Support large alignment requests

The linux kernel has some structures that are page aligned,
i.e. 4096.  Instead of enlarging the bit fields to specify this,
use the fact that alignment is always power of two, and store only
the log2 minus 1 of it.  The 5 bits are enough to specify an alignment
of 1 << 30.

tcc.h

@@ -379,7 +379,7 @@ typedef struct SValue {
 struct Attribute {
     unsigned
         func_call     : 3, /* calling convention (0..5), see below */
-        aligned       : 5, /* alignement (0..16) */
+        aligned       : 5, /* alignment as log2+1 (0 == unspecified) */
         packed        : 1,
         func_export   : 1,
         func_import   : 1,

tccgen.c

@@ -3062,6 +3062,24 @@ ST_FUNC void parse_mult_str (CString *astr, const char *msg)
     cstr_ccat(astr, '\0');
 }
 
+/* If I is >= 1 and a power of two, returns log2(i)+1.
+   If I is 0 returns 0.  */
+static int exact_log2p1(int i)
+{
+  int ret;
+  if (!i)
+    return 0;
+  for (ret = 1; i >= 1 << 8; ret += 8)
+    i >>= 8;
+  if (i >= 1 << 4)
+    ret += 4, i >>= 4;
+  if (i >= 1 << 2)
+    ret += 2, i >>= 2;
+  if (i >= 1 << 1)
+    ret++;
+  return ret;
+}
+
 /* Parse GNUC __attribute__ extension. Currently, the following
    extensions are recognized:
    - aligned(n) : set data/function alignment.
@@ -3131,7 +3149,9 @@ static void parse_attribute(AttributeDef *ad)
             } else {
                 n = MAX_ALIGN;
             }
-            ad->a.aligned = n;
+            ad->a.aligned = exact_log2p1(n);
+	    if (n != 1 << (ad->a.aligned - 1))
+	      tcc_error("alignment of %d is larger than implemented", n);
             break;
         case TOK_PACKED1:
         case TOK_PACKED2:
@@ -3270,7 +3290,7 @@ static void struct_layout(CType *type, AttributeDef *ad)
     int pcc = !tcc_state->ms_bitfields;
     Sym *f;
     if (ad->a.aligned)
-      maxalign = ad->a.aligned;
+      maxalign = 1 << (ad->a.aligned - 1);
     else
       maxalign = 1;
     offset = 0;
@@ -3293,6 +3313,8 @@ static void struct_layout(CType *type, AttributeDef *ad)
 	    align = f->r;
 	} else if (ad->a.packed || f->r == 1) {
 	    align = 1;
+	    /* Packed fields or packed records don't let the base type
+	       influence the records type alignment.  */
 	    typealign = 1;
 	} else {
 	    align = typealign;
@@ -3587,8 +3609,9 @@ static void struct_decl(CType *type, AttributeDef *ad, int u)
 		    /* Only remember non-default alignment.  */
 		    alignoverride = 0;
                     if (ad1.a.aligned) {
-                        if (align < ad1.a.aligned)
+			int speca = 1 << (ad1.a.aligned - 1);
-                            alignoverride = ad1.a.aligned;
+                        if (align < speca)
+                            alignoverride = speca;
                     } else if (ad1.a.packed || ad->a.packed) {
                         alignoverride = 1;
                     } else if (*tcc_state->pack_stack_ptr) {
@@ -6594,8 +6617,9 @@ static void decl_initializer_alloc(CType *type, AttributeDef *ad, int r,
 	        * pointed_size(&flexible_array->type);
     /* take into account specified alignment if bigger */
     if (ad->a.aligned) {
-        if (ad->a.aligned > align)
+	int speca = 1 << (ad->a.aligned - 1);
-            align = ad->a.aligned;
+        if (speca > align)
+            align = speca;
     } else if (ad->a.packed) {
         align = 1;
     }

tests/tcctest.c

@@ -1048,6 +1048,11 @@ int pad1;
    a copy of it).
 struct aligntest7 altest7[2] __attribute__((aligned(16)));*/
 
+struct aligntest8
+{
+  int i;
+} __attribute__((aligned(4096)));
+
 struct Large {
     unsigned long flags;
     union {
@@ -1129,6 +1134,8 @@ void struct_test()
            sizeof(struct aligntest6), __alignof__(struct aligntest6));
     printf("aligntest7 sizeof=%d alignof=%d\n",
            sizeof(struct aligntest7), __alignof__(struct aligntest7));
+    printf("aligntest8 sizeof=%d alignof=%d\n",
+           sizeof(struct aligntest8), __alignof__(struct aligntest8));
     printf("altest5 sizeof=%d alignof=%d\n",
            sizeof(altest5), __alignof__(altest5));
     printf("altest6 sizeof=%d alignof=%d\n",