fix optimizing two shifts into one; if the number of effective shifts overflows, it is not vali to just add them

3 files changed, 41 insertions, 14 deletions

diff --git a/src/ast/bits.h b/src/ast/bits.h
index c7d2ea4f0..11cf7b06d 100644
--- a/src/ast/bits.h
+++ b/src/ast/bits.h
@@ -42,11 +42,20 @@ struct Bits {
 
   // gets the number of effective shifts a shift operation does. In
   // wasm, only 5 bits matter for 32-bit shifts, and 6 for 64.
-  static uint32_t getEffectiveShifts(Const* amount) {
+  static Index getEffectiveShifts(Index amount, WasmType type) {
+    if (type == i32) {
+      return amount & 31;
+    } else if (type == i64) {
+      return amount & 63;
+    }
+    WASM_UNREACHABLE();
+  }
+
+  static Index getEffectiveShifts(Const* amount) {
     if (amount->type == i32) {
-      return amount->value.geti32() & 31;
+      return getEffectiveShifts(amount->value.geti32(), i32);
     } else if (amount->type == i64) {
-      return amount->value.geti64() & 63;
+      return getEffectiveShifts(amount->value.geti64(), i64);
     }
     WASM_UNREACHABLE();
   }
diff --git a/src/ast/literal-utils.h b/src/ast/literal-utils.h
index 7e75e8bc8..afa8146b9 100644
--- a/src/ast/literal-utils.h
+++ b/src/ast/literal-utils.h
@@ -23,21 +23,31 @@ namespace wasm {
 
 namespace LiteralUtils {
 
-inline Expression* makeZero(WasmType type, Module& wasm) {
-  Literal value;
+inline Literal makeLiteralFromInt32(int32_t x, WasmType type) {
   switch (type) {
-    case i32: value = Literal(int32_t(0)); break;
-    case i64: value = Literal(int64_t(0)); break;
-    case f32: value = Literal(float(0)); break;
-    case f64: value = Literal(double(0)); break;
+    case i32: return Literal(int32_t(x)); break;
+    case i64: return Literal(int64_t(x)); break;
+    case f32: return Literal(float(x)); break;
+    case f64: return Literal(double(x)); break;
     default: WASM_UNREACHABLE();
   }
+}
+
+inline Literal makeLiteralZero(WasmType type) {
+  return makeLiteralFromInt32(0, type);
+}
+
+inline Expression* makeFromInt32(int32_t x, WasmType type, Module& wasm) {
   auto* ret = wasm.allocator.alloc<Const>();
-  ret->value = value;
-  ret->type = value.type;
+  ret->value = makeLiteralFromInt32(x, type);
+  ret->type = type;
   return ret;
 }
 
+inline Expression* makeZero(WasmType type, Module& wasm) {
+  return makeFromInt32(0, type, wasm);
+}
+
 } // namespace LiteralUtils
 
 } // namespace wasm
diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index 424cbcc67..60222c3cd 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -29,6 +29,7 @@
 #include <ast/effects.h>
 #include <ast/manipulation.h>
 #include <ast/properties.h>
+#include <ast/literal-utils.h>
 
 namespace wasm {
 
@@ -533,9 +534,16 @@ struct OptimizeInstructions : public WalkerPass<PostWalker<OptimizeInstructions,
               } else if (left->op == OrInt32) {
                 leftRight->value = leftRight->value.or_(right->value);
                 return left;
-              } else if (left->op == ShlInt32 || left->op == ShrUInt32 || left->op == ShrSInt32) {
-                leftRight->value = leftRight->value.add(right->value);
-                return left;
+              } else if (left->op == ShlInt32 || left->op == ShrUInt32 || left->op == ShrSInt32 ||
+                         left->op == ShlInt64 || left->op == ShrUInt64 || left->op == ShrSInt64) {
+                // shifts only use an effective amount from the constant, so adding must
+                // be done carefully
+                auto total = Bits::getEffectiveShifts(leftRight) + Bits::getEffectiveShifts(right);
+                if (total == Bits::getEffectiveShifts(total, left->type)) {
+                  // no overflow, we can do this
+                  leftRight->value = LiteralUtils::makeLiteralFromInt32(total, left->type);
+                  return left;
+                } // TODO: handle overflows
               }
             }
           }