[PATCH v5 3/3] mm: Avoid complex heap growth math in hot path

[Zhang Boyang]

We do a lot of math about heap growth in hot path of grub_memalign().
However, the result is only used if out of memory is encountered, which
is seldom.

This patch moves these calculations away from hot path. These
calculations is now only done if out of memory is encountered. This
change can also help compiler to optimize interger overflow checks away.

Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
---
 grub-core/kern/mm.c | 34 ++++++++++++++++++++--------------
 1 file changed, 20 insertions(+), 14 deletions(-)

diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
index c3d96e2b0..94eb29108 100644
--- a/grub-core/kern/mm.c
+++ b/grub-core/kern/mm.c
@@ -467,20 +467,7 @@ grub_memalign (grub_size_t align, grub_size_t size)
   if (size > ~(grub_size_t) align)
     goto fail;
 
-  /*
-   * Pre-calculate the necessary size of heap growth (if applicable),
-   * with region management overhead taken into account.
-   */
-  if (grub_add (size + align, GRUB_MM_MGMT_OVERHEAD, &grow))
-    goto fail;
-
-  /* Preallocate some extra space if heap growth is small. */
-  grow = grub_max (grow, GRUB_MM_HEAP_GROW_EXTRA);
-
-  /* Align up heap growth to make it friendly to CPU/MMU. */
-  if (grow > ~(grub_size_t) (GRUB_MM_HEAP_GROW_ALIGN - 1))
-    goto fail;
-  grow = ALIGN_UP (grow, GRUB_MM_HEAP_GROW_ALIGN);
+  grow = size + align;
 
   /* We currently assume at least a 32-bit grub_size_t,
      so limiting allocations to <adress space size> - 1MiB
@@ -510,6 +497,25 @@ grub_memalign (grub_size_t align, grub_size_t size)
       /* Request additional pages, contiguous */
       count++;
 
+      /*
+       * Calculate the necessary size of heap growth (if applicable),
+       * with region management overhead taken into account.
+       */
+      if (grub_add (grow, GRUB_MM_MGMT_OVERHEAD, &grow))
+       goto fail;
+
+      /* Preallocate some extra space if heap growth is small. */
+      grow = grub_max (grow, GRUB_MM_HEAP_GROW_EXTRA);
+
+      /* Align up heap growth to make it friendly to CPU/MMU. */
+      if (grow > ~(grub_size_t) (GRUB_MM_HEAP_GROW_ALIGN - 1))
+       goto fail;
+      grow = ALIGN_UP (grow, GRUB_MM_HEAP_GROW_ALIGN);
+
+      /* Do the same sanity check again. */
+      if (grow > ~(grub_size_t) 0x100000)
+       goto fail;
+
       if (grub_mm_add_region_fn != NULL &&
           grub_mm_add_region_fn (grow, GRUB_MM_ADD_REGION_CONSECUTIVE) == 
GRUB_ERR_NONE)
        goto again;
-- 
2.30.2