From: Arvind Sankar <nivedita@alum.mit.edu>
To: Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: [PATCH] lib/string.c: Disable tree-loop-distribute-patterns
Date: Tue, 18 Aug 2020 19:43:07 -0400 [thread overview]
Message-ID: <20200818234307.3382306-1-nivedita@alum.mit.edu> (raw)
gcc can transform the loop in a naive implementation of memset/memcpy
etc into a call to the function itself. This optimization is enabled by
-ftree-loop-distribute-patterns.
This has been the case for a while (see eg [0]), but gcc-10.x enables
this option at -O2 rather than -O3 as in previous versions.
Add -ffreestanding, which implicitly disables this optimization with
gcc. It is unclear whether clang performs such optimizations, but
hopefully it will also not do so in a freestanding environment.
This by itself is insufficient for gcc if the optimization was
explicitly enabled by CFLAGS, so also add a flag to explicitly disable
it.
[0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56888
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
---
lib/Makefile | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/lib/Makefile b/lib/Makefile
index e290fc5707ea..80edea49613f 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -15,11 +15,18 @@ KCOV_INSTRUMENT_debugobjects.o := n
KCOV_INSTRUMENT_dynamic_debug.o := n
KCOV_INSTRUMENT_fault-inject.o := n
+# string.o implements standard library functions like memset/memcpy etc.
+# Use -ffreestanding to ensure that the compiler does not try to "optimize"
+# them into calls to themselves.
+# The optimization pass that does such transformations in gcc is
+# tree-loop-distribute-patterns. Explicitly disable it just in case.
+CFLAGS_string.o := -ffreestanding $(call cc-option,-fno-tree-loop-distribute-patterns)
+
# Early boot use of cmdline, don't instrument it
ifdef CONFIG_AMD_MEM_ENCRYPT
KASAN_SANITIZE_string.o := n
-CFLAGS_string.o := -fno-stack-protector
+CFLAGS_string.o += -fno-stack-protector
endif
# Used by KCSAN while enabled, avoid recursion.
--
2.26.2
next reply other threads:[~2020-08-18 23:43 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-18 23:43 Arvind Sankar [this message]
2020-08-19 0:44 ` [PATCH] lib/string.c: Disable tree-loop-distribute-patterns Linus Torvalds
2020-08-19 3:04 ` Arvind Sankar
2020-08-19 3:32 ` Linus Torvalds
2020-08-19 13:16 ` Arvind Sankar
2020-08-19 14:08 ` [PATCH v2] lib/string.c: Use freestanding environment Arvind Sankar
2020-08-19 18:35 ` Nick Desaulniers
2020-08-19 19:06 ` Arvind Sankar
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