Re: [PATCH 04/23] kallsyms: output binary data to speed output and kallsyms assembly

kallsyms generates an assembly file that consists mostly of .byte entries
containing compressed names, token strings and name-sorted sequence
numbers.

For an x86-64 build with 158k symbols that is a 37 MiB .S file which takes
0.57s to assemble each of the two to three times it is built over a kernel
build.

Each time it is generated it also takes kallsyms a similar amount of time
to output it.

Avoid this overhead by instead outputting this data as binary and importing
it into the assembly using the .incbin directive.

Tables that are wider than a byte remain part of the assembly to ensure
endianness and relative relocations are performed correctly.

With this change, the output assembly file shrinks from 37 MiB to 9.8 MiB,
with a 2.6 MiB binary data file alongside it, and the object remains
identical.

The generated binary file is deleted correctly on build clean along with
all other ephemeral data.

On an x86-64 system with CONFIG_KALLSYMS_ALL set:

                       before   after    delta
  scripts/kallsyms     0.24s    0.18s    0.06s
  assemble             0.57s    0.16s    0.41s

Per kallsyms invocation/assembly, for a total of 0.47s time saving upon
invocation.

An incremental build on the same system was reduced from 11.15s to 9.65s,
indicating a total of 1.5 seconds saved over the build.

The kallsyms runs and their assembly are on the serial tail of every build
that links vmlinux, no-op builds are unchanged.

Whole build, 128-thread Threadripper 9980X, best of N runs:

                                         before   after     delta
                                         -------------------------------
  x86 defconfig, touch mm/vma.c, gcc       10.8s     9.9s    -0.92s (-8%)
  x86 defconfig, touch mm/vma.c, clang     10.7s     9.5s     -1.2s (-11%)
  x86 defconfig, clean, gcc                29.5s    28.7s    -0.81s (-3%)
  x86 defconfig, clean, clang              29.7s    28.6s     -1.1s (-4%)
  x86 allmodconfig, touch mm/vma.c, gcc    45.3s    44.0s     -1.3s (-3%)
  x86 allmodconfig, touch mm/vma.c, clang  42.9s    40.2s     -2.7s (-6%)

Assisted-by: LLM
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 scripts/kallsyms.c      | 97 ++++++++++++++++++++++++++++++++++++++-----------
 scripts/link-vmlinux.sh |  2 +-
 2 files changed, 77 insertions(+), 22 deletions(-)

diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
index 350d118c3b9e..61c5eb537ed4 100644
--- a/scripts/kallsyms.c
+++ b/scripts/kallsyms.c

@@ -5,7 +5,10 @@
  * This software may be used and distributed according to the terms
  * of the GNU General Public License, incorporated herein by reference.
  *
- * Usage: kallsyms [--all-symbols] in.map > out.S
+ * Usage: kallsyms [--all-symbols] [--pc-relative] in.map out.bin > out.S
+ *
+ *      The byte tables go to out.bin and are pulled into out.S with .incbin;
+ *  wider tables stay assembler source for endianness and relocations.
  *
  *      Table compression uses all the unused char codes on the symbols and
  *  maps these to the most used substrings (tokens). For instance, it might

@@ -102,7 +105,7 @@ static void sym_arr_free(struct sym_arr *arr)
 
 static void usage(void)
 {
-	fprintf(stderr, "Usage: kallsyms [--all-symbols] in.map > out.S\n");
+	fprintf(stderr, "Usage: kallsyms [--all-symbols] [--pc-relative] in.map out.bin > out.S\n");
 	exit(1);
 }
 

@@ -319,6 +322,40 @@ static void output_label(const char *label)
 	printf("%s:\n", label);
 }
 
+static void write_bin(FILE *file, const void *data, size_t len)
+{
+	if (fwrite(data, 1, len, file) == len)
+		return;
+
+	perror("kallsyms: write");
+	exit(EXIT_FAILURE);
+}