates too many debug location entries to fit into 65KB required
 # by DWARF < 5. Check that the location is set to 0 instead of crashing.
 #
-# RUN: %python %s 4000 | llc -mtriple=x86_64-apple-darwin -filetype=obj -o %t
+# RUN: %python %s 10000 | llc -mtriple=x86_64-apple-darwin -filetype=obj -o %t
 # RUN: llvm-dwarfdump %t | FileCheck %s
 #
 # CHECK:       0x0000004d:       DW_TAG_formal_parameter
@@ -39,11 +39,11 @@ attributes #0 = {{ nounwind readnone speculatable }}
 !34 = !DILocation(line: 12, column: 8, scope: !24)
 """
 
-CALL = "call void @llvm.dbg.value(metadata i64 {0}, metadata !30, metadata !DIExpression(DW_OP_LLVM_fragment, {0}, 64)), !dbg !34"
+CALL = "call void @llvm.dbg.value(metadata i64 {0}, metadata !30, metadata !DIExpression(DW_OP_LLVM_fragment, {1}, 8)), !dbg !34"
 
 if __name__ == '__main__':
     N = int(sys.argv[1])
     calls = []
     for i in range(0, N):
-        calls.append(CALL.format(i * 10**12))
+        calls.append(CALL.format(i * 10**12, i*8))
     print(SKELETON.format('\n'.join(calls)))