util: Avoid waste space when linear alloc'ing large sizes

In the linear allocator, when a size larger than the minimum
buffer size is allocated, we currently create the new buffer
to fit exactly the requested size.

In that case, don't bother updating the `latest` pointer, since
this newly created buffer is already full.  In the worst case,
the current `latest` is full and it would be the same; in the
best case, there's still room for small allocations, so we avoid
wasting space if the next allocations would fit.

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25517>

src/util/tests/linear_test.cpp

@@ -53,3 +53,20 @@ TEST(LinearAlloc, RewriteTail)
 
    ralloc_free(ctx);
 }
+
+TEST(LinearAlloc, AvoidWasteAfterLargeAlloc)
+{
+   void *ctx = ralloc_context(NULL);
+   linear_ctx *lin_ctx = linear_context(ctx);
+
+   char *first = (char *) linear_alloc_child(lin_ctx, 32);
+
+   /* Large allocation that would force a larger buffer. */
+   linear_alloc_child(lin_ctx, 1024 * 16);
+
+   char *second = (char *) linear_alloc_child(lin_ctx, 32);
+
+   EXPECT_EQ(second - first, 32);
+
+   ralloc_free(ctx);
+}