enable testing for stage3 and stage4 being byte-for-byte identical

ci/aarch64-macos.sh

@@ -52,3 +52,19 @@ stage3-release/bin/zig build test docs \
 
 # Produce the experimental std lib documentation.
 stage3-release/bin/zig test ../lib/std/std.zig -femit-docs -fno-emit-bin --zig-lib-dir ../lib
+
+# Ensure that stage3 and stage4 are byte-for-byte identical.
+stage3-release/bin/zig build \
+  --prefix stage4-release \
+  -Denable-llvm \
+  -Dno-lib \
+  -Drelease \
+  -Dstrip \
+  -Dtarget=$TARGET \
+  -Duse-zig-libcxx \
+  -Dversion-string="$(stage3-release/bin/zig version)"
+
+# diff returns an error code if the files differ.
+echo "If the following command fails, it means nondeterminism has been"
+echo "introduced, making stage3 and stage4 no longer byte-for-byte identical."
+diff stage3-release/bin/zig stage4-release/bin/zig

ci/x86_64-macos.sh

@@ -60,3 +60,19 @@ stage3-release/bin/zig build test docs \
 
 # Produce the experimental std lib documentation.
 stage3-release/bin/zig test ../lib/std/std.zig -femit-docs -fno-emit-bin --zig-lib-dir ../lib
+
+# Ensure that stage3 and stage4 are byte-for-byte identical.
+stage3-release/bin/zig build \
+  --prefix stage4-release \
+  -Denable-llvm \
+  -Dno-lib \
+  -Drelease \
+  -Dstrip \
+  -Dtarget=$TARGET \
+  -Duse-zig-libcxx \
+  -Dversion-string="$(stage3-release/bin/zig version)"
+
+# diff returns an error code if the files differ.
+echo "If the following command fails, it means nondeterminism has been"
+echo "introduced, making stage3 and stage4 no longer byte-for-byte identical."
+diff stage3-release/bin/zig stage4-release/bin/zig