Add StubUnsupportedJSOps to remove operations that JS does not support (#3024)

This doesn't lower them - it just replaces the unsupported operation
with a drop. This will be useful for fuzzing, where to compare JS to the
correct semantics we must avoid operations where JS is not always
accurate.

Also fully document the i64 -> f32 conversion issue in JS.

2 files changed, 38 insertions, 0 deletions

diff --git a/test/passes/stub-unsupported-js.txt b/test/passes/stub-unsupported-js.txt
new file mode 100644
index 000000000..531c767c0
--- /dev/null
+++ b/test/passes/stub-unsupported-js.txt
@@ -0,0 +1,24 @@
+(module
+ (type $none_=>_f32 (func (result f32)))
+ (type $i32_=>_f32 (func (param i32) (result f32)))
+ (type $i64_=>_f32 (func (param i64) (result f32)))
+ (func $yes (param $x i64) (result f32)
+  (drop
+   (local.get $x)
+  )
+  (f32.const 0)
+ )
+ (func $no (param $x i32) (result f32)
+  (f32.convert_i32_u
+   (local.get $x)
+  )
+ )
+ (func $yes-unreach (result f32)
+  (unreachable)
+ )
+ (func $no-unreach (result f32)
+  (f32.convert_i32_u
+   (unreachable)
+  )
+ )
+)
diff --git a/test/passes/stub-unsupported-js.wast b/test/passes/stub-unsupported-js.wast
new file mode 100644
index 000000000..6cfc7bef2
--- /dev/null
+++ b/test/passes/stub-unsupported-js.wast
@@ -0,0 +1,14 @@
+(module
+ (func $yes (param $x i64) (result f32)
+  (f32.convert_i64_u (local.get $x))
+ )
+ (func $no (param $x i32) (result f32)
+  (f32.convert_i32_u (local.get $x))
+ )
+ (func $yes-unreach (result f32)
+  (f32.convert_i64_u (unreachable))
+ )
+ (func $no-unreach (result f32)
+  (f32.convert_i32_u (unreachable))
+ )
+)