[clang] [Sema] Add check for bitfield assignments to larger integral types (PR #68276)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

<details>
<summary>Changes</summary>

We noticed that clang does not check for bitfield assignment widths, while gcc does check this.

gcc produced a warning like so for it's -Wconversion flag:

$ gcc -Wconversion -c test.c
test.c: In function 'foo':
test.c:10:15: warning: conversion from 'int' to 'signed char:7' may change value [-Wconversion]
   10 |      vxx.bf = x; // no warning
      |               ^

This option is disabled by default, and is enabled by usign the -Wbitfield-width option.

---
Full diff: https://github.com/llvm/llvm-project/pull/68276.diff


3 Files Affected:

- (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+3) 
- (modified) clang/lib/Sema/SemaChecking.cpp (+10) 
- (added) clang/test/SemaCXX/bitfield-width.c (+30) 


``````````diff
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index e2eaeb885e2ea77..9ffb7baacac22d7 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -6165,6 +6165,9 @@ def warn_signed_bitfield_enum_conversion : Warning<
   "signed bit-field %0 needs an extra bit to represent the largest positive "
   "enumerators of %1">,
   InGroup<BitFieldEnumConversion>, DefaultIgnore;
+def warn_bitfield_too_small_for_integral_type : Warning<
+  "bit-field %0 is not wide enough to store type of %1 ">,
+  InGroup<BitFieldWidth>, DefaultIgnore;
 def note_change_bitfield_sign : Note<
   "consider making the bitfield type %select{unsigned|signed}0">;
 
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 3b0cc154c2e46ab..eb567bc4935e768 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -14300,6 +14300,16 @@ static bool AnalyzeBitFieldAssignment(Sema &S, FieldDecl *Bitfield, Expr *Init,
         S.Diag(WidthExpr->getExprLoc(), diag::note_widen_bitfield)
             << BitsNeeded << ED << WidthExpr->getSourceRange();
       }
+    } else if (OriginalInit->getType()->isIntegralType(S.Context)) {
+      TypeInfo TI = S.Context.getTypeInfo(OriginalInit->getType());
+      if (TI.Width > FieldWidth) {
+        Expr *WidthExpr = Bitfield->getBitWidth();
+        S.Diag(InitLoc, diag::warn_bitfield_too_small_for_integral_type)
+            << Bitfield << OriginalInit->getType();
+        S.Diag(WidthExpr->getExprLoc(), diag::note_widen_bitfield)
+            << TI.Width << OriginalInit->getType()
+            << WidthExpr->getSourceRange();
+      }
     }
 
     return false;
diff --git a/clang/test/SemaCXX/bitfield-width.c b/clang/test/SemaCXX/bitfield-width.c
new file mode 100644
index 000000000000000..69aa2e229869906
--- /dev/null
+++ b/clang/test/SemaCXX/bitfield-width.c
@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 -Wbitfield-width -fsyntax-only -verify %s
+
+typedef struct _xx {
+     int bf:9; // expected-note{{widen this field to 32 bits to store all values of 'int'}}
+     // expected-note at -1{{widen this field to 16 bits to store all values of 'short'}}
+     // expected-note at -2{{widen this field to 64 bits to store all values of 'long'}}
+     // expected-note at -3{{widen this field to 32 bits to store all values of 'int'}}
+ } xx, *pxx; 
+
+ xx vxx;
+
+ void foo1(int x) {     
+     vxx.bf = x; // expected-warning{{bit-field 'bf' is not wide enough to store type of 'int'}}
+ } 
+ void foo2(short x) {     
+     vxx.bf = x; // expected-warning{{bit-field 'bf' is not wide enough to store type of 'short'}}
+ } 
+ void foo3(char x) {     
+     vxx.bf = x;
+ } 
+ void foo4(long x) {     
+     vxx.bf = x; // expected-warning{{bit-field 'bf' is not wide enough to store type of 'long'}}
+ } 
+ void foo5(void * x) {     
+     vxx.bf = (int)x; // expected-warning{{cast to smaller integer type 'int' from 'void *'}}
+     // expected-warning at -1{{bit-field 'bf' is not wide enough to store type of 'int'}}
+ } 
+ int fee(void) {
+     return 0;
+ }

``````````

</details>


https://github.com/llvm/llvm-project/pull/68276