The __FILE__ macro is used everywhere in U-Boot to locate the file
printing the log message, such as WARN_ON(), etc.  If U-Boot is
built out of tree, this can be a long absolute path.

This is because Kbuild runs in the objtree instead of the srctree,
then __FILE__ is expanded to a file path prefixed with $(srctree)/.

A brand-new option from GCC, -fmacro-prefix-map, solves this problem.
If your compiler supports it, __FILE__ is the relative path from the
srctree regardless of O= option.  This provides more readable log,
more reproducible builds, and smaller image size.

[ Linux commit: a73619a845d5625079cc1b3b820f44c899618388 ]

Signed-off-by: Masahiro Yamada <>

GCC 8 is required to use this new option.

 Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Makefile b/Makefile
index 5cfe43f..243add1 100644
--- a/Makefile
+++ b/Makefile
@@ -595,6 +595,9 @@ endif
 KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector)
 KBUILD_CFLAGS += $(call cc-option,-fno-delete-null-pointer-checks)
+# change __FILE__ to the relative path from the srctree
+KBUILD_CFLAGS  += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
 # $(KBUILD_AFLAGS) sets -g, which causes gcc to pass a suitable -g<format>
 # option to the assembler.

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