Re: [U-Boot] instruction "xchgb" for x86_64

14 Aug 2019

      Hi James,
On Wed, Aug 14, 2019 at 1:08 PM James Wang James.Wang@rohde-schwarz.com
wrote:
...
Hi Simon,
This is James and am working on u-boot to bring back MinGW tool chain for
cross compiling host tools for Windows on Linux. After complete the code, I
plan to generate a patch for community review.
Do you mean cross compiling U-Boot's mkimage tool for Windows on Linux
using gcc-mingw-w64? Do you want to support building U-Boot on Windows as
well?
...
During work, I face an compiling error in file
<arch/x86/include/asm/byteorder.h> with macro *CONFIG_IS_ENABLED*. After
some background study, seems this macro should not be available when
compiling host tools.
After check history, it’s from 909780506773a9b09ae88cb267dbfa6a8d0caa7b on
2017-01-16 22:04:04, Below is the log message

x86: Fix up byteorder.h for x86_64*

Remove the very old x86 code and add support for 64-bit.*

Signed-off-by: Simon Glass <sjg@chromium.org sjg@chromium.org>*

*Reviewed-by: Bin Meng bmeng.cn@gmail.com bmeng.cn@gmail.com*
Below are the relevant changes.
*+#define _constant_swab16(x)
((__u16)(                                             *
*+      (((__u16)(x) & (__u16)0x00ffU) << 8) |
*
*+      (((__u16)(x) & (__u16)0xff00U) >> 8)))*
*+*
*static __inline__ __u16 ___arch__swab16(__u16 x)*
*{*
*+#if CONFIG_IS_ENABLED(X86_64)                          *
*+      return _constant_swab16(x);*
*+#else*

  __asm__("xchgb %b0,%h0"                   /* swap

bytes                */ *

              : "=q" (x) \*

              :  "0" (x)); \*

              return x;*

*+#endif*
*}*
I am not familiar with x86 instructions and not understand the purpose of
this change, is it because instruction “xchgb” not available on 64-bit
architecture or something else?
I can't remember the details of why Simon changed this. xchgb is available
in 64-bit. We probably need revert this change, Simon?
Regards,
Bin