pepyakin · February 23, 2018 10:48 · Feb 23, 2018
diff --git a/wasm-reduce.md b/wasm-reduce.md
@@ -0,0 +1,73 @@
+When one try to run `wasm-reduce` on macOS machine, they will see something like this:
+
+```
+wasm-reduce -v -f \
+	--timeout=15 \
+	--command="./predicate.sh" \
+	--test=test.wasm \
+	--working=working.wasm \
+	~/Downloads/wasm-collection-master/misc-valid/cv-wasm.wasm
+
+|wasm-reduce
+|input: /Users/pepyakin/Downloads/wasm-collection-master/misc-valid/cv-wasm.wasm
+|test: test.wasm
+|working: working.wasm
+|expected result:
+[ProgramResult] code: 32512 stdout:
+[/ProgramResult]
+
+|checking that command has different behavior on invalid binary
+|! running command on an invalid module should give different results
+[ProgramResult] code: 32512 stdout:
+[/ProgramResult]
+
+Fatal: |! stopping, as it is very unlikely reduction can succeed (use -f to ignore this check)
+```
+
+It took quite some to figure out why my predicate script always returns status code 32512! : )
+Here is my story:
+
+It turns out that exit code 32512 is returned when command not found. As I found out later, it is not my predicate script is not found but the some `timeout` command which is [used by wasm-reduce](https://github.com/WebAssembly/binaryen/blob/d9692277357ba6fd67a7e25ce16934209d049033/src/tools/wasm-reduce.cpp#L62) to cap the execution time of the predicate script.
+
+Unfortunately, macOS comes without installed `timeout`. Fortunately, it can easily be installed with [brew](https://brew.sh): `brew install coreutils`.
+
+Since macOS comes with __some__ of coreutils programs installed, all these programs installed with names prepended by `g` (e.g `timeout` is `gtimeout`). Luckily, these are just aliases and programs with original names are installed in `/usr/local/opt/coreutils/libexec/gnubin`. So it is sufficient to just export new `PATH` that will use `coreutils`
+
+```
+export PATH="/usr/local/opt/coreutils/libexec/gnubin:$PATH"
+```
+
+Next, after I try to launch reducing again, I stopped by this issue:
+
+```
+|checking that command has different behavior on invalid binary
+|checking that command has expected behavior on canonicalized (read-written) binary
+|! failed to read and write the binary
+[ProgramResult] code: 32512 stdout:
+[/ProgramResult]
+```
+
+So it seems that `wasm-reduce` trying to launch `wasm-opt` but for some reason can't.
+
+https://github.com/WebAssembly/binaryen/blob/d9692277357ba6fd67a7e25ce16934209d049033/src/tools/wasm-reduce.cpp#L685-L695
+
+That's weird since `wasm-opt` is on my `PATH` and `wasm-reduce` is launched from the same directory where `wasm-opt` is placed. Looking at the code, 
+
+https://github.com/WebAssembly/binaryen/blob/6cf7343bffa3d485161e221b23dfacd9243dec68/src/support/path.h#L41-L55
+
+I was able to see that `wasm-reduce` is only trying to search `wasm-opt` at `BINARYEN_ROOT`.
+So adding another `export` might help:
+
+```
+export BINARYEN_ROOT=~/dev/etc/binaryen
+```
+
+After that was done, I was able to launch reducing! Hope this will help for future reducers (incl. me).
+
+Here is my `predicate.sh` for the reference:
+
+```bash
+#!/bin/bash
+target/release/wasm-stack-height test.wasm result.wasm 2>&1 || exit 1
+wasm2wat result.wasm 2>&1 || exit 2
+```
No results found