[wasm] Avoid redundant validation checks

The only opcode which can pop the last control block is "end".
There is already validation in place to check that no code is following
the last "end". Thus all checks for a non-empty control stack in other
opcodes are redundant and can be replaced by DCHECKs.

Drive-by: Avoid the use of {PushControl} for pushing the very first
block, to avoid another check for an empty control stack there.

R=thibaudm@chromium.org

Change-Id: I8f04b225a61f832bca2dd7b706e31d1faa173767
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2678162Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72589}

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

@@ -2509,13 +2509,10 @@ class WasmFullDecoder : public WasmDecoder<validate> {
     CHECK_PROTOTYPE_OPCODE(eh);
     ExceptionIndexImmediate<validate> imm(this, this->pc_ + 1);
     if (!this->Validate(this->pc_ + 1, imm)) return 0;
-    if (!VALIDATE(!control_.empty())) {
-      this->DecodeError("catch does not match any try");
-      return 0;
-    }
+    DCHECK(!control_.empty());
     Control* c = &control_.back();
     if (!VALIDATE(c->is_try())) {
-      this->DecodeError("catch does not match any try");
+      this->DecodeError("catch does not match a try");
       return 0;
     }
     if (!VALIDATE(!c->is_try_catchall())) {
@@ -2568,13 +2565,10 @@ class WasmFullDecoder : public WasmDecoder<validate> {
 
   DECODE(Unwind) {
     CHECK_PROTOTYPE_OPCODE(eh);
-    if (!VALIDATE(!control_.empty())) {
-      this->DecodeError("unwind does not match any try");
-      return 0;
-    }
+    DCHECK(!control_.empty());
     Control* c = &control_.back();
     if (!VALIDATE(c->is_try())) {
-      this->DecodeError("unwind does not match any try");
+      this->DecodeError("unwind does not match a try");
       return 0;
     }
     if (!VALIDATE(!c->is_try_catch() && !c->is_try_catchall())) {
@@ -2687,13 +2681,10 @@ class WasmFullDecoder : public WasmDecoder<validate> {
 
   // Alias for "catch_all" if the current block is a try.
   DECODE(Else) {
-    if (!VALIDATE(!control_.empty())) {
-      this->DecodeError("else/catch_all does not match any if/try");
-      return 0;
-    }
+    DCHECK(!control_.empty());
     Control* c = &control_.back();
     if (!VALIDATE(c->is_if() || c->is_try())) {
-      this->DecodeError("else/catch_all does not match any if/try");
+      this->DecodeError("else/catch_all does not match an if/try");
       return 0;
     }
     if (c->is_if()) {
@@ -2725,10 +2716,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
   }
 
   DECODE(End) {
-    if (!VALIDATE(!control_.empty())) {
-      this->DecodeError("end does not match any if, try, or block");
-      return 0;
-    }
+    DCHECK(!control_.empty());
     Control* c = &control_.back();
     if (!VALIDATE(!c->is_incomplete_try())) {
       this->DecodeError("missing catch or catch-all in try");
@@ -3386,7 +3374,9 @@ class WasmFullDecoder : public WasmDecoder<validate> {
 
     // Set up initial function block.
     {
-      Control* c = PushControl(kControlBlock);
+      DCHECK(control_.empty());
+      control_.emplace_back(kControlBlock, 0, 0, this->pc_, kReachable);
+      Control* c = &control_.back();
       InitMerge(&c->start_merge, 0, [](uint32_t) -> Value { UNREACHABLE(); });
       InitMerge(&c->end_merge,
                 static_cast<uint32_t>(this->sig_->return_count()),
@@ -3485,8 +3475,8 @@ class WasmFullDecoder : public WasmDecoder<validate> {
   }
 
   Control* PushControl(ControlKind kind, uint32_t locals_count = 0) {
-    Reachability reachability =
-        control_.empty() ? kReachable : control_.back().innerReachability();
+    DCHECK(!control_.empty());
+    Reachability reachability = control_.back().innerReachability();
     control_.emplace_back(kind, locals_count, stack_size(), this->pc_,
                           reachability);
     current_code_reachable_ = this->ok() && reachability == kReachable;
@@ -3494,6 +3484,9 @@ class WasmFullDecoder : public WasmDecoder<validate> {
   }
 
   void PopControl(Control* c) {
+    // This cannot be the outermost control block.
+    DCHECK_LT(1, control_.size());
+
     DCHECK_EQ(c, &control_.back());
     CALL_INTERFACE_IF_PARENT_REACHABLE(PopControl, c);