[wasm] Remove non-trivially dead or redundant code

After the FallThruTo in kExprEnd, the current block {c} is never
unreachable. Hence, the check for {c->unreachable} afterwards can be
removed.
In the loop case, the {TypeCheckFallThru} already adds entries for
non-existing values to the stack, so no need to {PushEndValues}.
Also, add more tests for the loop case.

R=titzer@chromium.org

Change-Id: I8737affaeed2ea663bd6ddafa36532ca9a7379bb
Reviewed-on: https://chromium-review.googlesource.com/645859Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47765}

src/wasm/function-body-decoder-impl.h

@@ -1315,7 +1315,6 @@ class WasmFullDecoder : public WasmDecoder<validate> {
             if (c->is_loop()) {
               // A loop just leaves the values on the stack.
               TypeCheckFallThru(c);
-              if (c->unreachable) PushEndValues(c);
               PopControl(c);
               break;
             }
@@ -1343,16 +1342,13 @@ class WasmFullDecoder : public WasmDecoder<validate> {
                 break;
               }
               last_end_found_ = true;
-              if (c->unreachable) {
-                TypeCheckFallThru(c);
-              } else {
               // The result of the block is the return value.
               TRACE("  @%-8d #xx:%-20s|", startrel(this->pc_),
                     "(implicit) return");
               DoReturn();
               TRACE("\n");
             }
-            }
+
             PopControl(c);
             break;
           }

test/cctest/wasm/test-run-wasm.cc

@@ -3037,6 +3037,47 @@ WASM_EXEC_TEST(BranchOverUnreachableCode) {
   CHECK_EQ(18, r.Call());
 }
 
+WASM_EXEC_TEST(BranchOverUnreachableCodeInLoop0) {
+  WasmRunner<int32_t> r(execution_mode);
+  BUILD(r,
+        WASM_BLOCK_I(
+            // Start a loop which breaks in the middle (hence unreachable code
+            // afterwards) and continue execution after this loop.
+            // This should validate even though there is no value on the stack
+            // at the end of the loop.
+            WASM_LOOP_I(WASM_BRV(1, WASM_I32V_1(17)))),
+        // Add one to the 17 returned from the block.
+        WASM_ONE, kExprI32Add);
+  CHECK_EQ(18, r.Call());
+}
+
+WASM_EXEC_TEST(BranchOverUnreachableCodeInLoop1) {
+  WasmRunner<int32_t> r(execution_mode);
+  BUILD(r,
+        WASM_BLOCK_I(
+            // Start a loop which breaks in the middle (hence unreachable code
+            // afterwards) and continue execution after this loop.
+            // Even though unreachable, the loop leaves one value on the stack.
+            WASM_LOOP_I(WASM_BRV(1, WASM_I32V_1(17)), WASM_ONE)),
+        // Add one to the 17 returned from the block.
+        WASM_ONE, kExprI32Add);
+  CHECK_EQ(18, r.Call());
+}
+
+WASM_EXEC_TEST(BranchOverUnreachableCodeInLoop2) {
+  WasmRunner<int32_t> r(execution_mode);
+  BUILD(r,
+        WASM_BLOCK_I(
+            // Start a loop which breaks in the middle (hence unreachable code
+            // afterwards) and continue execution after this loop.
+            // The unreachable code is allowed to pop non-existing values off
+            // the stack and push back the result.
+            WASM_LOOP_I(WASM_BRV(1, WASM_I32V_1(17)), kExprI32Add)),
+        // Add one to the 17 returned from the block.
+        WASM_ONE, kExprI32Add);
+  CHECK_EQ(18, r.Call());
+}
+
 WASM_EXEC_TEST(BlockInsideUnreachable) {
   WasmRunner<int32_t> r(execution_mode);
   BUILD(r, WASM_RETURN1(WASM_I32V_1(17)), WASM_BLOCK(WASM_BR(0)));