Wasm: Better codegen for Long shifts with constant right-hand-side.

sjrd · sjrd · commit 01ee94a925f8 · 2025-03-26T11:19:45.000+01:00
In our IR, the rhs of Long shifts is an Int, but in Wasm it must be
a Long as well. In the common-case where the rhs is a constant, we
now constant-fold the extension to 64 bits.
diff --git a/linker/shared/src/main/scala/org/scalajs/linker/backend/wasmemitter/FunctionEmitter.scala b/linker/shared/src/main/scala/org/scalajs/linker/backend/wasmemitter/FunctionEmitter.scala
@@ -1648,9 +1648,18 @@ private class FunctionEmitter private (
 
     def genLongShiftOp(shiftInstr: wa.Instr): Type = {
       genTree(lhs, LongType)
-      genTree(rhs, IntType)
-      markPosition(tree)
-      fb += wa.I64ExtendI32S
+      rhs match {
+        case IntLiteral(r) =>
+          // common case: fold the extension to 64 bits into the literal
+          markPosition(rhs)
+          fb += wa.I64Const(r.toLong)
+          markPosition(tree)
+        case _ =>
+          // otherwise, extend at run-time
+          genTree(rhs, IntType)
+          markPosition(tree)
+          fb += wa.I64ExtendI32S
+      }
       fb += shiftInstr
       LongType
     }
