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[Qemu-devel] [PULL 17/25] osdep: Document differences in rounding macros


From: Paolo Bonzini
Subject: [Qemu-devel] [PULL 17/25] osdep: Document differences in rounding macros
Date: Tue, 2 Aug 2016 21:39:27 +0200

From: Eric Blake <address@hidden>

Make it obvious which macros are safe in which situations.

Useful since QEMU_ALIGN_UP and ROUND_UP both purport to do
the same thing, but differ on whether the alignment must be
a power of 2.

Signed-off-by: Eric Blake <address@hidden>
Message-Id: <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
---
 include/qemu/osdep.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index d7c111d..9e9fa61 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -158,7 +158,8 @@ extern int daemon(int, int);
 /* Round number down to multiple */
 #define QEMU_ALIGN_DOWN(n, m) ((n) / (m) * (m))
 
-/* Round number up to multiple */
+/* Round number up to multiple. Safe when m is not a power of 2 (see
+ * ROUND_UP for a faster version when a power of 2 is guaranteed) */
 #define QEMU_ALIGN_UP(n, m) QEMU_ALIGN_DOWN((n) + (m) - 1, (m))
 
 /* Check if n is a multiple of m */
@@ -175,6 +176,9 @@ extern int daemon(int, int);
 /* Check if pointer p is n-bytes aligned */
 #define QEMU_PTR_IS_ALIGNED(p, n) QEMU_IS_ALIGNED((uintptr_t)(p), (n))
 
+/* Round number up to multiple. Requires that d be a power of 2 (see
+ * QEMU_ALIGN_UP for a safer but slower version on arbitrary
+ * numbers) */
 #ifndef ROUND_UP
 #define ROUND_UP(n,d) (((n) + (d) - 1) & -(d))
 #endif
-- 
2.7.4





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