Zig cache hacks/optimizations

Zig does not always share `libc++.a` and the glibc shims. We have observed
to be caused for 2 reasons:

- For some commands Go `chdir`s to `/tmp/`. This causes `ZIG_LIB_DIR` to be
  absolute, which blows the cache key to find the right `libc++.a` (because Zig
  thinks we are using a different lib dir to compile libc++). This is currently
  worked around in `toolchain/defs.bzl` by overfitting to Go and returning
  early from that particular invocation.
- Sometimes Bazel's sandbox messes up Zig's cache keys. If one runs without the
  sandbox (`--spawn_strategy=standalone`), the cache hit rate and thus the
  build time are much better.

This is actively investigated. I am adding `--spawn_strategy=standalone`
to CI to see the speedup that it provides. I will rollback it later.

README.md

@@ -297,6 +297,20 @@ unlikely to implement them any time soon, but patches implementing those will
 be accepted. See [Questions & Contributions](#questions-amp-contributions) on
 how to contribute.
 
+## Sharing of libc shims/libc++.a
+
+Zig does not always share `libc++.a` and the glibc shims. We have observed
+to be caused for 2 reasons:
+
+- For some commands Go `chdir`s to `/tmp/`. This causes `ZIG_LIB_DIR` to be
+  absolute, which blows the cache key to find the right `libc++.a` (because Zig
+  thinks we are using a different lib dir to compile libc++). This is currently
+  worked around in `toolchain/defs.bzl` by overfitting to Go and returning
+  early from that particular invocation.
+- Sometimes Bazel's sandbox messes up Zig's cache keys. If one runs without the
+  sandbox (`--spawn_strategy=standalone`), the cache hit rate and thus the
+  build time are much better.
+
 ## Zig cache location
 
 Currently zig cache is in `$HOME`, so `bazel clean --expunge` does not clear

toolchain/defs.bzl

@@ -133,7 +133,7 @@ export ZIG_LIB_DIR
 export ZIG_LOCAL_CACHE_DIR="{cache_prefix}/bazel-zig-cc"
 export ZIG_GLOBAL_CACHE_DIR="{cache_prefix}/bazel-zig-cc"
 {maybe_gohack}
-exec "{zig}" "{zig_tool}" {maybe_target} "$@" $maybe_o2
+exec "{zig}" "{zig_tool}" {maybe_target} "$@"
 """
 
 _ZIG_TOOL_WRAPPER_CACHE_GUESS = """#!/bin/sh
@@ -158,7 +158,7 @@ export ZIG_LIB_DIR
 export ZIG_LOCAL_CACHE_DIR="$_cache_prefix/bazel-zig-cc"
 export ZIG_GLOBAL_CACHE_DIR=$ZIG_LOCAL_CACHE_DIR
 {maybe_gohack}
-exec "{zig}" "{zig_tool}" {maybe_target} "$@" $maybe_o2
+exec "{zig}" "{zig_tool}" {maybe_target} "$@"
 """
 
 # The abomination below adds "-O2" to Go's link-prober command. Saves around
@@ -169,9 +169,14 @@ exec "{zig}" "{zig_tool}" {maybe_target} "$@" $maybe_o2
 _ZIG_TOOL_GOHACK = """
 quote(){ echo "$1" | sed -e "s,','\\\\'',g"; }
 for arg in "$@"; do saved="${saved:+$saved }'$(quote "$arg")'"; done
-maybe_o2=
 while [ "$#" -gt 6 ]; do shift; done
-[ "$*" = "-Wl,--no-gc-sections -x c - -o /dev/null" ] && maybe_o2="-O2"
+if [ "$*" = "-Wl,--no-gc-sections -x c - -o /dev/null" ]; then
+  # This command probes if `--no-gc-sections` is accepted by the linker.
+  # Since it is executed in /tmp, the ZIG_LIB_DIR is absolute,
+  # glibc stubs and libc++ cannot be shared with other invocations (which use
+  # a relative ZIG_LIB_DIR).
+  exit 0;
+fi
 eval set -- "$saved"
 """