[U-Boot] instruction "xchgb" for x86_64

James Wang James.Wang at rohde-schwarz.com
Wed Aug 14 05:08:49 UTC 2019

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.

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 at chromium.org<mailto:sjg at chromium.org>>
Reviewed-by: Bin Meng bmeng.cn at gmail.com<mailto:bmeng.cn at 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)
+      return _constant_swab16(x);
       __asm__("xchgb %b0,%h0"                   /* swap bytes                */ \
                   : "=q" (x) \
                   :  "0" (x)); \
                   return x;

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?


Best regards
James Wang Jinmin

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