From 7fb44812fa307a5924e5ada58b4015de6736cce3 Mon Sep 17 00:00:00 2001
From: Max Graey <maxgraey@gmail.com>
Date: Mon, 29 Aug 2022 18:22:01 +0300
Subject: [OptimizeInstruction] Reorder rules in optimizeSelect (#4984)

To unblock some optimizations. For example this:
```wat
(select
 (i32.eqz (local.get $x))
 (i32.const 0)
 (i32.eqz (local.get $y))
)
```
Was previously optimized as:
```wat
(i32.eqz
 (select
  (i32.const 1)
  (local.get $x)
  (local.get $y)
 )
)
```
Because `optimizeSelect` applied `!x ? !y : 0  ->  x ? 0 : !y` then `!(x ? 1 : y)`, blocking the next rules which could have folded this to `or` or `and`.

After this PR the same example optimizes better:
```wat
(i32.eqz
 (i32.or
  (local.get $x)
  (local.get $y)
 )
)
```
---
 src/passes/OptimizeInstructions.cpp | 64 ++++++++++++++++++-------------------
 1 file changed, 32 insertions(+), 32 deletions(-)

(limited to 'src')

diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index b50f99fbc..9379d8588 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -2369,21 +2369,6 @@ private:
       // Don't bother when `ifFalse` isn't pure - we would need to reverse the
       // order using a temp local, which would be bad
     }
-    {
-      // Flip select to remove eqz if we can reorder
-      Select* s;
-      Expression *ifTrue, *ifFalse, *c;
-      if (matches(
-            curr,
-            select(
-              &s, any(&ifTrue), any(&ifFalse), unary(EqZInt32, any(&c)))) &&
-          canReorder(ifTrue, ifFalse)) {
-        s->ifTrue = ifFalse;
-        s->ifFalse = ifTrue;
-        s->condition = c;
-        return s;
-      }
-    }
     {
       // TODO: Remove this after landing SCCP pass. See: #4161
 
@@ -2436,6 +2421,25 @@ private:
         return curr->type == Type::i64 ? builder.makeUnary(ExtendUInt32, c) : c;
       }
     }
+    if (curr->type == Type::i32 &&
+        Bits::getMaxBits(curr->condition, this) <= 1 &&
+        Bits::getMaxBits(curr->ifTrue, this) <= 1 &&
+        Bits::getMaxBits(curr->ifFalse, this) <= 1) {
+      // The condition and both arms are i32 booleans, which allows us to do
+      // boolean optimizations.
+      Expression* x;
+      Expression* y;
+
+      // x ? y : 0   ==>   x & y
+      if (matches(curr, select(any(&y), ival(0), any(&x)))) {
+        return builder.makeBinary(AndInt32, y, x);
+      }
+
+      // x ? 1 : y   ==>   x | y
+      if (matches(curr, select(ival(1), any(&y), any(&x)))) {
+        return builder.makeBinary(OrInt32, y, x);
+      }
+    }
     {
       // Simplify x < 0 ? -1 : 1 or x >= 0 ? 1 : -1 to
       // i32(x) >> 31 | 1
@@ -2459,23 +2463,19 @@ private:
         }
       }
     }
-    if (curr->type == Type::i32 &&
-        Bits::getMaxBits(curr->condition, this) <= 1 &&
-        Bits::getMaxBits(curr->ifTrue, this) <= 1 &&
-        Bits::getMaxBits(curr->ifFalse, this) <= 1) {
-      // The condition and both arms are i32 booleans, which allows us to do
-      // boolean optimizations.
-      Expression* x;
-      Expression* y;
-
-      // x ? y : 0   ==>   x & y
-      if (matches(curr, select(any(&y), ival(0), any(&x)))) {
-        return builder.makeBinary(AndInt32, y, x);
-      }
-
-      // x ? 1 : y   ==>   x | y
-      if (matches(curr, select(ival(1), any(&y), any(&x)))) {
-        return builder.makeBinary(OrInt32, y, x);
+    {
+      // Flip select to remove eqz if we can reorder
+      Select* s;
+      Expression *ifTrue, *ifFalse, *c;
+      if (matches(
+            curr,
+            select(
+              &s, any(&ifTrue), any(&ifFalse), unary(EqZInt32, any(&c)))) &&
+          canReorder(ifTrue, ifFalse)) {
+        s->ifTrue = ifFalse;
+        s->ifFalse = ifTrue;
+        s->condition = c;
+        return s;
       }
     }
     {
-- 
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