[Sema] Add check for bitfield assignments to integral types

This change introduces the bitfield conversion check after fixes to the
test case. The original submission unfortunately did not comprehend
differences in architecture for the diagnostic messages, leading to
unanticipated failures in the arm build bots.

Original PR: llvm/llvm-project#68276

Clang does not check for bitfield assignment widths, while gcc checks 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 change simply adds this check for integral types under the
-Wbitfield-conversion compiler option.

Author: vabridgers

clang/docs/ReleaseNotes.rst

@@ -185,6 +185,9 @@ New Compiler Flags
   the preprocessed text to the output. This can greatly reduce the size of the
   preprocessed output, which can be helpful when trying to reduce a test case.
 
+* ``-Wbitfield-conversion`` was added to detect assignments of integral
+  types to a bitfield that may change the value.
+
 Deprecated Compiler Flags
 -------------------------
 

clang/include/clang/Basic/DiagnosticGroups.td

@@ -53,6 +53,7 @@ def SingleBitBitFieldConstantConversion :
 def BitFieldConstantConversion : DiagGroup<"bitfield-constant-conversion",
                                            [SingleBitBitFieldConstantConversion]>;
 def BitFieldEnumConversion : DiagGroup<"bitfield-enum-conversion">;
+def BitFieldConversion : DiagGroup<"bitfield-conversion">;
 def BitFieldWidth : DiagGroup<"bitfield-width">;
 def CompoundTokenSplitByMacro : DiagGroup<"compound-token-split-by-macro">;
 def CompoundTokenSplitBySpace : DiagGroup<"compound-token-split-by-space">;
@@ -933,6 +934,7 @@ def Conversion : DiagGroup<"conversion",
                             ConstantConversion,
                             EnumConversion,
                             BitFieldEnumConversion,
+                            BitFieldConversion,
                             FloatConversion,
                             Shorten64To32,
                             IntConversion,

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6171,6 +6171,9 @@ 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<
+  "conversion from %2 (%3 bits) to bit-field %0 (%1 bits) may change value">,
+  InGroup<BitFieldConversion>, DefaultIgnore;
 def note_change_bitfield_sign : Note<
   "consider making the bitfield type %select{unsigned|signed}0">;
 

clang/lib/Sema/SemaChecking.cpp

@@ -14298,6 +14298,18 @@ 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)) {
+      IntRange LikelySourceRange =
+          GetExprRange(S.Context, Init, S.isConstantEvaluatedContext(),
+                       /*Approximate=*/true);
+
+      if (LikelySourceRange.Width > FieldWidth) {
+        Expr *WidthExpr = Bitfield->getBitWidth();
+        S.Diag(InitLoc, diag::warn_bitfield_too_small_for_integral_type)
+            << Bitfield << FieldWidth << OriginalInit->getType()
+            << LikelySourceRange.Width;
+        S.Diag(WidthExpr->getExprLoc(), diag::note_declared_at);
+      }
     }
 
     return false;
@@ -15195,7 +15207,6 @@ static void CheckImplicitConversion(Sema &S, Expr *E, QualType T,
 
   if (LikelySourceRange.Width > TargetRange.Width) {
     // If the source is a constant, use a default-on diagnostic.
-    // TODO: this should happen for bitfield stores, too.
     Expr::EvalResult Result;
     if (E->EvaluateAsInt(Result, S.Context, Expr::SE_AllowSideEffects,
                          S.isConstantEvaluatedContext())) {

clang/test/SemaCXX/bitfield-width.c

@@ -0,0 +1,42 @@
+// RUN: %clang_cc1 -Wconversion -fsyntax-only -verify %s
+// RUN: %clang_cc1 -Wbitfield-conversion -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple armebv7-unknown-linux -Wbitfield-conversion \
+// RUN:     -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple arm64-unknown-linux  -Wbitfield-conversion \
+// RUN:     -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple arm-unknown-linux -Wbitfield-conversion \
+// RUN:     -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple aarch64-unknown-linux -Wbitfield-conversion \
+// RUN:     -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple mipsel-unknown-linux -Wbitfield-conversion \
+// RUN:     -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple mips64el-unknown-linux -Wbitfield-conversion \
+// RUN:     -fsyntax-only -verify %s
+
+typedef struct _xx {
+     int bf:9; // expected-note 3{{declared here}}
+ } xx, *pxx; 
+
+ xx vxx;
+
+ void foo1(int x) {     
+     vxx.bf = x; // expected-warning{{conversion from 'int' (32 bits) to bit-field 'bf' (9 bits) may change value}}
+ } 
+ void foo2(short x) {     
+     vxx.bf = x; // expected-warning{{conversion from 'short' (16 bits) to bit-field 'bf' (9 bits) may change value}}
+ } 
+ void foo3(char x) {     
+     vxx.bf = x; // no warning expected
+ } 
+ void foo4(short x) {     
+     vxx.bf = 0xff & x; // no warning expected 
+ } 
+ void foo5(short x) {     
+     vxx.bf = 0x1ff & x; // no warning expected 
+ } 
+ void foo6(short x) {     
+     vxx.bf = 0x3ff & x; // expected-warning{{conversion from 'int' (10 bits) to bit-field 'bf' (9 bits) may change value}}
+ } 
+ int fee(void) {
+     return 0;
+ }