[Optimize-Instructions] Simplify sign patterns like x < 0 ? -1 : 1 (#3756)

i32(x) < 0 ? i32(-1) : i32(1)  ->  x >> 31 | 1
i64(x) < 0 ? i64(-1) : i64(1)  ->  x >> 63 | 1

This shrinks 2 bytes.

1 files changed, 24 insertions, 0 deletions

diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index d3c3f26f6..106bfea19 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -1430,6 +1430,7 @@ private:
 
   Expression* optimizeSelect(Select* curr) {
     using namespace Match;
+    using namespace Abstract;
     Builder builder(*getModule());
     curr->condition = optimizeBoolean(curr->condition);
     {
@@ -1482,6 +1483,29 @@ private:
       }
     }
     {
+      // Simplify x < 0 ? -1 : 1 or x >= 0 ? 1 : -1 to
+      // i32(x) >> 31 | 1
+      // i64(x) >> 63 | 1
+      Binary* bin;
+      if (matches(
+            curr,
+            select(ival(-1), ival(1), binary(&bin, LtS, any(), ival(0)))) ||
+          matches(
+            curr,
+            select(ival(1), ival(-1), binary(&bin, GeS, any(), ival(0))))) {
+        auto c = bin->right->cast<Const>();
+        auto type = curr->ifTrue->type;
+        if (type == c->type) {
+          bin->type = type;
+          bin->op = Abstract::getBinary(type, ShrS);
+          c->value = Literal::makeFromInt32(type.getByteSize() * 8 - 1, type);
+          curr->ifTrue->cast<Const>()->value = Literal::makeOne(type);
+          return builder.makeBinary(
+            Abstract::getBinary(type, Or), bin, curr->ifTrue);
+        }
+      }
+    }
+    {
       // Sides are identical, fold
       Expression *ifTrue, *ifFalse, *c;
       if (matches(curr, select(any(&ifTrue), any(&ifFalse), any(&c))) &&