From bab7b73b85f865d20a5c4f2d78ac9e81b3d39109 Mon Sep 17 00:00:00 2001
From: Russ Cox <rsc@swtch.com>
Date: Mon, 24 Jan 2022 13:47:27 -0500
Subject: [PATCH] INSTALL: build arm64 binaries on arm64 Macs

If there is any x86 binary in the chain of parent processes from
the current one, then uname -m prints x86_64 and clang
defaults to building x86_64 binaries.

Detect arm64 using the kernel version and force building
an arm64 toolchain instead of perpetuating x86.

This is particularly important when the user shell is rc,
which has been built for x86, because then rebuilding
under rc continues to use x86.
---
 INSTALL | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/INSTALL b/INSTALL
index 79c0745f..92ac1959 100755
--- a/INSTALL
+++ b/INSTALL
@@ -95,7 +95,15 @@ if [ `uname` = Darwin ]; then
 		echo "See http://swtch.com/go/xcodegcc for details." >&2
 		exit 1
 	fi
-	echo "CC9='xcrun --sdk macosx clang'" >>$PLAN9/config
+	CC9="xcrun --sdk macosx clang"
+	case "$(uname -a)" in
+	*ARM64*)
+		CC9="$CC9 -arch arm64"
+		echo '	Forcing arm64 binaries with clang.'
+		;;
+	esac
+
+	echo "CC9='$CC9'" >>$PLAN9/config
 	rm -f ./a.out
 fi