gcc-10 inliner change

From 3df6e4b6318d3acb18b2336eec5340c47aa510b5 Mon Sep 17 00:00:00 2001
From: Sultan Alsawaf <sultan@kerneltoast.com>
Date: Tue, 12 May 2020 12:00:08 -0700
Subject: [PATCH] kbuild: Increase GCC automatic inline instruction limit to
 1000 for LTO

GCC 10 updated its interprocedural optimizer's logic to have it make
more conservative inlining decisions, resulting in worse syscall and
hackbench performance compared to GCC 9. Although the max-inline-insns-
auto parameter's value was not altered, increasing it from the -O3
default of 30 to 1000 instructions yields improved performance with LTO,
surpassing GCC 9.

Do this only for LTO though because for non-LTO builds, this causes GCC
to produce mountains of spurious -Wmaybe-used-uninitialized warnings.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
---
 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index de5047ed12dd..9ea681432d77 100644
--- a/Makefile
+++ b/Makefile
@@ -638,7 +638,7 @@ endif
 ifdef CONFIG_LTO
 LTO_CFLAGS    := -flto -flto=jobserver -fno-fat-lto-objects \
                  -fuse-linker-plugin -fwhole-program
-KBUILD_CFLAGS += $(LTO_CFLAGS)
+KBUILD_CFLAGS += $(LTO_CFLAGS) --param=max-inline-insns-auto=1000
 LTO_LDFLAGS   := $(LTO_CFLAGS) -Wno-lto-type-mismatch -Wno-psabi \
                  -Wno-stringop-overflow -flinker-output=nolto-rel
 LDFINAL       := $(CONFIG_SHELL) $(srctree)/scripts/gcc-ld $(LTO_LDFLAGS)