From f47b61d19e1c17bac9fff34e9b304bad4f132ffe Mon Sep 17 00:00:00 2001
From: "Felix S. Klock II" <pnkfelix@pnkfx.org>
Date: Fri, 19 Aug 2022 16:40:26 -0400
Subject: [PATCH] elaborate how revisions work with FileCheck stuff in
 src/test/codegen

---
 src/test/codegen/README.md | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/src/test/codegen/README.md b/src/test/codegen/README.md
index 00de55eeab1f4..8f2daaafcc77a 100644
--- a/src/test/codegen/README.md
+++ b/src/test/codegen/README.md
@@ -1,2 +1,24 @@
 The files here use the LLVM FileCheck framework, documented at
 <https://llvm.org/docs/CommandGuide/FileCheck.html>.
+
+One extension worth noting is the use of revisions as custom prefixes for
+FileCheck. If your codegen test has different behavior based on the chosen
+target or different compiler flags that you want to exercise, you can use a
+revisions annotation, like so:
+
+```rust
+// revisions: aaa bbb
+// [bbb] compile-flags: --flags-for-bbb
+```
+
+After specifying those variations, you can write different expected, or
+explicitly *unexpected* output by using `<prefix>-SAME:` and `<prefix>-NOT:`,
+like so:
+
+```rust
+// CHECK: expected code
+// aaa-SAME: emitted-only-for-aaa
+// aaa-NOT:                        emitted-only-for-bbb
+// bbb-NOT:  emitted-only-for-aaa
+// bbb-SAME:                       emitted-only-for-bbb
+```