Merge pull request #19339 from arjunroy/maybe_embiggen_v2

Reduced instruction count for maybe_embiggen common case.

src/core/lib/slice/slice_buffer.cc

@@ -32,36 +32,46 @@
 /* grow a buffer; requires GRPC_SLICE_BUFFER_INLINE_ELEMENTS > 1 */
 #define GROW(x) (3 * (x) / 2)
 
-static void maybe_embiggen(grpc_slice_buffer* sb) {
-  if (sb->count == 0) {
-    sb->slices = sb->base_slices;
-  }
-
-  /* How far away from sb->base_slices is sb->slices pointer */
-  size_t slice_offset = static_cast<size_t>(sb->slices - sb->base_slices);
-  size_t slice_count = sb->count + slice_offset;
-
-  if (slice_count == sb->capacity) {
-    if (sb->base_slices != sb->slices) {
+/* Typically, we do not actually need to embiggen (by calling
+ * memmove/malloc/realloc) - only if we were up against the full capacity of the
+ * slice buffer. If do_embiggen is inlined, the compiler clobbers multiple
+ * registers pointlessly in the common case. */
+static void GPR_ATTRIBUTE_NOINLINE do_embiggen(grpc_slice_buffer* sb,
+                                               const size_t slice_count,
+                                               const size_t slice_offset) {
+  if (slice_offset != 0) {
     /* Make room by moving elements if there's still space unused */
     memmove(sb->base_slices, sb->slices, sb->count * sizeof(grpc_slice));
     sb->slices = sb->base_slices;
   } else {
     /* Allocate more memory if no more space is available */
-      sb->capacity = GROW(sb->capacity);
-      GPR_ASSERT(sb->capacity > slice_count);
+    const size_t new_capacity = GROW(sb->capacity);
+    sb->capacity = new_capacity;
     if (sb->base_slices == sb->inlined) {
       sb->base_slices = static_cast<grpc_slice*>(
-            gpr_malloc(sb->capacity * sizeof(grpc_slice)));
+          gpr_malloc(new_capacity * sizeof(grpc_slice)));
       memcpy(sb->base_slices, sb->inlined, slice_count * sizeof(grpc_slice));
     } else {
       sb->base_slices = static_cast<grpc_slice*>(
-            gpr_realloc(sb->base_slices, sb->capacity * sizeof(grpc_slice)));
+          gpr_realloc(sb->base_slices, new_capacity * sizeof(grpc_slice)));
     }
 
     sb->slices = sb->base_slices + slice_offset;
   }
 }
+
+static void maybe_embiggen(grpc_slice_buffer* sb) {
+  if (sb->count == 0) {
+    sb->slices = sb->base_slices;
+    return;
+  }
+
+  /* How far away from sb->base_slices is sb->slices pointer */
+  size_t slice_offset = static_cast<size_t>(sb->slices - sb->base_slices);
+  size_t slice_count = sb->count + slice_offset;
+  if (GPR_UNLIKELY(slice_count == sb->capacity)) {
+    do_embiggen(sb, slice_count, slice_offset);
+  }
 }
 
 void grpc_slice_buffer_init(grpc_slice_buffer* sb) {