wasm-decompile: reworked how "stacky" code gets decompiled. (#1205)

For example: multi-value, and void exps while there are non-void
exps on the stack.

It now uses temp variables instead of pseudo push/pop, as the latter
weren't particularly readable and had an ordering problem that was
hard to make intuitive.

The new system covers all possible situations, generates as few
variables as possible, has clearer comments, and tests.

2 files changed, 80 insertions, 13 deletions

diff --git a/test/decompile/basic.txt b/test/decompile/basic.txt
index 7e529fc0..57856f4b 100644
--- a/test/decompile/basic.txt
+++ b/test/decompile/basic.txt
@@ -39,19 +39,12 @@
       i32.const 1
       br_if 0
     end
-    call $mv
-    i32.eq
-  )
-  (func $mv (param) (result i32 i32)
     i32.const 1
-    i32.const 2
   )
   (export "f" (func $f))
-  (export "mv" (func $mv))
 )
 
 (;; STDOUT ;;;
-
 function f(a:int, b:int):int {
   var c:long = 8L;
   var d:float = 6f;
@@ -65,12 +58,7 @@ function f(a:int, b:int):int {
     }
     if (1) continue L_b;
   }
-  push_all(mv());
-  return pop() == pop();
-}
-
-function mv():(int, int) {
-  return 1, 2
+  return 1;
 }
 
 ;;; STDOUT ;;)
diff --git a/test/decompile/stack-flush.txt b/test/decompile/stack-flush.txt
new file mode 100644
index 00000000..2ae09bda
--- /dev/null
+++ b/test/decompile/stack-flush.txt
@@ -0,0 +1,79 @@
+;;; TOOL: run-wasm-decompile
+
+(module
+  (memory 1)
+  (func $f (param i32 i32) (result i32) (local i64 f32 f64)
+    ;; Two-level flushing to stack with code that can't be reordered.
+    call $s
+    call $s
+    drop
+    call $s
+    call $s
+    call $s
+    drop
+    i32.add
+    drop
+    drop
+    ;; Two level flushing with constants that can be re-ordered.
+    i32.const 14
+    i32.const 15
+    drop
+    i32.const 11
+    i32.const 12
+    i32.const 13
+    drop
+    i32.add
+    drop
+    drop
+    ;; Multi-value examples.
+    call $mv
+    call $mv
+    drop
+    i32.add
+    drop
+    drop
+    call $mv
+    i32.eq
+  )
+  (func $s (param) (result i32)
+    i32.const 1
+  )
+  (func $mv (param) (result i32 i32)
+    i32.const 1
+    i32.const 2
+  )
+  (export "f" (func $f))
+  (export "s" (func $s))
+  (export "mv" (func $mv))
+)
+
+(;; STDOUT ;;;
+function f(a:int, b:int):int {
+  let t0 = s();
+  s();
+  let t1, t2 = s(), s();
+  s();
+  t1 + t2;
+  t0;
+  15;
+  13;
+  11 + 12;
+  14;
+  let t3, t4 = mv();
+  let t5, t6 = mv();
+  t6;
+  t4 + t5;
+  t3;
+  let t7, t8 = mv();
+  return t7 == t8;
+}
+
+function s():int {
+  return 1
+}
+
+function mv():(int, int) {
+  return 1, 2
+}
+
+;;; STDOUT ;;)