[PATCH 1/1] x86/boot/compressed: Use builtin mem functions for decompressor
From: Arvind Sankar
Date: Tue Aug 04 2020 - 19:48:29 EST
c041b5ad8640 ("x86, boot: Create a separate string.h file to provide standard string functions")
fb4cac573ef6 ("x86, boot: Move memcmp() into string.h and string.c")
the decompressor stub has been using the compiler's builtin memcpy,
memset and memcmp functions, _except_ where it would likely have the
largest impact, in the decompression code itself.
Remove the #undef's of memcpy and memset in misc.c so that the
decompressor code also uses the compiler builtins.
The rationale given in the comment doesn't really apply: just because
some functions use the out-of-line version is no reason to not use the
builtin version in the rest.
Replace the comment with an explanation of why memzero and memmove are
Drop the suggestion to #undef in boot/string.h as well: the out-of-line
versions are not really optimized versions, they're generic code that's
good enough for the preboot environment. The compiler will likely
generate better code for constant-size memcpy/memset/memcmp if it is
Most decompressors' performance is unchanged, with the exception of LZ4
and 64-bit ZSTD.
Before After ARCH
LZ4 73ms 10ms 32
LZ4 120ms 10ms 64
ZSTD 90ms 74ms 64
Measurements on QEMU on 2.2GHz Broadwell Xeon, using defconfig kernels.
Decompressor code size has small differences, with the largest being
that 64-bit ZSTD decreases just over 2k. The largest code size increase
was on 64-bit XZ, of about 400 bytes.
Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
Suggested-by: Nick Terrell <nickrterrell@xxxxxxxxx>
Tested-by: Nick Terrell <nickrterrell@xxxxxxxxx>
arch/x86/boot/compressed/misc.c | 7 ++-----
arch/x86/boot/string.h | 5 +----
2 files changed, 3 insertions(+), 9 deletions(-)
diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index 39e592d0e0b4..e478e40fbe5a 100644
@@ -30,12 +30,9 @@
#define STATIC static
- * Use normal definitions of mem*() from string.c. There are already
- * included header files which expect a definition of memset() and by
- * the time we define memset macro, it is too late.
+ * Provide definitions of memzero and memmove as some of the decompressors will
+ * try to define their own functions if these are not defined as macros.
#define memzero(s, n) memset((s), 0, (n))
#define memmove memmove
diff --git a/arch/x86/boot/string.h b/arch/x86/boot/string.h
index 995f7b7ad512..a232da487cd2 100644
@@ -11,10 +11,7 @@ void *memcpy(void *dst, const void *src, size_t len);
void *memset(void *dst, int c, size_t len);
int memcmp(const void *s1, const void *s2, size_t len);
- * Access builtin version by default. If one needs to use optimized version,
- * do "undef memcpy" in .c file and link against right string.c
+/* Access builtin version by default. */
#define memcpy(d,s,l) __builtin_memcpy(d,s,l)
#define memset(d,c,l) __builtin_memset(d,c,l)
#define memcmp __builtin_memcmp