OptimizeInstructions: Optimize boolean selects (#4147)

If all a select's inputs are boolean, we can sometimes turn the select
into an AND or an OR operation,

x ? y : 0  =>  x & y
x ? 1 : y  =>  x | y

I believe LLVM aggressively canonicalizes to this form. It makes sense
to do here too as it is smaller (save the constant 0 or 1). It also allows
further optimizations (which is why LLVM does it) but I don't think we
have those yet.

1 files changed, 19 insertions, 0 deletions

diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index 357f172d9..768ea5702 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -1819,6 +1819,25 @@ 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);
+      }
+    }
     {
       // Sides are identical, fold
       Expression *ifTrue, *ifFalse, *c;