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Re: [Qemu-devel] [PATCH 1/5] BIT_RANGE convenience macro
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH 1/5] BIT_RANGE convenience macro |
Date: |
Thu, 16 Jun 2016 20:05:03 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.0 |
On 16/06/2016 20:01, Peter Maydell wrote:
>> diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
>> index 755fdd1..e411688 100644
>> --- a/include/qemu/bitops.h
>> +++ b/include/qemu/bitops.h
>> @@ -23,6 +23,9 @@
>> #define BIT_MASK(nr) (1UL << ((nr) % BITS_PER_LONG))
>> #define BIT_WORD(nr) ((nr) / BITS_PER_LONG)
>> #define BITS_TO_LONGS(nr) DIV_ROUND_UP(nr, BITS_PER_BYTE *
>> sizeof(long))
>> +/* e.g. BIT_RANGE(15, 0) -> 0xff00 */
>> +#define BIT_RANGE(hb, lb) ((2ull << (hb)) - (1ull << (lb)))
>
> Isn't this undefined behaviour if the hb is 63?
No, these are unsigned values so there's no undefined behavior.
> I prefer a "start, length" macro to "position, position",
> because this matches what we already have for the deposit
> and extract functions in this header.
That's fine too, albeit in the case of this patch the code would be uglier.
Also, if you want a "start, length" mask you could always deposit -1
into 0...
Paolo