[clang] [TBAA] Skip all bitfields when generating !tbaa.struct metadata. (PR #82922)

[Florian Hahn via cfe-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/82922

At the moment, clang generates what I believe are incorrect !tbaa.struct fields for named bitfields. At the moment, the base type size is used for named bifields (e.g. sizeof(int)) instead of the bifield width per field. This results in overalpping fields in !tbaa.struct metadata.

This causes incorrect results when extracting individual copied fields from !tbaa.struct as in added in dc85719d5.

This patch fixes that by skipping all bitfields, not only unnamed ones (note that CollectFields has a TODO to support bitfields). As bitfields specify their widths in bits, while !tbaa metadata uses bytes for sizes and offsets, I don't think we would be able to generate correct metadata for them in general.

If this understanding is correct, I can also extend the verifier to check that !tbaa.struct fields aren't overlapping.

Fixes https://github.com/llvm/llvm-project/issues/82586

>From 556fcefed9645aa0a0a965ff8f22d8accdf9eefc Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Sun, 25 Feb 2024 13:23:17 +0000
Subject: [PATCH] [TBAA] Skip all bitfields when generating !tbaa.struct
 metadata.

At the moment, clang generates what I believe are incorrect !tbaa.struct
fields for named bitfields. At the moment, the base type size is used
for named bifields (e.g. sizeof(int)) instead of the bifield width per
field. This results in overalpping fields in !tbaa.struct metadata.

This causes incorrect results when extracting individual copied fields
from !tbaa.struct as in added in dc85719d5.

This patch fixes that by skipping all bitfields, not only unnamed ones
(note that CollectFields has a TODO to support bitfields). As bitfields
specify their widths in bits, while !tbaa metadata uses bytes for sizes
and offsets, I don't think we would be able to generate correct metadata
for them in general.

If this understanding is correct, I can also extend the verifier to
check that !tbaa.struct fields aren't overlapping.

Fixes https://github.com/llvm/llvm-project/issues/82586
---
 clang/lib/CodeGen/CodeGenTBAA.cpp  | 2 +-
 clang/test/CodeGen/tbaa-struct.cpp | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/clang/lib/CodeGen/CodeGenTBAA.cpp b/clang/lib/CodeGen/CodeGenTBAA.cpp
index dc288bc3f6157a..43a1aee3d73823 100644
--- a/clang/lib/CodeGen/CodeGenTBAA.cpp
+++ b/clang/lib/CodeGen/CodeGenTBAA.cpp
@@ -298,7 +298,7 @@ CodeGenTBAA::CollectFields(uint64_t BaseOffset,
     unsigned idx = 0;
     for (RecordDecl::field_iterator i = RD->field_begin(),
          e = RD->field_end(); i != e; ++i, ++idx) {
-      if ((*i)->isZeroSize(Context) || (*i)->isUnnamedBitfield())
+      if ((*i)->isZeroSize(Context) || (*i)->isBitField())
         continue;
       uint64_t Offset = BaseOffset +
                         Layout.getFieldOffset(idx) / Context.getCharWidth();
diff --git a/clang/test/CodeGen/tbaa-struct.cpp b/clang/test/CodeGen/tbaa-struct.cpp
index ff5521fcf3f604..17c9d6bf6a7260 100644
--- a/clang/test/CodeGen/tbaa-struct.cpp
+++ b/clang/test/CodeGen/tbaa-struct.cpp
@@ -130,7 +130,7 @@ void copy8(NamedBitfields *a1, NamedBitfields *a2) {
 // CHECK-OLD: [[TS3]] = !{i64 0, i64 8, !{{.*}}, i64 0, i64 2, !{{.*}}, i64 4, i64 8, !{{.*}}}
 // CHECK-OLD: [[TS4]] = !{i64 0, i64 1, [[TAG_CHAR]], i64 1, i64 1, [[TAG_CHAR]], i64 2, i64 1, [[TAG_CHAR]]}
 // CHECK-OLD: [[TS5]] = !{i64 0, i64 1, [[TAG_CHAR]], i64 4, i64 1, [[TAG_CHAR]], i64 5, i64 1, [[TAG_CHAR]]}
-// CHECK-OLD: [[TS6]] = !{i64 0, i64 4, [[TAG_INT]], i64 1, i64 4, [[TAG_INT]], i64 2, i64 1, [[TAG_CHAR]], i64 8, i64 8, [[TAG_DOUBLE:!.+]]}
+// CHECK-OLD: [[TS6]] = !{i64 2, i64 1, [[TAG_CHAR]], i64 8, i64 8, [[TAG_DOUBLE:!.+]]}
 // CHECK-OLD: [[TAG_DOUBLE]] = !{[[DOUBLE:!.+]], [[DOUBLE]], i64 0}
 // CHECK-OLD  [[DOUBLE]] = !{!"double", [[CHAR]], i64 0}