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From: Bin Meng <bmeng.cn@gmail.com>
To: u-boot@lists.denx.de
Subject: [U-Boot] [RFC PATCH 1/2] Makefile: Use -fno-strict-aliasing globally
Date: Mon, 10 Sep 2018 06:53:45 -0700	[thread overview]
Message-ID: <1536587626-3664-1-git-send-email-bmeng.cn@gmail.com> (raw)

The -fstrict-aliasing option is implicitly enabled at levels -O2,
-O3, -Os by GCC. This option allows the compiler to assume the
strictest aliasing rules applicable to the language being compiled.
For example, the practice of reading from a different union member
than the one most recently written to (called "type-punning") is
common. In this case, "type-punning" only works if the memory is
accessed through the union type, but might not work by taking the
address, casting the resulting pointer and dereferencing the result,
which is an undefined behavior per the "strict aliasing rules".

GCC's -Wstrict-aliasing (included in -Wall) option does not catch
all cases, but does attempt to catch the more common pitfalls. So
there are cases that GCC does not report but the codes are violating
the "strict aliasing rules".

Given lots of codes that may be written to rely on "type-punning",
and Linux kernel disables it by -fno-strict-aliasing globally, since
U-Boot currently does this on nds32/riscv/x86 builds only, extend
this for all architecture builds.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
---

 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index f30dd8e..994d957 100644
--- a/Makefile
+++ b/Makefile
@@ -372,7 +372,7 @@ KBUILD_CPPFLAGS := -D__KERNEL__ -D__UBOOT__
 KBUILD_CFLAGS   := -Wall -Wstrict-prototypes \
 		   -Wno-format-security \
 		   -fno-builtin -ffreestanding $(CSTD_FLAG)
-KBUILD_CFLAGS	+= -fshort-wchar
+KBUILD_CFLAGS	+= -fshort-wchar -fno-strict-aliasing
 KBUILD_AFLAGS   := -D__ASSEMBLY__
 
 # Don't generate position independent code
-- 
2.7.4

             reply	other threads:[~2018-09-10 13:53 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-10 13:53 Bin Meng [this message]
2018-09-10 13:53 ` [U-Boot] [RFC PATCH 2/2] config.mk: Remove duplicated -fno-strict-aliasing Bin Meng
     [not found]   ` <752D002CFF5D0F4FA35C0100F1D73F3F6BCBEC7A@ATCPCS16.andestech.com>
2018-09-11  1:54     ` Rick Chen
2018-09-10 21:49 ` [U-Boot] [RFC PATCH 1/2] Makefile: Use -fno-strict-aliasing globally Tom Rini
2018-09-14 10:53 ` Simon Glass

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