i#3699: Change HEAP_ALIGNMENT to 8 on 32-bit ARM. (#7175)

Some C++ library functions require 8-byte alignment. This made the test
drcacheoff.simple pass on 32-bit ARM.

Issue: #3699

Author: egrimley-arm

core/heap.c

@@ -105,7 +105,8 @@ static const uint BLOCK_SIZES[] = {
     8, /* for instr bits */
 #ifndef X64
     /* for x64 future_fragment_t is 24 bytes (could be 20 if we could put flags last) */
-    sizeof(future_fragment_t), /* 12 (24 x64) */
+    ALIGN_FORWARD(sizeof(future_fragment_t), /* 12 (24 x64) */
+                  HEAP_ALIGNMENT),
 #endif
     /* we have a lot of size 16 requests for IR but they are transient */
     24, /* fcache empties and vm_area_t are now 20, vm area extras still 24 */
@@ -119,8 +120,9 @@ static const uint BLOCK_SIZES[] = {
     sizeof(instr_t), /* 112 x64 */
 #    endif
 #else
-    sizeof(fragment_t) + sizeof(direct_linkstub_t) +
-        sizeof(cbr_fallthrough_linkstub_t), /* 60 dbg / 56 rel */
+    ALIGN_FORWARD(sizeof(fragment_t) + sizeof(direct_linkstub_t) +
+                      sizeof(cbr_fallthrough_linkstub_t), /* 60 dbg / 56 rel */
+                  HEAP_ALIGNMENT),
 #    ifndef DEBUG
     sizeof(instr_t), /* 72 */
 #    endif
@@ -156,11 +158,21 @@ DECLARE_NEVERPROT_VAR(static int block_peak_align_pad[BLOCK_TYPES], { 0 });
 DECLARE_NEVERPROT_VAR(static bool out_of_vmheap_once, false);
 #endif
 
-/* variable-length: we steal one int for the size */
-#define HEADER_SIZE (sizeof(size_t))
+/* The size of a variable-size allocation is stored as a size_t in the header.
+ * On 32-bit ARM, HEAP_ALIGNMENT is twice the size of a size_t but the wasted
+ * space for DR's own use is not expected to be significant as only allocs
+ * that do not fit in the buckets use headers. Client heap allocations
+ * probably bear the biggest hit.
+ */
+#define HEADER_SIZE HEAP_ALIGNMENT
 /* VARIABLE_SIZE is assignable */
 #define VARIABLE_SIZE(p) (*(size_t *)((p)-HEADER_SIZE))
-#define MEMSET_HEADER(p, value) VARIABLE_SIZE(p) = HEAP_TO_PTR_UINT(value)
+#define MEMSET_HEADER(p, value)                                             \
+    do {                                                                    \
+        ASSERT(HEADER_SIZE % sizeof(VARIABLE_SIZE(p)) == 0);                \
+        for (size_t k = 0; k < HEADER_SIZE / sizeof(VARIABLE_SIZE(p)); k++) \
+            (&VARIABLE_SIZE(p))[k] = HEAP_TO_PTR_UINT(value);               \
+    } while (0)
 #define GET_VARIABLE_ALLOCATION_SIZE(p) (VARIABLE_SIZE(p) + HEADER_SIZE)
 
 /* The heap is allocated in units.

core/heap.h

@@ -93,7 +93,11 @@ typedef enum {
 } map_flags_t;
 
 typedef byte *heap_pc;
-#define HEAP_ALIGNMENT sizeof(heap_pc *)
+#ifdef ARM
+#    define HEAP_ALIGNMENT 8 /* Some C++ functions require 8-byte alignment on ARM. */
+#else
+#    define HEAP_ALIGNMENT sizeof(heap_pc *)
+#endif
 extern vm_area_vector_t *landing_pad_areas;
 
 #ifdef X64

core/loader_shared.c

@@ -940,7 +940,13 @@ redirect_malloc(size_t size)
     /* We need extra space to store the size and alignment bit and ensure the returned
      * pointer is aligned.
      */
+#ifdef ARM
+    size_t alloc_size = size + STANDARD_HEAP_ALIGNMENT;
+    ASSERT(HEAP_ALIGNMENT == STANDARD_HEAP_ALIGNMENT);
+#else
     size_t alloc_size = size + sizeof(size_t) + STANDARD_HEAP_ALIGNMENT - HEAP_ALIGNMENT;
+    ASSERT(HEAP_ALIGNMENT * 2 == STANDARD_HEAP_ALIGNMENT);
+#endif
     /* Our header is the size itself, with the top bit stolen to indicate alignment. */
     if (TEST(REDIRECT_HEADER_SHIFTED, alloc_size)) {
         /* We do not support the top bit being set as that conflicts with the bit in
@@ -958,7 +964,6 @@ redirect_malloc(size_t size)
     ptr_uint_t res =
         ALIGN_FORWARD((ptr_uint_t)mem + sizeof(size_t), STANDARD_HEAP_ALIGNMENT);
     size_t header = alloc_size;
-    ASSERT(HEAP_ALIGNMENT * 2 == STANDARD_HEAP_ALIGNMENT);
     ASSERT(!TEST(REDIRECT_HEADER_SHIFTED, header));
     if (res == (ptr_uint_t)mem + sizeof(size_t)) {
         /* Already aligned. */