[DebugInfo] Emit DW_OP_implicit_value for Floating point constants

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/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -2501,8 +2501,21 @@ void DwarfDebug::emitDebugLocValue(const AsmPrinter &AP, const DIBasicType *BT,
       DwarfExpr.addExpression(std::move(ExprCursor));
       return;
   } 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(), AP);
+      return;
+    } 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));
 }

llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp

@@ -17,6 +17,7 @@
 #include "llvm/BinaryFormat/Dwarf.h"
 #include "llvm/CodeGen/Register.h"
 #include "llvm/CodeGen/TargetRegisterInfo.h"
+#include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <algorithm>
@@ -25,6 +26,8 @@
 
 using namespace llvm;
 
+#define DEBUG_TYPE "dwarfdebug"
+
 void DwarfExpression::emitConstu(uint64_t Value) {
   if (Value < 32)
     emitOp(dwarf::DW_OP_lit0 + Value);
@@ -219,6 +222,31 @@ void DwarfExpression::addUnsignedConstant(const APInt &Value) {
   }
 }
 
+void DwarfExpression::addConstantFP(const APFloat &APF, const AsmPrinter &AP) {
+  assert(isImplicitLocation() || isUnknownLocation());
+  APInt API = APF.bitcastToAPInt();
+  int NumBytes = API.getBitWidth() / 8;
+  if (NumBytes == 4 /*float*/ || NumBytes == 8 /*double*/) {
+    // FIXME: Add support for `long double`.
+    emitOp(dwarf::DW_OP_implicit_value);
+    emitUnsigned(NumBytes /*Size of the block in bytes*/);
+
+    const uint64_t *Value = API.getRawData();
+    const bool IsLittleEndian = AP.getDataLayout().isLittleEndian();
+    uint64_t Swapped = support::endian::byte_swap(
+        *Value, IsLittleEndian ? support::little : support::big);
+
+    const char *SwappedBytes = reinterpret_cast<const char *>(&Swapped);
+    for (int i = 0; i < NumBytes; ++i)
+      emitData1(SwappedBytes[i]);
+
+    return;
+  }
+  LLVM_DEBUG(
+      dbgs() << "Skipped DW_OP_implicit_value creation for ConstantFP of size: "
+             << API.getBitWidth() << " bits\n");
+}
+
 bool DwarfExpression::addMachineRegExpression(const TargetRegisterInfo &TRI,
                                               DIExpressionCursor &ExprCursor,
                                               unsigned MachineReg,

llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h

@@ -299,6 +299,9 @@ public:
   /// Emit an unsigned constant.
   void addUnsignedConstant(const APInt &Value);
 
+  /// Emit an floating point constant.
+  void addConstantFP(const APFloat &Value, const AsmPrinter &AP);
+
   /// Lock this down to become a memory location description.
   void setMemoryLocationKind() {
     assert(isUnknownLocation());

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"

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)

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)