[Qemu-devel] [RFC PATCH] utils: Add up_pow_of_two()

[Qemu-devel] [RFC PATCH] utils: Add up_pow_of_two()

[Alexey Kardashevskiy]

This adds a helper to get closest bigger power-of-two value.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---

It is a log2 of round up to power of two. Is there anything like this
in QEMU already? Any better name than up_pow_of_two()?

I'll use this little helper later to calculate a DMA window shift as
POWERPC PAPR architecture operates with shifts rather than bytes or TCE entries.

Thanks!


---
 include/qemu/host-utils.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/qemu/host-utils.h b/include/qemu/host-utils.h
index d4f21c9..b1ad164 100644
--- a/include/qemu/host-utils.h
+++ b/include/qemu/host-utils.h
@@ -361,6 +361,17 @@ static inline int ctpop64(uint64_t val)
 #endif
 }
 
+static inline int up_pow_of_two(uint64_t x)
+{
+    int m = 63 - clz64(x);
+
+    if ((1ULL << m) == x) {
+        return m;
+    }
+
+    return m + 1;
+}
+
 /* Host type specific sizes of these routines.  */
 
 #if ULONG_MAX == UINT32_MAX
-- 
2.0.0

Re: [Qemu-devel] [RFC PATCH] utils: Add up_pow_of_two()

[Peter Maydell]

On 23 February 2015 at 16:49, Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
> This adds a helper to get closest bigger power-of-two value.
>
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
>
> It is a log2 of round up to power of two. Is there anything like this
> in QEMU already? Any better name than up_pow_of_two()?

Maybe separate out the "round up to power of 2" and "log2" parts
into separate functions? If you do that then you could call the
first one of those pow2ceil(), to match our existing
pow2floor().

-- PMM

Re: [Qemu-devel] [RFC PATCH] utils: Add up_pow_of_two()

[Alexey Kardashevskiy]

On 02/23/2015 07:54 PM, Peter Maydell wrote:
> On 23 February 2015 at 16:49, Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
>> This adds a helper to get closest bigger power-of-two value.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> ---
>>
>> It is a log2 of round up to power of two. Is there anything like this
>> in QEMU already? Any better name than up_pow_of_two()?
>
> Maybe separate out the "round up to power of 2" and "log2" parts
> into separate functions? If you do that then you could call the
> first one of those pow2ceil(), to match our existing
> pow2floor().


good point, powe2ceil() is enough for me, I'll repost it in a sec. thanks!


-- 
Alexey