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nix: use -ld_new on macOS devshell

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The new parallel macOS linker reduces link time for the debug `jj` binary from
3s to 0.7s on my M2 Macbook Air, which is a significant reduction for nearly
no cost at all. This only assumes that you have a new enough Xcode environment
as your default (where `/usr/bin/ld` resides.)

This change requires Sonoma and Xcode 15, but in theory I think we could target
a lower macOS SDK version in order to produce binaries that are more backwards
compatible, so the only real cost is that developers who also use Nix would
require Sonoma.
This commit is contained in:
Austin Seipp 2024-06-13 15:29:05 -05:00
parent 3b2197c74a
commit fe40c9e364
1 changed files with 16 additions and 9 deletions
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25
flake.nix
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@ -54,16 +54,25 @@
export DYLD_FALLBACK_LIBRARY_PATH=$(${ourRustVersion}/bin/rustc --print sysroot)/lib
'';
# NOTE (aseipp): on Linux, go ahead and use mold by default to improve
# link times a bit; mostly useful for debug build speed, but will help
# over time if we ever get more dependencies, too
useMoldLinker = pkgs.stdenv.isLinux;
# these are needed in both devShell and buildInputs
linuxNativeDeps = with pkgs; lib.optionals stdenv.isLinux [
mold-wrapped
];
# on macOS and Linux, use faster parallel linkers that are much more
# efficient than the defaults. these noticeably improve link time even for
# medium sized rust projects like jj
rustLinkerFlags =
if pkgs.stdenv.isLinux then
[ "-fuse-ld=mold" "-Wl,--compress-debug-sections=zstd" ]
else if pkgs.stdenv.isDarwin then
[ "-fuse-ld=/usr/bin/ld" "-ld_new" ]
else
[ ];
rustLinkFlagsString = pkgs.lib.concatStringsSep " " (pkgs.lib.concatMap (x:
[ "-C" "link-arg=${x}" ]
) rustLinkerFlags);
in
{
packages = {
@ -98,7 +107,7 @@
ZSTD_SYS_USE_PKG_CONFIG = "1";
LIBSSH2_SYS_USE_PKG_CONFIG = "1";
RUSTFLAGS = pkgs.lib.optionalString useMoldLinker "-C link-arg=-fuse-ld=mold";
RUSTFLAGS = pkgs.lib.optionalString pkgs.stdenv.isLinux "-C link-arg=-fuse-ld=mold";
NIX_JJ_GIT_HASH = self.rev or "";
CARGO_INCREMENTAL = "0";
@ -175,9 +184,7 @@
export ZSTD_SYS_USE_PKG_CONFIG=1
export LIBSSH2_SYS_USE_PKG_CONFIG=1
export RUSTFLAGS="-Zthreads=0"
'' + pkgs.lib.optionalString useMoldLinker ''
export RUSTFLAGS+=" -C link-arg=-fuse-ld=mold -C link-arg=-Wl,--compress-debug-sections=zstd"
export RUSTFLAGS="-Zthreads=0 ${rustLinkFlagsString}"
'' + darwinNextestHack;
};
}));