[CLANG][MS-STRUCT] bitfield padding warning presents padding to exact bit count (#136062)

Aims to fix #131647.

Author: theomagellan

clang/lib/AST/RecordLayoutBuilder.cpp

@@ -1539,6 +1539,7 @@ void ItaniumRecordLayoutBuilder::LayoutBitField(const FieldDecl *D) {
   uint64_t StorageUnitSize = FieldInfo.Width;
   unsigned FieldAlign = FieldInfo.Align;
   bool AlignIsRequired = FieldInfo.isAlignRequired();
+  unsigned char PaddingInLastUnit = 0;
 
   // UnfilledBitsInLastUnit is the difference between the end of the
   // last allocated bitfield (i.e. the first bit offset available for
@@ -1611,6 +1612,7 @@ void ItaniumRecordLayoutBuilder::LayoutBitField(const FieldDecl *D) {
       if (!LastBitfieldStorageUnitSize && !FieldSize)
         FieldAlign = 1;
 
+      PaddingInLastUnit = UnfilledBitsInLastUnit;
       UnfilledBitsInLastUnit = 0;
       LastBitfieldStorageUnitSize = 0;
     }
@@ -1707,7 +1709,7 @@ void ItaniumRecordLayoutBuilder::LayoutBitField(const FieldDecl *D) {
   // For purposes of diagnostics, we're going to simultaneously
   // compute the field offsets that we would have used if we weren't
   // adding any alignment padding or if the field weren't packed.
-  uint64_t UnpaddedFieldOffset = FieldOffset;
+  uint64_t UnpaddedFieldOffset = FieldOffset - PaddingInLastUnit;
   uint64_t UnpackedFieldOffset = FieldOffset;
 
   // Check if we need to add padding to fit the bitfield within an

clang/test/SemaCXX/windows-Wpadded-bitfield.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -triple x86_64-windows-msvc -fsyntax-only -verify -Wpadded %s
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -fsyntax-only -verify -Wpadded %s
 
 struct __attribute__((ms_struct)) BitfieldStruct { // expected-warning {{padding size of 'BitfieldStruct' with 3 bytes to alignment boundary}}
   char c : 1;
@@ -24,9 +25,23 @@ struct __attribute__((ms_struct)) DifferentUnitSizeBitfield { // expected-warnin
   char i; // expected-warning {{padding struct 'DifferentUnitSizeBitfield' with 31 bits to align 'i'}}
 };
 
+struct __attribute__((ms_struct)) BitfieldBigPadding { // expected-warning {{padding size of 'BitfieldBigPadding' with 63 bits to alignment boundary}}
+  long long x;
+  char a : 1;
+  long long b : 1; // expected-warning {{padding struct 'BitfieldBigPadding' with 63 bits to align 'b'}}
+};
+
+struct __attribute__((ms_struct)) SameUnitSizeMultiple { // expected-warning {{padding size of 'SameUnitSizeMultiple' with 2 bits to alignment boundary}}
+  char c : 1;
+  char cc : 2;
+  char ccc : 3;
+};
+
 int main() {
   BitfieldStruct b;
   SevenBitfieldStruct s;
   SameUnitSizeBitfield su;
   DifferentUnitSizeBitfield du;
+  BitfieldBigPadding bbp;
+  SameUnitSizeMultiple susm;
 }