bitfields: add bitoffsetof, bitwidth, isBitfield

changelog/dmd.bitfield-introspection.dd

@@ -0,0 +1,9 @@
+Add Bitfield Introspection Capability
+
+Adds:
+
+* property `.bitoffsetof` to get the starting bit number of a bitfield
+
+* property `.bitwidth` to get the number of bits in a bitfield
+
+* `__traits(isBitfield, symbol)` returns true if a field symbol is a bitfield

compiler/src/dmd/expressionsem.d

@@ -14306,14 +14306,16 @@ private Expression dotIdSemanticPropX(DotIdExp exp, Scope* sc)
 
     if (auto te = exp.e1.isTupleExp())
     {
-        if (exp.ident == Id.offsetof)
+        if (exp.ident == Id.offsetof    ||
+            exp.ident == Id.bitoffsetof ||
+            exp.ident == Id.bitwidth)
         {
             /* 'distribute' the .offsetof to each of the tuple elements.
              */
             auto exps = new Expressions(te.exps.length);
             foreach (i, e; (*te.exps)[])
             {
-                (*exps)[i] = new DotIdExp(e.loc, e, Id.offsetof);
+                (*exps)[i] = new DotIdExp(e.loc, e, exp.ident);
             }
             // Don't evaluate te.e0 in runtime
             Expression e = new TupleExp(exp.loc, null, exps);
@@ -14636,6 +14638,8 @@ Expression dotIdSemanticProp(DotIdExp exp, Scope* sc, bool gag)
                !cfile &&
                 (exp.ident == Id._mangleof ||
                  exp.ident == Id.offsetof ||
+                 exp.ident == Id.bitoffsetof ||
+                 exp.ident == Id.bitwidth ||
                  exp.ident == Id._init ||
                  exp.ident == Id.stringof)
               ))

compiler/src/dmd/frontend.h

@@ -8407,6 +8407,8 @@ struct Id final
     static Identifier* dollar;
     static Identifier* ctfe;
     static Identifier* offset;
+    static Identifier* bitoffsetof;
+    static Identifier* bitwidth;
     static Identifier* ModuleInfo;
     static Identifier* ClassInfo;
     static Identifier* classinfo;
@@ -8698,6 +8700,7 @@ struct Id final
     static Identifier* isAbstractClass;
     static Identifier* isArithmetic;
     static Identifier* isAssociativeArray;
+    static Identifier* isBitfield;
     static Identifier* isFinalClass;
     static Identifier* isTemplate;
     static Identifier* isPOD;

compiler/src/dmd/id.d

@@ -102,6 +102,8 @@ immutable Msgtable[] msgtable =
     { "ctfe", "__ctfe" },
     { "offset" },
     { "offsetof" },
+    { "bitoffsetof" },
+    { "bitwidth" },
     { "ModuleInfo" },
     { "ClassInfo" },
     { "classinfo" },
@@ -455,6 +457,7 @@ immutable Msgtable[] msgtable =
     { "isAbstractClass" },
     { "isArithmetic" },
     { "isAssociativeArray" },
+    { "isBitfield" },
     { "isFinalClass" },
     { "isTemplate" },
     { "isPOD" },

compiler/src/dmd/traits.d

@@ -503,6 +503,17 @@ Expression semanticTraits(TraitsExp e, Scope* sc)
                 sm => sm.isTemplateDeclaration() !is null) != 0;
         });
     }
+    if (e.ident == Id.isBitfield)
+    {
+        if (dim != 1)
+            return dimError(1);
+
+        return isDsymX((s)
+        {
+            s = s.toAlias();
+            return s.isBitFieldDeclaration() !is null;
+        });
+    }
     if (e.ident == Id.isPOD)
     {
         if (dim != 1)

compiler/src/dmd/typesem.d

@@ -4083,7 +4083,9 @@ Expression dotExp(Type mt, Scope* sc, Expression e, Identifier ident, DotExpFlag
         }
         if (v)
         {
-            if (ident == Id.offsetof)
+            if (ident == Id.offsetof ||
+                ident == Id.bitoffsetof ||
+                ident == Id.bitwidth)
             {
                 v.dsymbolSemantic(null);
                 if (v.isField())
@@ -4093,7 +4095,20 @@ Expression dotExp(Type mt, Scope* sc, Expression e, Identifier ident, DotExpFlag
                     ad.size(e.loc);
                     if (ad.sizeok != Sizeok.done)
                         return ErrorExp.get();
-                    return new IntegerExp(e.loc, v.offset, Type.tsize_t);
+                    uint value;
+                    if (ident == Id.offsetof)
+                        value = v.offset;
+                    else // Id.bitoffsetof || Id.bitwidth
+                    {
+                        auto bf = v.isBitFieldDeclaration();
+                        if (bf)
+                        {
+                            value = ident == Id.bitoffsetof ? bf.bitOffset : bf.fieldWidth;
+                        }
+                        else
+                            error(v.loc, "`%s` is not a bitfield, cannot apply `%s`", v.toChars(), ident.toChars());
+                    }
+                    return new IntegerExp(e.loc, value, Type.tsize_t);
                 }
             }
             else if (ident == Id._init)
@@ -4512,6 +4527,8 @@ Expression dotExp(Type mt, Scope* sc, Expression e, Identifier ident, DotExpFlag
             ident != Id._mangleof &&
             ident != Id.stringof &&
             ident != Id.offsetof &&
+            ident != Id.bitoffsetof &&
+            ident != Id.bitwidth &&
             // https://issues.dlang.org/show_bug.cgi?id=15045
             // Don't forward special built-in member functions.
             ident != Id.ctor &&

compiler/test/fail_compilation/bitintro.d

@@ -0,0 +1,38 @@
+/* REQUIRED_ARGS: -preview=bitfields
+ */
+
+struct S
+{
+    int a;
+    int b:5, c:6;
+}
+
+static if (0)
+{
+    pragma(msg, __traits(isBitfield, S.a));
+    pragma(msg, __traits(isBitfield, S.b));
+    pragma(msg, S.b.bitoffsetof);
+    pragma(msg, S.b.bitwidth);
+    pragma(msg, S.c.bitoffsetof);
+    pragma(msg, S.c.bitwidth);
+    pragma(msg, S.a.bitoffsetof);
+    pragma(msg, S.a.bitwidth);
+}
+
+static assert(__traits(isBitfield, S.a) == false);
+static assert(__traits(isBitfield, S.b) == true);
+static assert(S.b.bitoffsetof == 0);
+static assert(S.b.bitwidth == 5);
+static assert(S.c.bitoffsetof == 5);
+static assert(S.c.bitwidth == 6);
+
+/* TEST_OUTPUT:
+---
+fail_compilation/bitintro.d(6): Error: `a` is not a bitfield, cannot apply `bitoffsetof`
+fail_compilation/bitintro.d(37):        while evaluating: `static assert(a.bitoffsetof)`
+fail_compilation/bitintro.d(6): Error: `a` is not a bitfield, cannot apply `bitwidth`
+fail_compilation/bitintro.d(38):        while evaluating: `static assert(a.bitwidth)`
+---
+*/
+static assert(S.a.bitoffsetof);
+static assert(S.a.bitwidth);