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

Fixes https://github.com/llvm/llvm-project/issues/50962.

Reviewed By: whisperity, aaron.ballman

Differential Revision: https://reviews.llvm.org/D114292

diff --git a/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp b/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
index 6ae5351..b5f8d08 100644 (file)
--- a/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
+++ b/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
@@ -77,7 +77,8 @@ void StructPackAlignCheck::check(const MatchFinder::MatchResult &Result) {
   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 =

diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 50d5b8e..491bc92 100644 (file)
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -170,6 +170,9 @@ Changes in existing checks
   <clang-tidy/checks/performance-inefficient-vector-operation>` to work when
   the vector is a member of a structure.
 
+- Fixed nonsensical suggestion of :doc:`altera-struct-pack-align
+  <clang-tidy/checks/altera-struct-pack-align>` check for empty structs.
+
 Removed checks
 ^^^^^^^^^^^^^^
 

diff --git a/clang-tools-extra/test/clang-tidy/checkers/altera-struct-pack-align.cpp b/clang-tools-extra/test/clang-tidy/checkers/altera-struct-pack-align.cpp
index 615b6ca..472372f 100644 (file)
--- a/clang-tools-extra/test/clang-tidy/checkers/altera-struct-pack-align.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/altera-struct-pack-align.cpp
@@ -99,3 +99,22 @@ void no_trigger_on_instantiation() {
   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)));