commit b9d977b0ca60c54f11615ca9d144c9f08b29fd85
Author: Jeremy Morse <jeremy.morse@sony.com>
Date:   Wed Jul 8 10:01:14 2020 +0100

    [DWARF] Add cuttoff guarding quadratic validThroughout behaviour
    
    Occasionally we see absolutely massive basic blocks, typically in global
    constructors that are vulnerable to heavy inlining. When these blocks are
    dense with DBG_VALUE instructions, we can hit near quadratic complexity in
    DwarfDebug's validThroughout function. The problem is caused by:
    
      * validThroughout having to step through all instructions in the block to
        examine their lexical scope,
      * and a high proportion of instructions in that block being DBG_VALUEs
        for a unique variable fragment,
    
    Leading to us stepping through every instruction in the block, for (nearly)
    each instruction in the block.
    
    By adding this guard, we force variables in large blocks to use a location
    list rather than a single-location expression, as shown in the added test.
    This shouldn't change the meaning of the output DWARF at all: instead we
    use a less efficient DWARF encoding to avoid a poor-performance code path.
    
    Differential Revision: https://reviews.llvm.org/D83236

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 80935f8a454..45ed5256deb 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -167,6 +167,11 @@ static cl::opt<LinkageNameOption>
                                             "Abstract subprograms")),
                       cl::init(DefaultLinkageNames));
 
+static cl::opt<unsigned> LocationAnalysisSizeLimit(
+    "singlevarlocation-input-bb-limit",
+    cl::desc("Maximum block size to analyze for single-location variables"),
+    cl::init(30000), cl::Hidden);
+
 static const char *const DWARFGroupName = "dwarf";
 static const char *const DWARFGroupDescription = "DWARF Emission";
 static const char *const DbgTimerName = "writer";
@@ -1637,8 +1642,10 @@ static bool validThroughout(LexicalScopes &LScopes,
 // [1-3)    [(reg0, fragment 0, 32), (reg1, fragment 32, 32)]
 // [3-4)    [(reg1, fragment 32, 32), (123, fragment 64, 32)]
 // [4-)     [(@g, fragment 0, 96)]
-bool DwarfDebug::buildLocationList(SmallVectorImpl<DebugLocEntry> &DebugLoc,
-                                   const DbgValueHistoryMap::Entries &Entries) {
+bool DwarfDebug::buildLocationList(
+    SmallVectorImpl<DebugLocEntry> &DebugLoc,
+    const DbgValueHistoryMap::Entries &Entries,
+    DenseSet<const MachineBasicBlock *> &VeryLargeBlocks) {
   using OpenRange =
       std::pair<DbgValueHistoryMap::EntryIndex, DbgValueLoc>;
   SmallVector<OpenRange, 4> OpenRanges;
@@ -1734,8 +1741,14 @@ bool DwarfDebug::buildLocationList(SmallVectorImpl<DebugLocEntry> &DebugLoc,
       DebugLoc.pop_back();
   }
 
-  return DebugLoc.size() == 1 && isSafeForSingleLocation &&
-         validThroughout(LScopes, StartDebugMI, EndMI);
+  // If there's a single entry, safe for a single location, and not part of
+  // an over-sized basic block, then ask validThroughout whether this
+  // location can be represented as a single variable location.
+  if (DebugLoc.size() != 1 || !isSafeForSingleLocation)
+    return false;
+  if (VeryLargeBlocks.count(StartDebugMI->getParent()))
+    return false;
+  return validThroughout(LScopes, StartDebugMI, EndMI);
 }
 
 DbgEntity *DwarfDebug::createConcreteEntity(DwarfCompileUnit &TheCU,
@@ -1767,6 +1780,13 @@ void DwarfDebug::collectEntityInfo(DwarfCompileUnit &TheCU,
   // Grab the variable info that was squirreled away in the MMI side-table.
   collectVariableInfoFromMFTable(TheCU, Processed);
 
+  // Identify blocks that are unreasonably sized, so that we can later
+  // skip lexical scope analysis over them.
+  DenseSet<const MachineBasicBlock *> VeryLargeBlocks;
+  for (const auto &MBB : *CurFn)
+    if (MBB.size() > LocationAnalysisSizeLimit)
+      VeryLargeBlocks.insert(&MBB);
+
   for (const auto &I : DbgValues) {
     InlinedEntity IV = I.first;
     if (Processed.count(IV))
@@ -1803,7 +1823,8 @@ void DwarfDebug::collectEntityInfo(DwarfCompileUnit &TheCU,
     if (HistSize == 1 || SingleValueWithClobber) {
       const auto *End =
           SingleValueWithClobber ? HistoryMapEntries[1].getInstr() : nullptr;
-      if (validThroughout(LScopes, MInsn, End)) {
+      if (VeryLargeBlocks.count(MInsn->getParent()) == 0 &&
+          validThroughout(LScopes, MInsn, End)) {
         RegVar->initializeDbgValue(MInsn);
         continue;
       }
@@ -1818,7 +1839,8 @@ void DwarfDebug::collectEntityInfo(DwarfCompileUnit &TheCU,
 
     // Build the location list for this variable.
     SmallVector<DebugLocEntry, 8> Entries;
-    bool isValidSingleLocation = buildLocationList(Entries, HistoryMapEntries);
+    bool isValidSingleLocation =
+        buildLocationList(Entries, HistoryMapEntries, VeryLargeBlocks);
 
     // Check whether buildLocationList managed to merge all locations to one
     // that is valid throughout the variable's scope. If so, produce single
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
index bb0f550b965..d7a4b2abf52 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
@@ -592,8 +592,10 @@ class DwarfDebug : public DebugHandlerBase {
   /// function that describe the same variable. If the resulting 
   /// list has only one entry that is valid for entire variable's
   /// scope return true.
-  bool buildLocationList(SmallVectorImpl<DebugLocEntry> &DebugLoc,
-                         const DbgValueHistoryMap::Entries &Entries);
+  bool buildLocationList(
+      SmallVectorImpl<DebugLocEntry> &DebugLoc,
+      const DbgValueHistoryMap::Entries &Entries,
+      DenseSet<const MachineBasicBlock *> &VeryLargeBlocks);
 
   /// Collect variable information from the side table maintained by MF.
   void collectVariableInfoFromMFTable(DwarfCompileUnit &TheCU,
diff --git a/llvm/test/DebugInfo/MIR/X86/singlelocation-cutoffs.mir b/llvm/test/DebugInfo/MIR/X86/singlelocation-cutoffs.mir
new file mode 100644
index 00000000000..6ad64d9d74b
--- /dev/null
+++ b/llvm/test/DebugInfo/MIR/X86/singlelocation-cutoffs.mir
@@ -0,0 +1,65 @@
+# Test cutoffs for single-location variable analysis.
+# Disable validThroughout if the input size exceeds the specified limit
+
+# RUN: llc %s -o - -start-after=livedebugvalues -mtriple=x86_64-unknown-unknown \
+# RUN:   --singlevarlocation-input-bb-limit=0 -filetype=obj\
+# RUN:   | llvm-dwarfdump -v -\
+# RUN:   | FileCheck %s -check-prefix=LIMITED
+
+# RUN: llc %s -o - -start-after=livedebugvalues -mtriple=x86_64-unknown-unknown \
+# RUN: --singlevarlocation-input-bb-limit=20 -filetype=obj | llvm-dwarfdump -v -\
+# RUN:   | FileCheck %s -check-prefix=UNLIMITED
+
+# LIMITED: DW_AT_location [DW_FORM_sec_offset]
+
+# UNLIMITED: DW_AT_location [DW_FORM_exprloc]
+
+--- |
+  target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+  declare i32 @use(i32)
+
+  define i32 @foo(i32 %x) !dbg !6 {
+  entry:
+    ret i32 1, !dbg !15
+  }
+
+  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: "/tmp/t.ll", directory: "/")
+  !2 = !{}
+  !3 = !{i32 4}
+  !4 = !{i32 2}
+  !5 = !{i32 2, !"Debug Info Version", i32 3}
+  !6 = distinct !DISubprogram(name: "foo", linkageName: "foo", scope: null, file: !1, line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !8)
+  !7 = !DISubroutineType(types: !2)
+  !8 = !{!9, !11}
+  !9 = !DILocalVariable(name: "1", scope: !6, file: !1, line: 1, type: !10)
+  !10 = !DIBasicType(name: "ty32", size: 32, encoding: DW_ATE_unsigned)
+  !11 = !DILocalVariable(name: "2", scope: !6, file: !1, line: 3, type: !10)
+  !12 = !DILocation(line: 1, column: 1, scope: !6)
+  !13 = !DILocation(line: 2, column: 1, scope: !6)
+  !14 = !DILocation(line: 3, column: 1, scope: !6)
+  !15 = !DILocation(line: 4, column: 1, scope: !6)
+
+...
+---
+name:            foo
+liveins:
+  - { reg: '$edi', virtual-reg: '' }
+stack:
+  - { id: 0, name: '', type: spill-slot, offset: -12, size: 4, alignment: 4,
+      stack-id: default, callee-saved-register: '', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+body:             |
+  bb.0.entry:
+    liveins: $edi
+    DBG_VALUE renamable $edi, $noreg, !11, !DIExpression(), debug-location !14
+    RETQ debug-location !14
+
+...