[stable] Prepare TargetC for ARM bit-fields special case

compiler/src/dmd/dsymbolsem.d

@@ -7186,12 +7186,19 @@
             error(bfd.loc, "bit field width %d is larger than type", bfd.fieldWidth);
 
         const style = target.c.bitFieldStyle;
+        if (style != TargetC.BitFieldStyle.MS && style != TargetC.BitFieldStyle.Gcc_Clang)
+            assert(0, "unsupported bit-field style");
+
+        const isMicrosoftStyle = style == TargetC.BitFieldStyle.MS;
+        const contributesToAggregateAlignment = target.c.contributesToAggregateAlignment(bfd);
 
         void startNewField()
         {
             if (log) printf("startNewField()\n");
             uint alignsize;
-            if (style == TargetC.BitFieldStyle.Gcc_Clang)
+            if (isMicrosoftStyle)
+               alignsize = memsize; // not memalignsize
+            else
             {
                 if (bfd.fieldWidth > 32)
                     alignsize = memalignsize;
@@ -7202,15 +7209,13 @@
                 else
                     alignsize = 1;
             }
-            else
-                alignsize = memsize; // not memalignsize
 
             uint dummy;
             bfd.offset = placeField(
                 fieldState.offset,
                 memsize, alignsize, bfd.alignment,
                 ad.structsize,
-                (anon && style == TargetC.BitFieldStyle.Gcc_Clang) ? dummy : ad.alignsize,
+                contributesToAggregateAlignment ? ad.alignsize : dummy,
                 isunion);
 
             fieldState.inFlight = true;
@@ -7219,53 +7224,38 @@
             fieldState.fieldSize = memsize;
         }
 
-        if (style == TargetC.BitFieldStyle.Gcc_Clang)
+        if (ad.alignsize == 0)
+            ad.alignsize = 1;
+        if (!isMicrosoftStyle && contributesToAggregateAlignment && ad.alignsize < memalignsize)
+            ad.alignsize = memalignsize;
+
+        if (bfd.fieldWidth == 0)
         {
-            if (bfd.fieldWidth == 0)
+            if (!isMicrosoftStyle && !isunion)
             {
-                if (!isunion)
-                {
-                    // Use type of zero width field to align to next field
-                    fieldState.offset = (fieldState.offset + memalignsize - 1) & ~(memalignsize - 1);
-                    ad.structsize = fieldState.offset;
-                }
-
-                fieldState.inFlight = false;
-                return;
+                // Use type of zero width field to align to next field
+                fieldState.offset = (fieldState.offset + memalignsize - 1) & ~(memalignsize - 1);
+                ad.structsize = fieldState.offset;
             }
-
-            if (ad.alignsize == 0)
-                ad.alignsize = 1;
-            if (!anon &&
-                  ad.alignsize < memalignsize)
-                ad.alignsize = memalignsize;
-        }
-        else if (style == TargetC.BitFieldStyle.MS)
-        {
-            if (ad.alignsize == 0)
-                ad.alignsize = 1;
-            if (bfd.fieldWidth == 0)
+            else if (isMicrosoftStyle && fieldState.inFlight && !isunion)
             {
-                if (fieldState.inFlight && !isunion)
-                {
-                    // documentation says align to next int
-                    //const alsz = cast(uint)Type.tint32.size();
-                    const alsz = memsize; // but it really does this
-                    fieldState.offset = (fieldState.offset + alsz - 1) & ~(alsz - 1);
-                    ad.structsize = fieldState.offset;
-                }
-
-                fieldState.inFlight = false;
-                return;
+                // documentation says align to next int
+                //const alsz = cast(uint)Type.tint32.size();
+                const alsz = memsize; // but it really does this
+                fieldState.offset = (fieldState.offset + alsz - 1) & ~(alsz - 1);
+                ad.structsize = fieldState.offset;
             }
+
+            fieldState.inFlight = false;
+            return;
         }
 
         if (!fieldState.inFlight)
         {
             //printf("not in flight\n");
             startNewField();
         }
-        else if (style == TargetC.BitFieldStyle.Gcc_Clang)
+        else if (!isMicrosoftStyle)
         {
             // If the bit-field spans more units of alignment than its type,
             // start a new field at the next alignment boundary.
@@ -7288,7 +7278,7 @@
                 }
             }
         }
-        else if (style == TargetC.BitFieldStyle.MS)
+        else
         {
             if (memsize != fieldState.fieldSize ||
                 fieldState.bitOffset + bfd.fieldWidth > fieldState.fieldSize * 8)
@@ -7297,14 +7287,14 @@
                 startNewField();
             }
         }
-        else
-            assert(0);
 
         bfd.offset = fieldState.fieldOffset;
         bfd.bitOffset = fieldState.bitOffset;
 
         const pastField = bfd.bitOffset + bfd.fieldWidth;
-        if (style == TargetC.BitFieldStyle.Gcc_Clang)
+        if (isMicrosoftStyle)
+            fieldState.fieldSize = memsize;
+        else
         {
             auto size = (pastField + 7) / 8;
             fieldState.fieldSize = size;
@@ -7318,8 +7308,6 @@
             else
                 ad.structsize = bfd.offset + size;
         }
-        else
-            fieldState.fieldSize = memsize;
         //printf("at end: ad.structsize = %d\n", cast(int)ad.structsize);
         //print(fieldState);
 

compiler/src/dmd/frontend.h

@@ -5950,6 +5950,7 @@ struct TargetC final
     uint8_t wchar_tsize;
     Runtime runtime;
     BitFieldStyle bitFieldStyle;
+    bool contributesToAggregateAlignment(BitFieldDeclaration* bfd);
     TargetC() :
         crtDestructorsSupported(true),
         boolsize(),

compiler/src/dmd/target.d

@@ -1360,6 +1360,8 @@
  */
 struct TargetC
 {
+    import dmd.declaration : BitFieldDeclaration;
+
     enum Runtime : ubyte
     {
         Unspecified,
@@ -1449,6 +1451,24 @@
             crtDestructorsSupported = false;
         }
     }
+
+    /**
+     * Indicates whether the specified bit-field contributes to the alignment
+     * of the containing aggregate.
+     * E.g., (not all) ARM ABIs do NOT ignore anonymous (incl. 0-length)
+     * bit-fields.
+     */
+    extern (C++) bool contributesToAggregateAlignment(BitFieldDeclaration bfd)
+    {
+        if (bitFieldStyle == BitFieldStyle.MS)
+            return true;
+        if (bitFieldStyle == BitFieldStyle.Gcc_Clang)
+        {
+            // sufficient for DMD's currently supported architectures
+            return !bfd.isAnonymous();
+        }
+        assert(0);
+    }
 }
 
 ////////////////////////////////////////////////////////////////////////////////

compiler/src/dmd/target.h

@@ -16,6 +16,7 @@
 #include "globals.h"
 #include "tokens.h"
 
+class BitFieldDeclaration;
 class ClassDeclaration;
 class Dsymbol;
 class Expression;
@@ -77,6 +78,8 @@ struct TargetC
     uint8_t wchar_tsize;         // size of a C 'wchar_t' type
     Runtime runtime;
     BitFieldStyle bitFieldStyle; // different C compilers do it differently
+
+    bool contributesToAggregateAlignment(BitFieldDeclaration *bfd);
 };
 
 struct TargetCPP