[globalopt] Don't emit DWARF fragments for members of a struct that cover the whole struct

[DavidSpickett]

Summary

This can happen when the rest of the members of are zero length. Following the same pattern applied to the SROA pass in:
d7f6f1636d53c3e2faf55cdf20fbb44a1a149df1

Fixes: https://bugs.llvm.org/show_bug.cgi?id=45335

[nathanchance]

Thanks! I can confirm that I do not see a crash anymore with the config I used when initially reporting. I will try to run this patch through all of my kernel build tests this afternoon to make sure nothing else has regressed.

I am not an expert in this area by any means so I don't feel confident reviewing or approving the patch.

[nickdesaulniers]

llvm/lib/Transforms/IPO/GlobalOpt.cpp

479

Should this be:
if (VarSize && FragmentSizeInBits < *VarSize)?

If we don't have a size, then !VarSize is true, and we proceed to emit a fragment? Seems in juxtaposition with the comment?

llvm/test/DebugInfo/Generic/global-sra-struct-zero-length.ll

76

Is there maybe some of this debug info that's irrelevant and could be cleaned up to help make the test case more concise?

[aprantl]

llvm/lib/Transforms/IPO/GlobalOpt.cpp

478–482

// If the FragmentSize is smaller than the variable, emit a fragment expression.
// If the variable size is unknown a fragment must be emitted to be safe.

Otherwise this raises more questions.

llvm/test/DebugInfo/Generic/global-sra-struct-zero-length.ll

3

please explain what is being tested in this test.

41

please try to remove all non-essential attributes from the test.

[DavidSpickett]

• Fixed comments as suggested
• Added comment to explain purpose of test case
• Removed unimportant info from the test case

[DavidSpickett]

llvm/lib/Transforms/IPO/GlobalOpt.cpp

479

Updated the comment as aprantl suggested, now matches the code.

llvm/test/DebugInfo/Generic/global-sra-struct-zero-length.ll

3

Added, please check that this is clear enough.

[nickdesaulniers]

ah got it, thanks for updating the comment, it makes more sense to me now.

Minor nits on the comments (sorry to bikeshed those), but LGTM.

llvm/test/DebugInfo/Generic/global-sra-struct-zero-length.ll

8

indentation and missing semicolon

12

indentation

[DavidSpickett]

@nathanchance Post back here or on the Bugzilla issue if you continue to see failures.

llvm/lib/Transforms/IPO/GlobalOpt.cpp

if (isa<StructType>(Init->getType())) {
     return false;
 
   return GlobalUsersSafeToSRA(GV);
 }
 
 /// Copy over the debug info for a variable to its SRA replacements.
 static void transferSRADebugInfo(GlobalVariable *GV, GlobalVariable *NGV,
                                  uint64_t FragmentOffsetInBits,
-                                 uint64_t FragmentSizeInBits,
-                                 unsigned NumElements) {
+                                 uint64_t FragmentSizeInBits) {
   SmallVector<DIGlobalVariableExpression *, 1> GVs;
   GV->getDebugInfo(GVs);
   for (auto *GVE : GVs) {
     DIVariable *Var = GVE->getVariable();
+    Optional<uint64_t> VarSize = Var->getSizeInBits();
+
     DIExpression *Expr = GVE->getExpression();
-    if (NumElements > 1) {
+    // If the FragmentSize is smaller than the variable,
+    // emit a fragment expression.
+    // If the variable size is unknown a fragment must be
+    // emitted to be safe.
+    if (!VarSize || FragmentSizeInBits < *VarSize) {
       if (auto E = DIExpression::createFragmentExpression(
               Expr, FragmentOffsetInBits, FragmentSizeInBits))
         Expr = *E;
       else
         return;
     }
     auto *NGVE = DIGlobalVariableExpression::get(GVE->getContext(), Var, Expr);
     NGV->addDebugInfo(NGVE);
if (StructType *STy = dyn_cast<StructType>(Ty)) {
       Align NewAlign(MinAlign(StartAlignment, FieldOffset));
       if (NewAlign >
           Align(DL.getABITypeAlignment(STy->getElementType(ElementIdx))))
         NGV->setAlignment(NewAlign);
 
       // Copy over the debug info for the variable.
       uint64_t Size = DL.getTypeAllocSizeInBits(NGV->getValueType());
       uint64_t FragmentOffsetInBits = Layout.getElementOffsetInBits(ElementIdx);
-      transferSRADebugInfo(GV, NGV, FragmentOffsetInBits, Size,
-                           STy->getNumElements());
+      transferSRADebugInfo(GV, NGV, FragmentOffsetInBits, Size);
     } else {
       uint64_t EltSize = DL.getTypeAllocSize(ElTy);
       Align EltAlign(DL.getABITypeAlignment(ElTy));
       uint64_t FragmentSizeInBits = DL.getTypeAllocSizeInBits(ElTy);
 
       // Calculate the known alignment of the field.  If the original aggregate
       // had 256 byte alignment for example, something might depend on that:
       // propagate info to each field.
       Align NewAlign(MinAlign(StartAlignment, EltSize * ElementIdx));
       if (NewAlign > EltAlign)
         NGV->setAlignment(NewAlign);
       transferSRADebugInfo(GV, NGV, FragmentSizeInBits * ElementIdx,
-                           FragmentSizeInBits, GetSRASequentialNumElements(Ty));
+                           FragmentSizeInBits);
     }
   }
 
   if (NewGlobals.empty())
     return nullptr;
 
   Module::GlobalListType &Globals = GV->getParent()->getGlobalList();
   for (auto NewGlobalVar : NewGlobals)

llvm/test/DebugInfo/Generic/global-sra-struct-zero-length.ll

+; RUN: opt -S -globalopt < %s | FileCheck %s
+; Generated at -O2 -g from:
+; typedef struct {
+; } a;
+; static struct {
+;     long b;
+;     a c;
+; } d;
+; e() {
+;     long f = d.b + 1;
+;     d.b = f;
+; }
+; (with some simplification by hand)
+
+; Check that the global variable "d" is not
+; emitted as a fragment after the member "c"
+; is removed.
+; d.b is referenced, but d.c has zero length.
+; So a fragment covering d.b would be the same
+; size as d itself.
+
+source_filename = "pr45335.c"
+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-unknown-linux-gnu"
+
+%struct.anon = type { i64, %struct.a }
+%struct.a = type {}
+
+; CHECK: @d.0 = internal unnamed_addr global i64 0, align 8, !dbg ![[GVE:.*]]
+@d = internal global %struct.anon zeroinitializer, align 8, !dbg !0
+
+; Function Attrs: noinline nounwind uwtable
+define dso_local i32 @e() #0 !dbg !18 {
+entry:
+  %0 = load i64, i64* getelementptr inbounds (%struct.anon, %struct.anon* @d, i32 0, i32 0), align 8
+  %add = add nsw i64 %0, 1
+  call void @llvm.dbg.value(metadata i64 %add, metadata !24, metadata !DIExpression()), !dbg !25
+  store i64 %add, i64* getelementptr inbounds (%struct.anon, %struct.anon* @d, i32 0, i32 0), align 8
+  ret i32 undef
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.declare(metadata, metadata, metadata)
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!14, !15}
+
+; CHECK: ![[GVE]] = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "d", scope: !2, file: !3, line: 6, type: !7, isLocal: true, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !{}, globals: !{!0}, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "pr45335.c", directory: "/")
+!7 = distinct !DICompositeType(tag: DW_TAG_structure_type, file: !3, line: 3, size: 64, elements: !8)
+!8 = !{!9, !11}
+!9 = !DIDerivedType(tag: DW_TAG_member, name: "b", scope: !7, file: !3, line: 4, baseType: !10, size: 64)
+!10 = !DIBasicType(name: "long int", size: 64, encoding: DW_ATE_signed)
+!11 = !DIDerivedType(tag: DW_TAG_member, name: "c", scope: !7, file: !3, line: 5, baseType: !12, offset: 64)
+!12 = !DIDerivedType(tag: DW_TAG_typedef, name: "a", file: !3, line: 2, baseType: !13)
+!13 = distinct !DICompositeType(tag: DW_TAG_structure_type, file: !3, line: 1, elements: !{})
+!14 = !{i32 7, !"Dwarf Version", i32 4}
+!15 = !{i32 2, !"Debug Info Version", i32 3}
+!18 = distinct !DISubprogram(name: "e", scope: !3, file: !3, line: 7, type: !19, scopeLine: 7, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !{})
+!19 = !DISubroutineType(types: !{!21})
+!21 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!24 = !DILocalVariable(name: "f", scope: !18, file: !3, line: 8, type: !10)
+!25 = !DILocation(line: 0, scope: !18)