1 files changed, 39 insertions, 0 deletions
diff --git a/src/test/mir-opt/README.md b/src/test/mir-opt/README.md
new file mode 100644
index 000000000..a0550466c
--- /dev/null
+++ b/src/test/mir-opt/README.md
@@ -0,0 +1,39 @@
+This folder contains tests for MIR optimizations.
+
+The `mir-opt` test format emits MIR to extra files that you can automatically update by specifying
+`--bless` on the command line (just like `ui` tests updating `.stderr` files).
+
+# `--bless`able test format
+
+By default 32 bit and 64 bit targets use the same dump files, which can be problematic in the
+presence of pointers in constants or other bit width dependent things. In that case you can add
+
+```
+// EMIT_MIR_FOR_EACH_BIT_WIDTH
+```
+
+to your test, causing separate files to be generated for 32bit and 64bit systems.
+
+## Emit a diff of the mir for a specific optimization
+
+This is what you want most often when you want to see how an optimization changes the MIR.
+
+```
+// EMIT_MIR $file_name_of_some_mir_dump.diff
+```
+
+## Emit mir after a specific optimization
+
+Use this if you are just interested in the final state after an optimization.
+
+```
+// EMIT_MIR $file_name_of_some_mir_dump.after.mir
+```
+
+## Emit mir before a specific optimization
+
+This exists mainly for completeness and is rarely useful.
+
+```
+// EMIT_MIR $file_name_of_some_mir_dump.before.mir
+```