[DebugInfo] Emit DW_OP_implicit_value for Floating point constants

Summary: llvm is missing support for DW_OP_implicit_value operation. DW_OP_implicit_value op is indispensable for cases such as optimized out long double variables. For intro refer: DWARFv5 Spec Pg: 40 2.6.1.1.4 Implicit Location Descriptions Consider the following example: ``` int main() { long double ld = 3.14; printf("dummy\n"); ld *= ld; return 0; } ``` when compiled with tunk `clang` as `clang test.c -g -O1` produces following location description of variable `ld`: ``` DW_AT_location (0x00000000: [0x0000000000201691, 0x000000000020169b): DW_OP_constu 0xc8f5c28f5c28f800, DW_OP_stack_value, DW_OP_piece 0x8, DW_OP_constu 0x4000, DW_OP_stack_value, DW_OP_bit_piece 0x10 0x40, DW_OP_stack_value) DW_AT_name ("ld") ``` Here one may notice that this representation is incorrect(DWARF4 stack could only hold integers(and only up to the size of address)). Here the variable size itself is `128` bit. GDB and LLDB confirms this: ``` (gdb) p ld $1 = <invalid float value> (lldb) frame variable ld (long double) ld = <extracting data from value failed> ``` GCC represents/uses DW_OP_implicit_value in these sort of situations. Based on the discussion with Jakub Jelinek regarding GCC's motivation for using this, I concluded that DW_OP_implicit_value is most appropriate in this case. Link: https://gcc.gnu.org/pipermail/gcc/2020-July/233057.html GDB seems happy after this patch:(LLDB doesn't have support for DW_OP_implicit_value) ``` (gdb) p ld p ld $1 = 3.14000000000000012434 ``` Reviewed By: aprantl Differential Revision: https://reviews.llvm.org/D83560
llvm · Jul 23, 2020 · 6b55a95 · 6b55a95
1 parent 99ad956
commit 6b55a95
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Showing 7 changed files with 252 additions and 5 deletions.
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -2512,8 +2512,20 @@ void DwarfDebug::emitDebugLocValue(const AsmPrinter &AP, const DIBasicType *BT,
     // encoding is supported.
     DwarfExpr.addWasmLocation(Loc.Index, static_cast<uint64_t>(Loc.Offset));
   } else if (Value.isConstantFP()) {
-    APInt RawBytes = Value.getConstantFP()->getValueAPF().bitcastToAPInt();
-    DwarfExpr.addUnsignedConstant(RawBytes);
+    if (AP.getDwarfVersion() >= 4 && AP.getDwarfDebug()->tuneForGDB())
+      DwarfExpr.addConstantFP(Value.getConstantFP()->getValueAPF());
+    else if (Value.getConstantFP()
+                 ->getValueAPF()
+                 .bitcastToAPInt()
+                 .getBitWidth() <= 64 /*bits*/)
+      DwarfExpr.addUnsignedConstant(
+          Value.getConstantFP()->getValueAPF().bitcastToAPInt());
+    else
+      LLVM_DEBUG(
+          dbgs()
+          << "Skipped DwarfExpression creation for ConstantFP of size"
+          << Value.getConstantFP()->getValueAPF().bitcastToAPInt().getBitWidth()
+          << " bits\n");
   }
   DwarfExpr.addExpression(std::move(ExprCursor));
 }

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
@@ -25,6 +25,8 @@
 
 using namespace llvm;
 
+#define DEBUG_TYPE "dwarfdebug"
+
 void DwarfExpression::emitConstu(uint64_t Value) {
   if (Value < 32)
     emitOp(dwarf::DW_OP_lit0 + Value);
@@ -219,6 +221,36 @@ void DwarfExpression::addUnsignedConstant(const APInt &Value) {
   }
 }
 
+void DwarfExpression::addConstantFP(const APFloat &Value) {
+  assert(isImplicitLocation() || isUnknownLocation());
+  APInt RawBytes = Value.bitcastToAPInt();
+  int NumBytes = RawBytes.getBitWidth() / 8;
+  const char *Data = (const char *)RawBytes.getRawData();
+  emitOp(dwarf::DW_OP_implicit_value);
+  if (NumBytes == 4 /*float*/ || NumBytes == 8 /*double*/) {
+    emitUnsigned(NumBytes /*Size of the block in bytes*/);
+    for (int i = 0; i < NumBytes; ++i)
+      emitData1(Data[i]);
+    return;
+  }
+  if (NumBytes == 10 /*long double*/) {
+    // long double IEEE representation uses 80 bits(10 bytes).
+    // 6 bytes are padded to make it 128 bits(16 bytes) due to
+    // addressing restrictions.
+    emitUnsigned(16 /*Size of the block in bytes*/);
+    // Emit the block of bytes.
+    for (int i = 0; i < NumBytes; ++i)
+      emitData1(Data[i]);
+    // Emit the rest as padding bytes.
+    for (int i = 0; i < 16 - NumBytes; ++i)
+      emitData1(0);
+    return;
+  }
+  LLVM_DEBUG(
+      dbgs() << "Skipped DW_OP_implicit_value creation for ConstantFP of size"
+             << RawBytes.getBitWidth() << " bits\n");
+}
+
 bool DwarfExpression::addMachineRegExpression(const TargetRegisterInfo &TRI,
                                               DIExpressionCursor &ExprCursor,
                                               unsigned MachineReg,

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h
@@ -299,6 +299,9 @@ class DwarfExpression {
   /// Emit an unsigned constant.
   void addUnsignedConstant(const APInt &Value);
 
+  /// Emit floating point constant.
+  void addConstantFP(const APFloat &Value);
+
   /// Lock this down to become a memory location description.
   void setMemoryLocationKind() {
     assert(isUnknownLocation());

diff --git a/llvm/test/DebugInfo/X86/float_const_loclist.ll b/llvm/test/DebugInfo/X86/float_const_loclist.ll
@@ -20,12 +20,10 @@
 ;
 ; CHECK: .debug_info contents:
 ; CHECK: DW_TAG_variable
-; CHECK-NEXT:  DW_AT_location {{.*}} (
-; CHECK-NEXT:    [0x[[START:.*]], 0x[[END:.*]]): DW_OP_constu 0xc8f5c28f5c28f800, DW_OP_piece 0x8, DW_OP_constu 0x4000, DW_OP_bit_piece 0x10 0x40)
 ; CHECK-NEXT:  DW_AT_name {{.*}}"ld"
 ; CHECK: DW_TAG_variable
 ; CHECK-NEXT:  DW_AT_location {{.*}} (
-; CHECK-NEXT:    [0x[[START]], 0x[[END]]): DW_OP_constu 0x4048f5c3)
+; CHECK-NEXT:    [0x{{.*}}, 0x{{.*}}): DW_OP_constu 0x4048f5c3)
 ; CHECK-NEXT:  DW_AT_name {{.*}}"f"
 
 source_filename = "test.c"

diff --git a/llvm/test/DebugInfo/X86/implicit_value-double.ll b/llvm/test/DebugInfo/X86/implicit_value-double.ll
@@ -0,0 +1,66 @@
+;; This test checks for emission of DW_OP_implicit_value operation
+;; for double type.
+
+; RUN: llc -debugger-tune=gdb -filetype=obj %s -o - | llvm-dwarfdump - | FileCheck %s
+
+; CHECK: .debug_info contents:
+; CHECK: DW_TAG_variable
+; CHECK-NEXT:  DW_AT_location        ({{.*}}
+; CHECK-NEXT:                     [{{.*}}): DW_OP_implicit_value 0x8 0x1f 0x85 0xeb 0x51 0xb8 0x1e 0x09 0x40)
+; CHECK-NEXT:  DW_AT_name    ("d")
+
+;; Generated from: clang -ggdb -O1
+;;int main() {
+;;        double d = 3.14;
+;;        printf("dummy\n");
+;;        d *= d;
+;;        return 0;
+;;}
+
+; ModuleID = 'implicit_value-double.c'
+source_filename = "implicit_value-double.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"
+
+@str = private unnamed_addr constant [6 x i8] c"dummy\00", align 1
+
+; Function Attrs: nofree nounwind uwtable
+define dso_local i32 @main() local_unnamed_addr #0 !dbg !7 {
+entry:
+  call void @llvm.dbg.value(metadata double 3.140000e+00, metadata !12, metadata !DIExpression()), !dbg !14
+  %puts = call i32 @puts(i8* nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @str, i64 0, i64 0)), !dbg !15
+  call void @llvm.dbg.value(metadata double undef, metadata !12, metadata !DIExpression()), !dbg !14
+  ret i32 0, !dbg !16
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+; Function Attrs: nofree nounwind
+declare i32 @puts(i8* nocapture readonly) local_unnamed_addr #2
+
+attributes #0 = { nofree nounwind uwtable }
+attributes #1 = { nounwind readnone speculatable willreturn }
+attributes #2 = { nofree nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 11.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "implicit_value-double.c", directory: "/home/")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 11.0.0"}
+!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !{!12}
+!12 = !DILocalVariable(name: "d", scope: !7, file: !1, line: 2, type: !13)
+!13 = !DIBasicType(name: "double", size: 64, encoding: DW_ATE_float)
+!14 = !DILocation(line: 0, scope: !7)
+!15 = !DILocation(line: 3, column: 2, scope: !7)
+!16 = !DILocation(line: 5, column: 2, scope: !7)
diff --git a/llvm/test/DebugInfo/X86/implicit_value-float.ll b/llvm/test/DebugInfo/X86/implicit_value-float.ll
@@ -0,0 +1,65 @@
+;; This test checks for emission of DW_OP_implicit_value operation
+;; for float type.
+
+; RUN: llc -debugger-tune=gdb -filetype=obj %s -o - | llvm-dwarfdump - | FileCheck %s
+
+; CHECK: .debug_info contents:
+; CHECK: DW_TAG_variable
+; CHECK-NEXT:  DW_AT_location        ({{.*}}
+; CHECK-NEXT:                     [{{.*}}): DW_OP_implicit_value 0x4 0xc3 0xf5 0x48 0x40)
+; CHECK-NEXT:  DW_AT_name    ("f")
+
+;; Generated from: clang -ggdb -O1
+;;int main() {
+;;        float f = 3.14f;
+;;        printf("dummy\n");
+;;        f *= f;
+;;        return 0;
+;;}
+; ModuleID = 'implicit_value-float.c'
+source_filename = "implicit_value-float.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"
+
+@str = private unnamed_addr constant [6 x i8] c"dummy\00", align 1
+
+; Function Attrs: nofree nounwind uwtable
+define dso_local i32 @main() local_unnamed_addr #0 !dbg !7 {
+entry:
+  call void @llvm.dbg.value(metadata float 0x40091EB860000000, metadata !12, metadata !DIExpression()), !dbg !14
+  %puts = call i32 @puts(i8* nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @str, i64 0, i64 0)), !dbg !15
+  call void @llvm.dbg.value(metadata float undef, metadata !12, metadata !DIExpression()), !dbg !14
+  ret i32 0, !dbg !16
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+; Function Attrs: nofree nounwind
+declare i32 @puts(i8* nocapture readonly) local_unnamed_addr #2
+
+attributes #0 = { nofree nounwind uwtable }
+attributes #1 = { nounwind readnone speculatable willreturn }
+attributes #2 = { nofree nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 11.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "implicit_value-float.c", directory: "/home/")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 11.0.0"}
+!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !{!12}
+!12 = !DILocalVariable(name: "f", scope: !7, file: !1, line: 2, type: !13)
+!13 = !DIBasicType(name: "float", size: 32, encoding: DW_ATE_float)
+!14 = !DILocation(line: 0, scope: !7)
+!15 = !DILocation(line: 3, column: 2, scope: !7)
+!16 = !DILocation(line: 5, column: 2, scope: !7)
diff --git a/llvm/test/DebugInfo/X86/implicit_value-ld.ll b/llvm/test/DebugInfo/X86/implicit_value-ld.ll
@@ -0,0 +1,71 @@
+;; This test checks for emission of DW_OP_implicit_value operation
+;; for long double type.
+
+; RUN: llc -debugger-tune=gdb -filetype=obj %s -o - | llvm-dwarfdump - | FileCheck %s
+
+; CHECK: .debug_info contents:
+; CHECK: DW_TAG_variable
+; CHECK-NEXT:  DW_AT_location        ({{.*}}
+; CHECK-NEXT:                     [{{.*}}): DW_OP_implicit_value 0x10 0x00 0xf8 0x28 0x5c 0x8f 0xc2 0xf5 0xc8 0x00 0x40 0x00 0x00 0x00 0x00 0x00 0x00)
+; CHECK-NEXT:  DW_AT_name    ("ld")
+
+;; Generated from: clang -ggdb -O1
+;;int main() {
+;;        long double ld = 3.14;
+;;        printf("dummy\n");
+;;        ld *= ld;
+;;        return 0;
+;;}
+
+; ModuleID = 'implicit_value-ld.c'
+source_filename = "implicit_value-ld.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"
+
+@str = private unnamed_addr constant [6 x i8] c"dummy\00", align 1
+
+; Function Attrs: nofree nounwind uwtable
+define dso_local i32 @main() local_unnamed_addr #0 !dbg !7 {
+entry:
+  call void @llvm.dbg.declare(metadata [6 x i8]* undef, metadata !12, metadata !DIExpression(DW_OP_LLVM_fragment, 80, 48)), !dbg !14
+  call void @llvm.dbg.value(metadata x86_fp80 0xK4000C8F5C28F5C28F800, metadata !12, metadata !DIExpression()), !dbg !15
+  %puts = call i32 @puts(i8* nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @str, i64 0, i64 0)), !dbg !16
+  call void @llvm.dbg.value(metadata x86_fp80 undef, metadata !12, metadata !DIExpression()), !dbg !15
+  ret i32 0, !dbg !17
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+; Function Attrs: nofree nounwind
+declare i32 @puts(i8* nocapture readonly) local_unnamed_addr #2
+
+attributes #0 = { nofree nounwind uwtable }
+attributes #1 = { nounwind readnone speculatable willreturn }
+attributes #2 = { nofree nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 11.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "implicit_value-ld.c", directory: "/home/")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 11.0.0"}
+!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !{!12}
+!12 = !DILocalVariable(name: "ld", scope: !7, file: !1, line: 2, type: !13)
+!13 = !DIBasicType(name: "long double", size: 128, encoding: DW_ATE_float)
+!14 = !DILocation(line: 2, column: 14, scope: !7)
+!15 = !DILocation(line: 0, scope: !7)
+!16 = !DILocation(line: 3, column: 2, scope: !7)
+!17 = !DILocation(line: 5, column: 2, scope: !7)