[PATCH dwarves v3] pahole: enable --reproducible_build when SOURCE_DATE_EPOCH is set

[Dominique Martinet <asmadeus@codewreck.org>]

The SOURCE_DATE_EPOCH environment variable is used in reproducible
builds to set build timestamps and is generally synonymous with
reproducible builds: instead of making users figure out how to pass
pahole flags (e.g. explicitly setting PAHOLE_FLAGS in linux) just assume
that the user wants a reproducible build if this variable is set,
printing a debug message in verbose mode.

The impact on build time appears to be minimal enough even if we
incorrectly make this decision (on a Ryzen 7 7735HS):
$ hyperfine 'sh pahole-test -j --reproducible_build' 'sh pahole-test -j1' 'sh pahole-test -j' -p 'cp .tmp_vmlinux.orig .tmp_vmlinux.btf' -m 4
Benchmark 1: sh pahole-test -j --reproducible_build
  Time (mean ± σ):      3.991 s ±  0.045 s    [User: 7.223 s, System: 3.741 s]
  Range (min … max):    3.950 s …  4.042 s    4 runs

Benchmark 2: sh pahole-test -j1
  Time (mean ± σ):      7.083 s ±  0.095 s    [User: 4.805 s, System: 2.242 s]
  Range (min … max):    6.964 s …  7.191 s    4 runs

Benchmark 3: sh pahole-test -j
  Time (mean ± σ):      3.858 s ±  0.089 s    [User: 13.447 s, System: 7.078 s]
  Range (min … max):    3.763 s …  3.978 s    4 runs

Summary
  sh pahole-test -j ran
    1.03 ± 0.03 times faster than sh pahole-test -j --reproducible_build
    1.84 ± 0.05 times faster than sh pahole-test -j1

Where pahole-test is the pahole command of the linux build (minus -j):
LLVM_OBJCOPY=objcopy pahole -J --btf_gen_floats --lang_exclude=rust --skip_encoding_btf_inconsistent_proto --btf_gen_optimized .tmp_vmlinux.btf "$@"

And .tmp_vmlinux.orig was the .tmp_vmlinux.btf binary before being
processed:
ld -m elf_x86_64 -z noexecstack --no-warn-rwx-segments --emit-relocs --discard-none -z max-page-size=0x200000 --build-id=sha1 --orphan-handling=warn --script=./arch/x86/kernel/vmlinux.lds -o .tmp_vmlinux.btf --whole-archive vmlinux.a .vmlinux.export.o init/version-timestamp.o --no-whole-archive --start-group --end-group

Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
---
v2 -> v3:
 - added message about the env var to man page.
 - fixed typo / style of comment in code

v1 -> v2:
 - added message in verbose mode if setting reproducible build through
   this.

 man-pages/pahole.1 |  3 +++
 pahole.c           | 11 +++++++++++
 2 files changed, 14 insertions(+)

diff --git a/man-pages/pahole.1 b/man-pages/pahole.1
index f0605935a9f1..d9c2c1ac6912 100644
--- a/man-pages/pahole.1
+++ b/man-pages/pahole.1
@@ -327,6 +327,9 @@ If one wants to add an extra feature to the set of standard ones, the '+' prefix
 \-\-btf_features=+reproducible_build will add all default features plus the 'reproducible_build' extra
 feature.
 
+Alternatively, reproducible_build will also be enabled if the SOURCE_DATE_EPOCH environment variable
+is set, as that is commonly set in build systems that aim for reproducible builds.
+
 .TP
 .B \-\-btf_features_strict
 Identical to \-\-btf_features above, but pahole will exit if it encounters an unrecognized feature.
diff --git a/pahole.c b/pahole.c
index c866095c8be6..7724980ff106 100644
--- a/pahole.c
+++ b/pahole.c
@@ -3705,6 +3705,17 @@ int main(int argc, char *argv[])
 		goto out;
 	}
 
+	/*
+	 * SOURCE_DATE_EPOCH being set means whoever called us likely tries
+	 * to do a reproducible build. The setting only impacts btf_encode code
+	 * so only check in this case to avoid needless messages.
+	 */
+	if (btf_encode && getenv("SOURCE_DATE_EPOCH")) {
+		if (!conf_load.reproducible_build && global_verbose)
+			printf("pahole: auto-enabling reproducible build (SOURCE_DATE_EPOCH set)\n");
+		conf_load.reproducible_build = true;
+	}
+
 	if (languages.str && parse_languages())
 		return rc;
 
-- 
2.45.1