build: reduce linux release binary size by 87%

Our Linux release binary was hilariously large, weighing in at nearly
800MB (!). Nearly all of the bloat was from DWARF debug info:

    $ bloaty materialized -n 10
        FILE SIZE        VM SIZE
     --------------  --------------
      24.5%   194Mi   0.0%       0    .debug_info
      24.1%   191Mi   0.0%       0    .debug_loc
      13.8%   109Mi   0.0%       0    .debug_pubtypes
      10.1%  79.9Mi   0.0%       0    .debug_pubnames
       8.8%  70.0Mi   0.0%       0    .debug_str
       8.3%  66.3Mi   0.0%       0    .debug_ranges
       4.4%  35.3Mi   0.0%       0    .debug_line
       3.1%  24.8Mi  66.3%  24.8Mi    .text
       1.8%  14.4Mi  25.1%  9.39Mi    [41 Others]
       0.6%  4.79Mi   0.0%       0    .strtab
       0.4%  3.22Mi   8.6%  3.22Mi    .eh_frame
     100.0%   793Mi 100.0%  37.4Mi    TOTAL

This patch gets a handle on this by attacking the problem
from several angles:

  1. We instruct the linker to compress debug info sections. Most of the
     debug info is redundant and compresses exceptionally well. Part of
     the reason we didn't notice the issue is because our Docker images
     and gzipped tarballs were relatively small (~150MB).

  2. We strip out the unnecessary `.debug_pubnames` and `.debug_pubtypes`
     sections from the binary. This works around a known Rust bug
     (rust-lang/rust#46034).

  3. We ask Rust to generate less debug info for release builds,
     limiting it to line info. This is enough information to symbolicate
     a backtrace, but not enough information to run an interactive
     debugger. This is usually the right tradeoff for a release build.

    $ bloaty materialized -n 10
        FILE SIZE       VM SIZE
     --------------   --------------
      33.8%  31.9Mi     0.0%       0  .debug_info
      26.5%  25.0Mi    70.5%  25.0Mi  .text
       8.0%  7.54Mi     0.0%       0  .debug_str
       6.7%  6.36Mi     0.0%       0  .debug_line
       5.7%  5.36Mi     9.4%  3.33Mi  [38 Others]
       5.0%  4.71Mi     0.0%       0  .strtab
       3.8%  3.55Mi     0.0%       0  .debug_ranges
       3.3%  3.11Mi     8.8%  3.11Mi  .eh_frame
       3.0%  2.87Mi     0.0%       0  .symtab
       2.2%  2.12Mi     6.0%  2.12Mi  .rodata
       2.0%  1.92Mi     5.4%  1.92Mi  .gcc_except_table
     100.0%  94.4Mi   100.0%  35.5Mi  TOTAL

One issue remains unsolved, which is that Rust/LLVM cannot currently
garbage collect DWARF that refers to unused symbols/types. The actual
symbols get cut from the binary, but their debug info remains. Follow
rust-lang/rust#56068 and LLVM D74169 [0] if curious. I tested with the
aforementioned lld patch and the resulting binary is even small, at
71MB, so there's another 25MB of savings to be had there. (That patch on
its own, without the other changes, cuts the ~800MB binary to a ~300MB
binary, so it's an impressive piece of work. Unfortunately it also
increases link time by 15-25x.)

[0]: https://reviews.llvm.org/D74169

.cargo/config

@@ -0,0 +1,8 @@
+[target."x86_64-unknown-linux-gnu"]
+# Compressing debug information can yield hundreds of megabytes of savings.
+# The Rust toolchain does not currently perform dead code elimination on
+# debug info.
+#
+# See: https://github.com/rust-lang/rust/issues/56068
+# See: https://reviews.llvm.org/D74169#1990180
+rustflags = ["-C", "link-arg=-Wl,--compress-debug-sections=zlib-gabi"]

.gitignore

@@ -11,7 +11,6 @@
 
 target
 miri-target
-/.cargo
 .mtrlz.log
 **/*.rs.bk
 .netlify

Cargo.toml

@@ -23,7 +23,13 @@ members = [
 ]
 
 [profile.release]
-debug = true
+# Emit only the line info tables, not full debug info, in release builds, to
+# substantially reduce the size of the debug info. Line info tables are enough
+# to symbolicate a backtrace, but not enough to use a debugger interactively.
+# This seems to be the right tradeoff for release builds: it's unlikely we're
+# going to get interactive access to a debugger in production installations, but
+# we still want useful crash reports.
+debug = 1
 
 [patch.crates-io]
 # Waiting on a release with this commit:

bin/lint

@@ -60,6 +60,7 @@ copyright_files=$(grep -vE \
     -e '(^|/)\.gitmodules$' \
     -e '(^|/)go\.sum$' \
     -e '(^|/)Cargo\.toml$' \
+    -e '^\.cargo/config$' \
     -e '^Cargo\.lock$' \
     -e '^deny\.toml$' \
     -e '^netlify\.toml$' \

misc/python/mzbuild.py

@@ -98,11 +98,15 @@ def xcargo_target_dir(root: Path) -> Path:
         return root / "target" / "x86_64-unknown-linux-gnu"
 
 
-def xstrip(root: Path) -> str:
+def xbinutil(tool: str) -> str:
     if sys.platform == "linux":
-        return "strip"
+        return tool
     else:
-        return "x86_64-unknown-linux-gnu-strip"
+        return f"x86_64-unknown-linux-gnu-{tool}"
+
+
+xobjcopy = xbinutil("objcopy")
+xstrip = xbinutil("strip")
 
 
 def docker_images() -> Set[str]:
@@ -157,13 +161,32 @@ def run(self, root: Path, path: Path) -> None:
             # down CI, since we're packaging these binaries up into Docker
             # images and shipping them around. A bit unfortunate, since it'd be
             # nice to have useful backtraces if the binary crashes.
-            runv([xstrip(root), path / self.bin])
+            runv([xstrip, path / self.bin])
+        else:
+            # Even if we've been asked not to strip the binary, remove the
+            # `.debug_pubnames` and `.debug_pubtypes` sections. These are just
+            # indexes that speed up launching a debugger against the binary,
+            # and we're happy to have slower debugger start up in exchange for
+            # smaller binaries. Plus the sections have been obsoleted by a
+            # `.debug_names` section in DWARF 5, and so debugger support for
+            # `.debug_pubnames`/`.debug_pubtypes` is minimal anyway.
+            # See: https://github.com/rust-lang/rust/issues/46034
+            runv(
+                [
+                    xobjcopy,
+                    "-R",
+                    ".debug_pubnames",
+                    "-R",
+                    ".debug_pubtypes",
+                    path / self.bin,
+                ]
+            )
 
     def depends(self, root: Path, path: Path) -> List[bytes]:
         # TODO(benesch): this should be much smarter about computing the Rust
         # files that actually contribute to this binary target.
         return super().depends(root, path) + git_ls_files(
-            root, "src/**", "Cargo.toml", "Cargo.lock"
+            root, "src/**", "Cargo.toml", "Cargo.lock", ".cargo"
         )
 
 
@@ -216,7 +239,7 @@ def run(self, root: Path, path: Path) -> None:
         with open(path / "tests" / "manifest", "w") as manifest:
             for (executable, slug, crate_path) in tests:
                 shutil.copy(executable, path / "tests" / slug)
-                runv([xstrip(root), path / "tests" / slug])
+                runv([xstrip, path / "tests" / slug])
                 manifest.write(f"{slug} {crate_path}\n")
         shutil.move(str(path / "testdrive"), path / "tests")
         shutil.copy(
@@ -229,7 +252,7 @@ def depends(self, root: Path, path: Path) -> List[bytes]:
         # TODO(benesch): this should be much smarter about computing the Rust
         # files that actually contribute to this binary target.
         return super().depends(root, path) + git_ls_files(
-            root, "src/**", "Cargo.toml", "Cargo.lock"
+            root, "src/**", "Cargo.toml", "Cargo.lock", ".cargo"
         )