[PATCH] D32745: Correct debug info bit offset calculation for big-endian targets

[Frej Drejhammar via Phabricator via cfe-commits]

frej created this revision.
Herald added subscribers: arichardson, aprantl.

The change "[CodeView] Implement support for bit fields in
Clang" (r274201, https://reviews.llvm.org/rL274201) broke the
calculation of bit offsets for the debug info describing bitfields on
big-endian targets.

Prior to commit r274201 the debug info for bitfields got their offsets
from the ASTRecordLayout in CGDebugInfo::CollectRecordFields(), the
current field offset was then passed on to
CGDebugInfo::CollectRecordNormalField() and used directly in the
DIDerivedType.

Since commit r274201, the bit offset ending up in the DIDerivedType no
longer comes directly from the ASTRecordLayout. Instead
CGDebugInfo::CollectRecordNormalField() calls the new method
CGDebugInfo::createBitFieldType(), which in turn calls
CodeGenTypes::getCGRecordLayout().getBitFieldInfo() to fetch a
CGBitFieldInfo describing the field. The 'Offset' member of
CGBitFieldInfo is then used to calculate the bit offset of the
DIDerivedType. Unfortunately the previous and current method of
calculating the bit offset are only equivalent for little endian
targets, as CGRecordLowering::setBitFieldInfo() reverses the bit
offsets for big endian targets as the last thing it does.

A simple reproducer for this error is the following module:

struct fields {

  unsigned a : 4;
  unsigned b : 4;

} flags = {0x0f, 0x1};

Compiled for Mips, with commit r274200 both the DIDerivedType bit
offsets on the IR-level and the DWARF information on the ELF-level
will have the expected values: the offsets of 'a' and 'b' are 0 and 4
respectively. With r274201 the offsets are switched to 4 and 0. By
noting that the static initialization of 'flags' in both cases is the
same, we can eliminate a change in record layout as the cause of the
change in the debug info. Also compiling this example with gcc,
produces the same record layout and debug info as commit r274200.

In order to restore the previous function we extend
CGDebugInfo::createBitFieldType() to compensate for the reversal done
in CGRecordLowering::setBitFieldInfo().


https://reviews.llvm.org/D32745

Files:
  clang/lib/CodeGen/CGDebugInfo.cpp
  clang/test/CodeGen/mips-debug-info-bitfield.c


Index: clang/test/CodeGen/mips-debug-info-bitfield.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/mips-debug-info-bitfield.c
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -x c -debug-info-kind=limited -triple mips-none-linux-gnu -emit-llvm -o - %s | FileCheck %s
+
+struct fields
+{
+  unsigned a : 4;
+  unsigned b : 4;
+} flags;
+
+// CHECK: !DIDerivedType(tag: DW_TAG_member,
+// CHECK-SAME: {{.*}}name: "a"
+// CHECK-NOT: {{.*}}offset:
+// CHECK-SAME: {{.*}}flags: DIFlagBitField
+
+// CHECK: !DIDerivedType(tag: DW_TAG_member,
+// CHECK-SAME: {{.*}}name: "b"
+// CHECK-SAME: {{.*}}offset: 4
+// CHECK-SAME: {{.*}}flags: DIFlagBitField
Index: clang/lib/CodeGen/CGDebugInfo.cpp
===================================================================
--- clang/lib/CodeGen/CGDebugInfo.cpp
+++ clang/lib/CodeGen/CGDebugInfo.cpp
@@ -1041,7 +1041,13 @@
   assert(SizeInBits > 0 && "found named 0-width bitfield");
   uint64_t StorageOffsetInBits =
       CGM.getContext().toBits(BitFieldInfo.StorageOffset);
-  uint64_t OffsetInBits = StorageOffsetInBits + BitFieldInfo.Offset;
+  uint64_t Offset = BitFieldInfo.Offset;
+  // The bit offsets for big endian machines are reversed for big
+  // endian target, compensate for that as the DIDerivedType requires
+  // un-reversed offsets.
+  if (CGM.getDataLayout().isBigEndian())
+    Offset = BitFieldInfo.StorageSize - BitFieldInfo.Size - Offset;
+  uint64_t OffsetInBits = StorageOffsetInBits + Offset;
   llvm::DINode::DIFlags Flags = getAccessFlag(BitFieldDecl->getAccess(), RD);
   return DBuilder.createBitFieldMemberType(
       RecordTy, Name, File, Line, SizeInBits, OffsetInBits, StorageOffsetInBits,


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