Re: [PATCH 10/27] Replace config-time define HOST_WORDS_BIGENDIAN

[Thomas Huth <thuth@redhat.com>]

On 16/03/2022 12.15, Halil Pasic wrote:
> On Wed, 16 Mar 2022 11:28:59 +0100
> Thomas Huth <thuth@redhat.com> wrote:
> 
>> On 16/03/2022 10.53, marcandre.lureau@redhat.com wrote:
>>> From: Marc-André Lureau <marcandre.lureau@redhat.com>
>>>
>>> Replace a config-time define with a compile time condition
>>> define (compatible with clang and gcc) that must be declared prior to
>>> its usage. This avoids having a global configure time define, but also
>>> prevents from bad usage, if the config header wasn't included before.
>>>
>>> This can help to make some code independent from qemu too.
>>>
>>> gcc supports __BYTE_ORDER__ from about 4.6 and clang from 3.2.
>>>
>>> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
>>> ---
>> [...]
>>> @@ -188,7 +188,7 @@ CPU_CONVERT(le, 64, uint64_t)
>>>     * a compile-time constant if you pass in a constant.  So this can be
>>>     * used to initialize static variables.
>>>     */
>>> -#if defined(HOST_WORDS_BIGENDIAN)
>>> +#if HOST_BIG_ENDIAN
>>>    # define const_le32(_x)                          \
>>>        ((((_x) & 0x000000ffU) << 24) |              \
>>>         (((_x) & 0x0000ff00U) <<  8) |              \
>>> @@ -211,7 +211,7 @@ typedef union {
>>>    
>>>    typedef union {
>>>        float64 d;
>>> -#if defined(HOST_WORDS_BIGENDIAN)
>>> +#if HOST_BIG_ENDIAN
>>>        struct {
>>>            uint32_t upper;
>>>            uint32_t lower;
>>> @@ -235,7 +235,7 @@ typedef union {
>>>    
>>>    typedef union {
>>>        float128 q;
>>> -#if defined(HOST_WORDS_BIGENDIAN)
>>> +#if HOST_BIG_ENDIAN
>>>        struct {
>>>            uint32_t upmost;
>>>            uint32_t upper;
>>> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
>>> index 0a5e67fb970e..7fdd88adb368 100644
>>> --- a/include/qemu/compiler.h
>>> +++ b/include/qemu/compiler.h
>>> @@ -7,6 +7,8 @@
>>>    #ifndef COMPILER_H
>>>    #define COMPILER_H
>>>    
>>> +#define HOST_BIG_ENDIAN (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)
>>
>> Why don't you do it this way instead:
>>
>> #if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
>> #define HOST_WORDS_BIGENDIAN 1
>> #endif
>>
>> ... that way you could avoid the churn in all the other files?
>>
> 
> I guess "prevents from bad usage, if the config header wasn't included
> before" from the commit message is the answer to that question. I agree
> that it is more robust. If we keep the #if defined we really can't
> differentiate between "not defined because not big-endian" and "not
> defined because the appropriate header was not included."
> 

Ok, fair point, now I got it.

Acked-by: Thomas Huth <thuth@redhat.com>