Index: llvm/utils/gn/build/BUILD.gn
===================================================================
--- llvm/utils/gn/build/BUILD.gn
+++ llvm/utils/gn/build/BUILD.gn
@@ -75,6 +75,18 @@
   if (host_os != "win") {
     if (is_debug) {
       cflags += [ "-g" ]
+
+      # --gdb-index makes links ~15% slower, and gdb symbol reading time
+      # 15x faster (lld links in 4.4 instead of 3.9s, and gdb loads and runs
+      # it in 2s instead of in 30s). Since the GN build defaults to release
+      # builds, it's likely that people doing debug builds want to run a
+      # debugger. So this seems like the right tradeoff.
+      if (host_os != "mac") {
+        if (use_lld) {
+          cflags += [ "-ggnu-pubnames" ]  # PR34820
+        }
+        ldflags += [ "-Wl,--gdb-index" ]
+      }
     }
     if (is_optimized) {
       cflags += [ "-O3" ]