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authorAndrii Nakryiko <andriin@fb.com>2019-10-10 19:38:47 -0700
committerDaniel Borkmann <daniel@iogearbox.net>2019-10-11 22:35:46 +0200
commit409017847d2014db8ab1da49dd48182af88344b7 (patch)
treecd6f2070bf31969ce71d5f88a4b41dc66d67a9f3
parentbaead859edbb3cd53b8e388c1f33641ce01d4c01 (diff)
downloadlinux-409017847d2014db8ab1da49dd48182af88344b7.tar.bz2
libbpf: Generate more efficient BPF_CORE_READ code
Existing BPF_CORE_READ() macro generates slightly suboptimal code. If there are intermediate pointers to be read, initial source pointer is going to be assigned into a temporary variable and then temporary variable is going to be uniformly used as a "source" pointer for all intermediate pointer reads. Schematically (ignoring all the type casts), BPF_CORE_READ(s, a, b, c) is expanded into: ({ const void *__t = src; bpf_probe_read(&__t, sizeof(*__t), &__t->a); bpf_probe_read(&__t, sizeof(*__t), &__t->b); typeof(s->a->b->c) __r; bpf_probe_read(&__r, sizeof(*__r), &__t->c); }) This initial `__t = src` makes calls more uniform, but causes slightly less optimal register usage sometimes when compiled with Clang. This can cascase into, e.g., more register spills. This patch fixes this issue by generating more optimal sequence: ({ const void *__t; bpf_probe_read(&__t, sizeof(*__t), &src->a); /* <-- src here */ bpf_probe_read(&__t, sizeof(*__t), &__t->b); typeof(s->a->b->c) __r; bpf_probe_read(&__r, sizeof(*__r), &__t->c); }) Fixes: 7db3822ab991 ("libbpf: Add BPF_CORE_READ/BPF_CORE_READ_INTO helpers") Signed-off-by: Andrii Nakryiko <andriin@fb.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20191011023847.275936-1-andriin@fb.com
-rw-r--r--tools/lib/bpf/bpf_core_read.h4
1 files changed, 2 insertions, 2 deletions
diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
index ae877e3ffb51..4daf04c25493 100644
--- a/tools/lib/bpf/bpf_core_read.h
+++ b/tools/lib/bpf/bpf_core_read.h
@@ -88,11 +88,11 @@
read_fn((void *)(dst), sizeof(*(dst)), &((src_type)(src))->accessor)
/* "recursively" read a sequence of inner pointers using local __t var */
+#define ___rd_first(src, a) ___read(bpf_core_read, &__t, ___type(src), src, a);
#define ___rd_last(...) \
___read(bpf_core_read, &__t, \
___type(___nolast(__VA_ARGS__)), __t, ___last(__VA_ARGS__));
-#define ___rd_p0(src) const void *__t = src;
-#define ___rd_p1(...) ___rd_p0(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
+#define ___rd_p1(...) const void *__t; ___rd_first(__VA_ARGS__)
#define ___rd_p2(...) ___rd_p1(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
#define ___rd_p3(...) ___rd_p2(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)
#define ___rd_p4(...) ___rd_p3(___nolast(__VA_ARGS__)) ___rd_last(__VA_ARGS__)