Do not precompute v128 expressions (#1839)

Without this change, sequences like `i32.const 0, i32x4.splat` will
get precomputed to v128.const ops, which are much larger and also not
implemented in V8 yet. Until we have SIMD-aware optimization passes or
at least engine support for v128.const, do not perform such
transformations.

Author: tlively

src/passes/Precompute.cpp

@@ -161,6 +161,10 @@ struct Precompute : public WalkerPass<PostWalker<Precompute, UnifiedExpressionVi
   void visitExpression(Expression* curr) {
     // TODO: if get_local, only replace with a constant if we don't care about size...?
     if (curr->is<Const>() || curr->is<Nop>()) return;
+    // Until engines implement v128.const and we have SIMD-aware optimizations
+    // that can break large v128.const instructions into smaller consts and
+    // splats, do not try to precompute v128 expressions.
+    if (curr->type == v128) return;
     // try to evaluate this into a const
     Flow flow = precomputeExpression(curr);
     if (flow.breaking()) {

test/passes/precompute.txt

@@ -3,6 +3,7 @@
  (type $1 (func (result i32)))
  (type $2 (func))
  (type $3 (func (result f64)))
+ (type $4 (func (result v128)))
  (memory $0 0)
  (global $global i32 (i32.const 1))
  (global $global-mut (mut i32) (i32.const 2))
@@ -211,4 +212,9 @@
    (i32.const 2)
   )
  )
+ (func $no-simd-precompute (; 11 ;) (type $4) (result v128)
+  (i32x4.splat
+   (i32.const 0)
+  )
+ )
 )

test/passes/precompute.wast

@@ -306,4 +306,9 @@
     )
    )
   )
+  (func $no-simd-precompute (result v128)
+   (i32x4.splat
+    (i32.const 0)
+   )
+  )
 )