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

[Halil Pasic <pasic@linux.ibm.com>]

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."