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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-codegen

Author: Florian Hahn (fhahn)

<details>
<summary>Changes</summary>

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

---
Full diff: https://github.com/llvm/llvm-project/pull/82922.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CodeGenTBAA.cpp (+1-1) 
- (modified) clang/test/CodeGen/tbaa-struct.cpp (+1-1) 


``````````diff
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}
 

``````````

</details>


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