linux-kernel - [PATCH] Makefile: evaluate LDFLAGS_BUILD

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Date:   Sun, 30 Apr 2017 18:16:00 +0200
From:   Rabin Vincent <rabin@....in>
To:     yamada.masahiro@...ionext.com, mmarek@...e.com
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Rabin Vincent <rabin@....in>
Subject: [PATCH] Makefile: evaluate LDFLAGS_BUILD_ID only once

Evaluate LDFLAGS_BUILD_ID (which involves invoking the compiler) only
once instead of over and over.

This provides a ~20% reduction in null build time with x86 allnoconfig:

$ make allnoconfig && make -j8
$ perf stat -r5 -e sched:sched_process_exec make -j8
-       2 119      sched:sched_process_exec
+       1 878      sched:sched_process_exec

-       1,238817018 seconds time elapsed
+       0,971020553 seconds time elapsed

Signed-off-by: Rabin Vincent <rabin@....in>
---
 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 779302695453..c78a266594c1 100644
--- a/Makefile
+++ b/Makefile
@@ -815,7 +815,7 @@ KBUILD_AFLAGS   += $(ARCH_AFLAGS)   $(KAFLAGS)
 KBUILD_CFLAGS   += $(ARCH_CFLAGS)   $(KCFLAGS)
 
 # Use --build-id when available.
-LDFLAGS_BUILD_ID = $(patsubst -Wl$(comma)%,%,\
+LDFLAGS_BUILD_ID := $(patsubst -Wl$(comma)%,%,\
 			      $(call cc-ldoption, -Wl$(comma)--build-id,))
 KBUILD_LDFLAGS_MODULE += $(LDFLAGS_BUILD_ID)
 LDFLAGS_vmlinux += $(LDFLAGS_BUILD_ID)
-- 
2.11.0