[llvm] r364736 - [DebugInfo] Avoid adding too much indirection to pointer-valued variables
Jeremy Morse via llvm-commits
llvm-commits at lists.llvm.org
Mon Jul 1 02:38:23 PDT 2019
Author: jmorse
Date: Mon Jul 1 02:38:23 2019
New Revision: 364736
URL: http://llvm.org/viewvc/llvm-project?rev=364736&view=rev
Log:
[DebugInfo] Avoid adding too much indirection to pointer-valued variables
This patch addresses PR41675, where a stack-pointer variable is dereferenced
too many times by its location expression, presenting a value on the stack as
the pointer to the stack.
The difference between a stack *pointer* DBG_VALUE and one that refers to a
value on the stack, is currently the indirect flag. However the DWARF backend
will also try to guess whether something is a memory location or not, based
on whether there is any computation in the location expression. By simply
prepending the stack offset to existing expressions, we can accidentally
convert a register location into a memory location, which introduces a
suprise (and unintended) dereference.
The solution is to add DW_OP_stack_value whenever we add a DIExpression
computation to a stack *pointer*. It's an implicit location computed on the
expression stack, thus needs to be flagged as a stack_value.
For the edge case where the offset is zero and the location could be a register
location, DIExpression::prepend will still generate opcodes, and thus
DW_OP_stack_value must still be added.
Differential Revision: https://reviews.llvm.org/D63429
Added:
llvm/trunk/test/DebugInfo/MIR/X86/prolog-epilog-indirection.mir
Modified:
llvm/trunk/include/llvm/IR/DebugInfoMetadata.h
llvm/trunk/lib/CodeGen/PrologEpilogInserter.cpp
llvm/trunk/lib/IR/DebugInfoMetadata.cpp
Modified: llvm/trunk/include/llvm/IR/DebugInfoMetadata.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/DebugInfoMetadata.h?rev=364736&r1=364735&r2=364736&view=diff
==============================================================================
--- llvm/trunk/include/llvm/IR/DebugInfoMetadata.h (original)
+++ llvm/trunk/include/llvm/IR/DebugInfoMetadata.h Mon Jul 1 02:38:23 2019
@@ -2464,6 +2464,10 @@ public:
/// Return whether this is an implicit location description.
bool isImplicit() const;
+ /// Return whether the location is computed on the expression stack, meaning
+ /// it cannot be a simple register location.
+ bool isComplex() const;
+
/// Append \p Ops with operations to apply the \p Offset.
static void appendOffset(SmallVectorImpl<uint64_t> &Ops, int64_t Offset);
Modified: llvm/trunk/lib/CodeGen/PrologEpilogInserter.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/PrologEpilogInserter.cpp?rev=364736&r1=364735&r2=364736&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/PrologEpilogInserter.cpp (original)
+++ llvm/trunk/lib/CodeGen/PrologEpilogInserter.cpp Mon Jul 1 02:38:23 2019
@@ -1200,6 +1200,16 @@ void PEI::replaceFrameIndices(MachineBas
MI.getOperand(0).setIsDebug();
const DIExpression *DIExpr = MI.getDebugExpression();
+
+ // If we have a direct DBG_VALUE, and its location expression isn't
+ // currently complex, then adding an offset will morph it into a
+ // complex location that is interpreted as being a memory address.
+ // This changes a pointer-valued variable to dereference that pointer,
+ // which is incorrect. Fix by adding DW_OP_stack_value.
+ unsigned PrependFlags = DIExpression::ApplyOffset;
+ if (!MI.isIndirectDebugValue() && !DIExpr->isComplex())
+ PrependFlags |= DIExpression::StackValue;
+
// If we have DBG_VALUE that is indirect and has a Implicit location
// expression need to insert a deref before prepending a Memory
// location expression. Also after doing this we change the DBG_VALUE
@@ -1211,8 +1221,7 @@ void PEI::replaceFrameIndices(MachineBas
// Make the DBG_VALUE direct.
MI.getOperand(1).ChangeToRegister(0, false);
}
- DIExpr =
- DIExpression::prepend(DIExpr, DIExpression::ApplyOffset, Offset);
+ DIExpr = DIExpression::prepend(DIExpr, PrependFlags, Offset);
MI.getOperand(3).setMetadata(DIExpr);
continue;
}
Modified: llvm/trunk/lib/IR/DebugInfoMetadata.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/DebugInfoMetadata.cpp?rev=364736&r1=364735&r2=364736&view=diff
==============================================================================
--- llvm/trunk/lib/IR/DebugInfoMetadata.cpp (original)
+++ llvm/trunk/lib/IR/DebugInfoMetadata.cpp Mon Jul 1 02:38:23 2019
@@ -926,6 +926,27 @@ bool DIExpression::isImplicit() const {
return false;
}
+bool DIExpression::isComplex() const {
+ if (!isValid())
+ return false;
+
+ if (getNumElements() == 0)
+ return false;
+
+ // If there are any elements other than fragment or tag_offset, then some
+ // kind of complex computation occurs.
+ for (const auto &It : expr_ops()) {
+ switch (It.getOp()) {
+ case dwarf::DW_OP_LLVM_tag_offset:
+ case dwarf::DW_OP_LLVM_fragment:
+ continue;
+ default: return true;
+ }
+ }
+
+ return false;
+}
+
Optional<DIExpression::FragmentInfo>
DIExpression::getFragmentInfo(expr_op_iterator Start, expr_op_iterator End) {
for (auto I = Start; I != End; ++I)
Added: llvm/trunk/test/DebugInfo/MIR/X86/prolog-epilog-indirection.mir
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/DebugInfo/MIR/X86/prolog-epilog-indirection.mir?rev=364736&view=auto
==============================================================================
--- llvm/trunk/test/DebugInfo/MIR/X86/prolog-epilog-indirection.mir (added)
+++ llvm/trunk/test/DebugInfo/MIR/X86/prolog-epilog-indirection.mir Mon Jul 1 02:38:23 2019
@@ -0,0 +1,130 @@
+# RUN: llc %s -x mir -o - -mtriple=x86_64-unknown-unknown -run-pass=prologepilog | FileCheck %s
+#
+# Check when the DBG_VALUE on a stack slot below (for var "c") has its stack
+# slot replaced with $rsp and a complex expression, it has DW_OP_stack_value
+# added. A direct reference to the stack slot is considered to be the _address_
+# of that stack slot, wheras its contents would be an indirect DBG_VALUE.
+#
+# Check too that for the same DBG_VALUE inst, with an indirect reference to
+# the stack slot, we do _not_ get DW_OP_plus_uconst added. This expression
+# should remain indirect, referring to the contents of the stack slot.
+#
+# CHECK: ![[VAR:[0-9]+]] = !DILocalVariable(name: "c"
+# CHECK: ![[VAR2:[0-9]+]] = !DILocalVariable(name: "asdf"
+# CHECK: ![[VAR3:[0-9]+]] = !DILocalVariable(name: "bees"
+#
+# CHECK: LEA64r $rsp
+# CHECK-NEXT: DBG_VALUE $rsp, $noreg, ![[VAR]], !DIExpression(DW_OP_plus_uconst, {{[0-9]+}}, DW_OP_stack_value)
+# CHECK-NEXT: DBG_VALUE $rsp, $noreg, ![[VAR2]], !DIExpression(DW_OP_plus_uconst, {{[0-9]+}}, DW_OP_stack_value, DW_OP_LLVM_fragment, 1, 2)
+# CHECK-NEXT: DBG_VALUE $rsp, $noreg, ![[VAR3]], !DIExpression(DW_OP_plus_uconst, {{[0-9]+}}, DW_OP_LLVM_tag_offset, 0, DW_OP_stack_value)
+# CHECK-NEXT: DBG_VALUE 1834104526
+# CHECK-NEXT: MOV64mr
+# CHECK-NEXT: DBG_VALUE $rsp, 0, ![[VAR]], !DIExpression(DW_OP_plus_uconst, {{[0-9]+}})
+
+--- |
+ ; ModuleID = 'out.ll'
+ source_filename = "abc.c"
+ target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+ target triple = "x86_64-unknown-linux-gnu"
+
+ @b = common dso_local local_unnamed_addr global i32* null, align 8, !dbg !0
+ @a = common dso_local local_unnamed_addr global i32 0, align 4, !dbg !6
+
+ ; Function Attrs: nounwind uwtable
+ define dso_local i32 @main() local_unnamed_addr !dbg !14 {
+ entry:
+ %l_1081 = alloca i32, align 4
+ %0 = bitcast i32* %l_1081 to i8*, !dbg !20
+ call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %0), !dbg !20
+ call void @llvm.dbg.value(metadata i32 1834104526, metadata !18, metadata !DIExpression()), !dbg !21
+ call void @llvm.dbg.value(metadata i32* %l_1081, metadata !19, metadata !DIExpression()), !dbg !21
+ store i32* %l_1081, i32** @b, align 8, !dbg !22, !tbaa !23
+ store i32 9, i32* @a, align 4, !dbg !27, !tbaa !28
+ store i32 9, i32* %l_1081, align 4, !dbg !30, !tbaa !28
+ %call = call i32 (...) @optimize_me_not(), !dbg !31
+ call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %0), !dbg !32
+ ret i32 0, !dbg !32
+ }
+
+ ; Function Attrs: argmemonly nounwind
+ declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture)
+
+ declare dso_local i32 @optimize_me_not(...) local_unnamed_addr
+
+ ; Function Attrs: argmemonly nounwind
+ declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture)
+
+ ; Function Attrs: nounwind readnone speculatable
+ declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+ !llvm.dbg.cu = !{!2}
+ !llvm.module.flags = !{!10, !11, !12}
+ !llvm.ident = !{!13}
+
+ !0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+ !1 = distinct !DIGlobalVariable(name: "b", scope: !2, file: !3, line: 2, type: !9, isLocal: false, isDefinition: true)
+ !2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
+ !3 = !DIFile(filename: "abc.c", directory: ".")
+ !4 = !{}
+ !5 = !{!6, !0}
+ !6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
+ !7 = distinct !DIGlobalVariable(name: "a", scope: !2, file: !3, line: 1, type: !8, isLocal: false, isDefinition: true)
+ !8 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+ !9 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !8, size: 64)
+ !10 = !{i32 2, !"Dwarf Version", i32 4}
+ !11 = !{i32 2, !"Debug Info Version", i32 3}
+ !12 = !{i32 1, !"wchar_size", i32 4}
+ !13 = !{!"clang"}
+ !14 = distinct !DISubprogram(name: "main", scope: !3, file: !3, line: 3, type: !15, scopeLine: 3, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !17)
+ !15 = !DISubroutineType(types: !16)
+ !16 = !{!8}
+ !17 = !{!18, !19, !33, !34}
+ !18 = !DILocalVariable(name: "l_1081", scope: !14, file: !3, line: 4, type: !8)
+ !19 = !DILocalVariable(name: "c", scope: !14, file: !3, line: 5, type: !9)
+ !20 = !DILocation(line: 4, column: 3, scope: !14)
+ !21 = !DILocation(line: 0, scope: !14)
+ !22 = !DILocation(line: 6, column: 5, scope: !14)
+ !23 = !{!24, !24, i64 0}
+ !24 = !{!"any pointer", !25, i64 0}
+ !25 = !{!"omnipotent char", !26, i64 0}
+ !26 = !{!"Simple C/C++ TBAA"}
+ !27 = !DILocation(line: 7, column: 10, scope: !14)
+ !28 = !{!29, !29, i64 0}
+ !29 = !{!"int", !25, i64 0}
+ !30 = !DILocation(line: 7, column: 6, scope: !14)
+ !31 = !DILocation(line: 8, column: 3, scope: !14)
+ !32 = !DILocation(line: 9, column: 1, scope: !14)
+ !33 = !DILocalVariable(name: "asdf", scope: !14, file: !3, line: 4, type: !8)
+ !34 = !DILocalVariable(name: "bees", scope: !14, file: !3, line: 4, type: !8)
+
+...
+---
+name: main
+alignment: 4
+tracksRegLiveness: true
+frameInfo:
+ maxAlignment: 4
+ hasCalls: true
+stack:
+ - { id: 0, name: l_1081, type: default, offset: 0, size: 4, alignment: 4,
+ callee-saved-register: '', callee-saved-restored: true,
+ debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+body: |
+ bb.0.entry:
+ renamable $rax = LEA64r %stack.0.l_1081, 1, $noreg, 0, $noreg
+ DBG_VALUE %stack.0.l_1081, $noreg, !19, !DIExpression(), debug-location !21
+ DBG_VALUE %stack.0.l_1081, $noreg, !33, !DIExpression(DW_OP_LLVM_fragment, 1, 2), debug-location !21
+ DBG_VALUE %stack.0.l_1081, $noreg, !34, !DIExpression(DW_OP_LLVM_tag_offset, 0), debug-location !21
+ DBG_VALUE 1834104526, $noreg, !18, !DIExpression(), debug-location !21
+ MOV64mr $rip, 1, $noreg, @b, $noreg, killed renamable $rax, debug-location !22 :: (store 8 into @b, !tbaa !23)
+ DBG_VALUE %stack.0.l_1081, 0, !19, !DIExpression(), debug-location !21
+ MOV32mi $rip, 1, $noreg, @a, $noreg, 9, debug-location !27 :: (store 4 into @a, !tbaa !28)
+ MOV32mi %stack.0.l_1081, 1, $noreg, 0, $noreg, 9, debug-location !30 :: (store 4 into %ir.l_1081, !tbaa !28)
+ ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !31
+ dead $eax = MOV32r0 implicit-def dead $eflags, implicit-def $al, debug-location !31
+ CALL64pcrel32 @optimize_me_not, csr_64, implicit $rsp, implicit $ssp, implicit $al, implicit-def $rsp, implicit-def $ssp, implicit-def dead $eax, debug-location !31
+ ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp, debug-location !31
+ $eax = MOV32r0 implicit-def dead $eflags, debug-location !32
+ RET 0, $eax, debug-location !32
+
+...
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