[PATCH] D114292: [clang-tidy] Fix `altera-struct-pack-align` check for empty structs

[Fabian Wolff via Phabricator via cfe-commits]

fwolff updated this revision to Diff 400634.
fwolff added a comment.

I've added the `static_cast` and an entry in the release notes. I'm not sure how to handle `[[no_unique_address]]`, so I'd rather leave this to future work.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D114292/new/

https://reviews.llvm.org/D114292

Files:
  clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
  clang-tools-extra/docs/ReleaseNotes.rst
  clang-tools-extra/test/clang-tidy/checkers/altera-struct-pack-align.cpp


Index: clang-tools-extra/test/clang-tidy/checkers/altera-struct-pack-align.cpp
===================================================================
--- clang-tools-extra/test/clang-tidy/checkers/altera-struct-pack-align.cpp
+++ clang-tools-extra/test/clang-tidy/checkers/altera-struct-pack-align.cpp
@@ -99,3 +99,22 @@
   struct bad_align3 instantiated { 'a', 0.001, 'b' };
 }
 
+// Make sure that we don't recommend aligning an empty struct to zero bytes (PR#51620)
+struct StructWithNoFields {};
+
+struct ContainsStructWithNoFields {
+  StructWithNoFields s;
+};
+
+// Make sure that an empty struct is treated like "char" for padding and alignment purposes
+struct ContainsStructWithNoFields2 {
+  StructWithNoFields s;
+  double d;
+  StructWithNoFields t;
+};
+// CHECK-MESSAGES: :[[@LINE-5]]:8: warning: accessing fields in struct 'ContainsStructWithNoFields2' is inefficient due to padding; only needs 10 bytes but is using 24 bytes [altera-struct-pack-align]
+// CHECK-MESSAGES: :[[@LINE-6]]:8: note: use "__attribute__((packed))" to reduce the amount of padding applied to struct 'ContainsStructWithNoFields2'
+// CHECK-MESSAGES: :[[@LINE-7]]:8: warning: accessing fields in struct 'ContainsStructWithNoFields2' is inefficient due to poor alignment; currently aligned to 8 bytes, but recommended alignment is 16 bytes [altera-struct-pack-align]
+// CHECK-MESSAGES: :[[@LINE-8]]:8: note: use "__attribute__((aligned(16)))" to align struct 'ContainsStructWithNoFields2' to 16 bytes
+// CHECK-FIXES: __attribute__((packed))
+// CHECK-FIXES: __attribute__((aligned(16)));
Index: clang-tools-extra/docs/ReleaseNotes.rst
===================================================================
--- clang-tools-extra/docs/ReleaseNotes.rst
+++ clang-tools-extra/docs/ReleaseNotes.rst
@@ -83,6 +83,9 @@
 - Generalized the `modernize-use-default-member-init` check to handle non-default
   constructors.
 
+- Fixed nonsensical suggestion of `altera-struct-pack-align` check for empty
+  structs.
+
 New checks
 ^^^^^^^^^^
 
Index: clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
===================================================================
--- clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
+++ clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
@@ -77,7 +77,8 @@
   uint64_t CharSize = Result.Context->getCharWidth();
   CharUnits CurrSize = Result.Context->getASTRecordLayout(Struct).getSize();
   CharUnits MinByteSize =
-      CharUnits::fromQuantity(ceil((float)TotalBitSize / CharSize));
+      CharUnits::fromQuantity(std::max<clang::CharUnits::QuantityType>(
+          ceil(static_cast<float>(TotalBitSize) / CharSize), 1));
   CharUnits MaxAlign = CharUnits::fromQuantity(
       ceil((float)Struct->getMaxAlignment() / CharSize));
   CharUnits CurrAlign =


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