Index: llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
+++ llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
@@ -223,8 +223,22 @@
         Entries.front().getInstr()->getDebugVariable();
     if (DIVar->isParameter() &&
         getDISubprogram(DIVar->getScope())->describes(&MF->getFunction())) {
+      // FIXME: If the basic block containing the first mention is not in the
+      // same section as the entry basic block, we cannot use the
+      // CurrentFnBegin label and we use the Section begin label instead.
+      // Using the Section Label will result in loss of debug loc fidelity as
+      // we do not capture the basic blocks before the start of this section.
+      auto FindLabelBeforeInsn = [this](const MachineInstr *MI) {
+        const auto *MBB = MI->getParent();
+        if (MBB->sameSection(&Asm->MF->front()))
+          return Asm->getFunctionBegin();
+        else
+          return Asm->MBBSectionRanges[MBB->getSectionIDNum()].BeginLabel;
+      };
       if (!IsDescribedByReg(Entries.front().getInstr()))
-        LabelsBeforeInsn[Entries.front().getInstr()] = Asm->getFunctionBegin();
+        LabelsBeforeInsn[Entries.front().getInstr()] =
+            FindLabelBeforeInsn(Entries.front().getInstr());
+
       if (Entries.front().getInstr()->getDebugExpression()->isFragment()) {
         // Mark all non-overlapping initial fragments.
         for (auto I = Entries.begin(); I != Entries.end(); ++I) {
@@ -244,7 +258,7 @@
           // registers, we must bail out when encountering such a fragment.
           if (IsDescribedByReg(I->getInstr()))
             break;
-          LabelsBeforeInsn[I->getInstr()] = Asm->getFunctionBegin();
+          LabelsBeforeInsn[I->getInstr()] = FindLabelBeforeInsn(I->getInstr());
         }
       }
     }
Index: llvm/test/DebugInfo/X86/basic-block-sections-debug-loc.ll
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/X86/basic-block-sections-debug-loc.ll
@@ -0,0 +1,88 @@
+;; When params loaded on the stack like va_args are used first in a non-entry
+;; basic block, the debug_loc entry is computed as the difference between the
+;; non-entry basic block and function begin address. When these two basic
+;; blocks are in different sections, it is an error.  With basic block
+;; sections, compute the difference from the start of the section.
+; RUN: llc %s --dwarf-version=4 --basic-block-sections=all -filetype=asm -o - | FileCheck %s
+
+; CHECK:      .section .debug_loc
+; CHECK-NEXT: .Ldebug_loc{{[0-9]+}}:
+; CHECK-NEXT: .quad
+; CHECK-NEXT: .quad  _ZL4ncatPcjz.2 # base address
+; CHECK-NEXT: .quad  .Ltmp{{[0-9]+}}-_ZL4ncatPcjz.2
+; CHECK-NEXT: .quad  .Ltmp{{[0-9]+}}-_ZL4ncatPcjz.2
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-grtev4-linux-gnu"
+
+define dso_local void @_ZL4ncatPcjz(i8* %0, i32 %1, ...) unnamed_addr #0 align 32 !dbg !22 {
+.critedge3:
+  call void @llvm.dbg.value(metadata i32 157, metadata !27, metadata !DIExpression()), !dbg !46
+  call void @llvm.va_start(i8* nonnull undef), !dbg !47
+  br label %2
+
+2:                                                ; preds = %2, %.critedge3
+  br label %2
+}
+
+; Function Attrs: nounwind
+declare void @llvm.va_start(i8*) #1
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #2
+
+attributes #0 = { "use-soft-float"="false" }
+attributes #1 = { nounwind }
+attributes #2 = { nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!20, !21}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version google3-trunk (9d2da6759b4d05d834371bcaaa8fc3d9b3385b18)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, retainedTypes: !2)
+!1 = !DIFile(filename: "bug.cpp", directory: "/proc/self/cwd")
+!2 = !{!3, !6, !9, !10, !11, !13, !16, !17, !15}
+!3 = !DIDerivedType(tag: DW_TAG_typedef, name: "int32_t", file: !4, line: 38, baseType: !5)
+!4 = !DIFile(filename: "stdint.h", directory: "/proc/self/cwd")
+!5 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!6 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !7, size: 64)
+!7 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !8)
+!8 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_unsigned_char)
+!9 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !8, size: 64)
+!10 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 64)
+!11 = !DIDerivedType(tag: DW_TAG_typedef, name: "uint32_t", file: !4, line: 51, baseType: !12)
+!12 = !DIBasicType(name: "unsigned int", size: 32, encoding: DW_ATE_unsigned)
+!13 = !DIDerivedType(tag: DW_TAG_typedef, name: "UChar", file: !14, line: 372, baseType: !15)
+!14 = !DIFile(filename: "umachine.h", directory: "/proc/self/cwd")
+!15 = !DIBasicType(name: "char16_t", size: 16, encoding: DW_ATE_UTF)
+!16 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !3, size: 64)
+!17 = !DIDerivedType(tag: DW_TAG_typedef, name: "UBool", file: !14, line: 261, baseType: !18)
+!18 = !DIDerivedType(tag: DW_TAG_typedef, name: "int8_t", file: !4, line: 36, baseType: !19)
+!19 = !DIBasicType(name: "signed char", size: 8, encoding: DW_ATE_signed_char)
+!20 = !{i32 2, !"Debug Info Version", i32 3}
+!21 = !{i32 7, !"PIC Level", i32 2}
+!22 = distinct !DISubprogram(name: "ncat", linkageName: "_ZL4ncatPcjz", scope: !1, file: !1, line: 37, type: !23, scopeLine: 37, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !25)
+!23 = !DISubroutineType(types: !24)
+!24 = !{!3, !9, !11, null}
+!25 = !{!26, !27, !28, !41, !42, !43, !44}
+!26 = !DILocalVariable(name: "buffer", arg: 1, scope: !22, file: !1, line: 37, type: !9)
+!27 = !DILocalVariable(name: "buflen", arg: 2, scope: !22, file: !1, line: 37, type: !11)
+!28 = !DILocalVariable(name: "args", scope: !22, file: !1, line: 38, type: !29)
+!29 = !DIDerivedType(tag: DW_TAG_typedef, name: "va_list", file: !30, line: 14, baseType: !31)
+!30 = !DIFile(filename: "stdarg.h", directory: "/proc/self/cwd")
+!31 = !DIDerivedType(tag: DW_TAG_typedef, name: "__builtin_va_list", file: !1, line: 38, baseType: !32)
+!32 = !DICompositeType(tag: DW_TAG_array_type, baseType: !33, size: 192, elements: !39)
+!33 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "__va_list_tag", file: !1, line: 38, size: 192, flags: DIFlagTypePassByValue, elements: !34, identifier: "_ZTS13__va_list_tag")
+!34 = !{!35, !36, !37, !38}
+!35 = !DIDerivedType(tag: DW_TAG_member, name: "gp_offset", scope: !33, file: !1, line: 38, baseType: !12, size: 32)
+!36 = !DIDerivedType(tag: DW_TAG_member, name: "fp_offset", scope: !33, file: !1, line: 38, baseType: !12, size: 32, offset: 32)
+!37 = !DIDerivedType(tag: DW_TAG_member, name: "overflow_arg_area", scope: !33, file: !1, line: 38, baseType: !10, size: 64, offset: 64)
+!38 = !DIDerivedType(tag: DW_TAG_member, name: "reg_save_area", scope: !33, file: !1, line: 38, baseType: !10, size: 64, offset: 128)
+!39 = !{!40}
+!40 = !DISubrange(count: 1)
+!41 = !DILocalVariable(name: "str", scope: !22, file: !1, line: 39, type: !9)
+!42 = !DILocalVariable(name: "p", scope: !22, file: !1, line: 40, type: !9)
+!43 = !DILocalVariable(name: "e", scope: !22, file: !1, line: 41, type: !6)
+!44 = !DILocalVariable(name: "c", scope: !45, file: !1, line: 49, type: !8)
+!45 = distinct !DILexicalBlock(scope: !22, file: !1, line: 48, column: 45)
+!46 = !DILocation(line: 0, scope: !22)
+!47 = !DILocation(line: 47, column: 3, scope: !22)