Add a pass to lower sign-ext operations to MVP (#5254)

Fixes #5250

1 files changed, 75 insertions, 0 deletions

diff --git a/src/passes/SignExtLowering.cpp b/src/passes/SignExtLowering.cpp
new file mode 100644
index 000000000..7d03046ea
--- /dev/null
+++ b/src/passes/SignExtLowering.cpp
@@ -0,0 +1,75 @@
+/*
+ * Copyright 2022 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+//
+// Lowers sign-ext operations into wasm MVP operations.
+//
+
+#include "pass.h"
+#include "wasm-builder.h"
+#include "wasm.h"
+
+namespace wasm {
+
+struct SignExtLowering : public WalkerPass<PostWalker<SignExtLowering>> {
+  template<typename T>
+  void lowerToShifts(Expression* value,
+                     BinaryOp leftShift,
+                     BinaryOp rightShift,
+                     T originalBits) {
+    // To sign-extend, we shift all the way left so the effective sign bit is
+    // where the actual sign bit is. For example, when sign-extending i8, the
+    // effective sign bit is at bit 8, and we shift it to the actual place of
+    // the sign bit, which is 32 for i32 or 64 for i64. Then we do a signed
+    // shift in the other direction, which fills with the proper bit all the
+    // way back, so e.g.
+    //
+    //  0x000000ff  =(shift left)=>  0xff000000  =(shift right)=>  0xffffffff
+    //
+    T shiftBits = (sizeof(T) * 8) - originalBits;
+    Builder builder(*getModule());
+    replaceCurrent(builder.makeBinary(
+      rightShift,
+      builder.makeBinary(leftShift, value, builder.makeConst(shiftBits)),
+      builder.makeConst(shiftBits)));
+  }
+
+  void visitUnary(Unary* curr) {
+    switch (curr->op) {
+      case ExtendS8Int32:
+        lowerToShifts(curr->value, ShlInt32, ShrSInt32, int32_t(8));
+        break;
+      case ExtendS16Int32:
+        lowerToShifts(curr->value, ShlInt32, ShrSInt32, int32_t(16));
+        break;
+      case ExtendS8Int64:
+        lowerToShifts(curr->value, ShlInt64, ShrSInt64, int64_t(8));
+        break;
+      case ExtendS16Int64:
+        lowerToShifts(curr->value, ShlInt64, ShrSInt64, int64_t(16));
+        break;
+      case ExtendS32Int64:
+        lowerToShifts(curr->value, ShlInt64, ShrSInt64, int64_t(32));
+        break;
+      default: {
+      }
+    }
+  }
+};
+
+Pass* createSignExtLoweringPass() { return new SignExtLowering(); }
+
+} // namespace wasm