[DebugInfo] Limit the size of DIExpressions that we will salvage up to

Fixes: https://bugs.llvm.org/show_bug.cgi?id=51841

This patch places an arbitrary limit on the size of DIExpressions that
we will produce via salvaging, for performance reasons. This helps to
fix a performance issue observed in the bug above, in which debug values
would be salvaged hundreds of times, producing expressions with over
1000 elements and causing the compiler to hang. Limiting the size of
debug values that we will produce to 128 largely fixes this issue.

Reviewed By: dblaikie, jmorse

Differential Revision: https://reviews.llvm.org/D110332

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 7533f4f..122977c 100644 (file)
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1733,9 +1733,11 @@ void llvm::salvageDebugInfo(Instruction &I) {
 
 void llvm::salvageDebugInfoForDbgValues(
     Instruction &I, ArrayRef<DbgVariableIntrinsic *> DbgUsers) {
-  // This is an arbitrary chosen limit on the maximum number of values we can
-  // salvage up to in a DIArgList, used for performance reasons.
+  // These are arbitrary chosen limits on the maximum number of values and the
+  // maximum size of a debug expression we can salvage up to, used for
+  // performance reasons.
   const unsigned MaxDebugArgs = 16;
+  const unsigned MaxExpressionSize = 128;
   bool Salvaged = false;
 
   for (auto *DII : DbgUsers) {
@@ -1772,9 +1774,10 @@ void llvm::salvageDebugInfoForDbgValues(
       break;
 
     DII->replaceVariableLocationOp(&I, Op0);
-    if (AdditionalValues.empty()) {
+    bool IsValidSalvageExpr = SalvagedExpr->getNumElements() <= MaxExpressionSize;
+    if (AdditionalValues.empty() && IsValidSalvageExpr) {
       DII->setExpression(SalvagedExpr);
-    } else if (isa<DbgValueInst>(DII) &&
+    } else if (isa<DbgValueInst>(DII) && IsValidSalvageExpr &&
                DII->getNumVariableLocationOps() + AdditionalValues.size() <=
                    MaxDebugArgs) {
       DII->addVariableLocationOps(AdditionalValues, SalvagedExpr);

diff --git a/llvm/test/DebugInfo/salvage-limit-expr-size.ll b/llvm/test/DebugInfo/salvage-limit-expr-size.ll
new file mode 100644 (file)
index 0000000..51cd06f
--- /dev/null
+++ b/llvm/test/DebugInfo/salvage-limit-expr-size.ll
@@ -0,0 +1,47 @@
+; RUN: opt %s -dce -S | FileCheck %s
+
+;; Tests that a DIExpression will only be salvaged up to a certain length, and
+;; will produce an undef value if an expression would need to exceed that length.
+
+define i32 @"?multiply@@YAHHH@Z"(i32 %a, i32 %b) !dbg !8 {
+entry:
+  %add.1 = add nsw i32 %a, 5, !dbg !14
+  %add.2 = add nsw i32 %a, %b, !dbg !14
+  ;; These expressions should salvage successfully, up to exactly 128 elements.
+  ; CHECK: call void @llvm.dbg.value(metadata i32 %a, metadata ![[VAR_C:[0-9]+]]
+  ; CHECK-NEXT: call void @llvm.dbg.value(metadata !DIArgList(i32 %a, i32 %b), metadata ![[VAR_C]]
+  call void @llvm.dbg.value(metadata i32 %add.1, metadata !12, metadata !DIExpression(DW_OP_constu, 1, DW_OP_plus, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_stack_value)), !dbg !13
+  call void @llvm.dbg.value(metadata i32 %add.2, metadata !12, metadata !DIExpression(DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_stack_value)), !dbg !13
+  ;; These expressions should be set undef, as they would salvage up to exactly 129 elements.
+  ; CHECK-NEXT: call void @llvm.dbg.value(metadata i32 undef, metadata ![[VAR_C]]
+  ; CHECK-NEXT: call void @llvm.dbg.value(metadata i32 undef, metadata ![[VAR_C]]
+  call void @llvm.dbg.value(metadata i32 %add.1, metadata !12, metadata !DIExpression(DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_stack_value)), !dbg !13
+  call void @llvm.dbg.value(metadata i32 %add.2, metadata !12, metadata !DIExpression(DW_OP_constu, 1, DW_OP_plus, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_plus_uconst, 1, DW_OP_stack_value)), !dbg !13
+  %mul = mul nsw i32 %a, %b, !dbg !15
+  ret i32 %mul, !dbg !15
+}
+
+; CHECK: ![[VAR_C]] = !DILocalVariable(name: "c"
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 11.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "/")
+!2 = !{}
+!3 = !{i32 2, !"CodeView", i32 1}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 2}
+!6 = !{i32 7, !"PIC Level", i32 2}
+!7 = !{!"clang version 11.0.0"}
+!8 = distinct !DISubprogram(name: "multiply", linkageName: "?multiply@@YAHHH@Z", scope: !1, file: !1, line: 1, type: !9, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!9 = !DISubroutineType(types: !10)
+!10 = !{!11, !11, !11}
+!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!12 = !DILocalVariable(name: "c", scope: !8, file: !1, line: 2, type: !11)
+!13 = !DILocation(line: 0, scope: !8)
+!14 = !DILocation(line: 2, scope: !8)
+!15 = !DILocation(line: 3, scope: !8)