From 4d60e14ad247a134822b05b0bce29f5641d1ee6f Mon Sep 17 00:00:00 2001
From: Mauricio Collares <mauricio@collares.org>
Date: Fri, 19 Feb 2021 17:15:59 -0300
Subject: [PATCH] emacs: precompile trampolines in parallel

---
 pkgs/applications/editors/emacs/generic.nix | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/pkgs/applications/editors/emacs/generic.nix b/pkgs/applications/editors/emacs/generic.nix
index 619bde59874c..70e253dd6dfb 100644
--- a/pkgs/applications/editors/emacs/generic.nix
+++ b/pkgs/applications/editors/emacs/generic.nix
@@ -159,10 +159,14 @@ let emacs = stdenv.mkDerivation (lib.optionalAttrs nativeComp {
   '' + lib.optionalString (nativeComp && withNS) ''
     ln -snf $out/lib/emacs/*/native-lisp $out/Applications/Emacs.app/Contents/native-lisp
   '' + lib.optionalString nativeComp ''
-    $out/bin/emacs --batch \
-      -l comp --eval "(mapatoms (lambda (s) \
-                        (when (subr-primitive-p (symbol-function s)) \
-                          (comp-trampoline-compile s))))"
+    echo "Generating native-compiled trampolines..."
+    # precompile trampolines in parallel, but avoid spawning one process per trampoline.
+    # 1000 is a rough lower bound on the number of trampolines compiled.
+    $out/bin/emacs --batch --eval "(mapatoms (lambda (s) \
+      (when (subr-primitive-p (symbol-function s)) (print s))))" \
+      | xargs -n $((1000/NIX_BUILD_CORES + 1)) -P $NIX_BUILD_CORES \
+        $out/bin/emacs --batch -l comp --eval "(while argv \
+          (comp-trampoline-compile (intern (pop argv))))"
     mkdir -p $out/share/emacs/native-lisp
     $out/bin/emacs --batch \
       --eval "(add-to-list 'comp-eln-load-path \"$out/share/emacs/native-lisp\")" \