[clang][bytecode] Special-case ConstantExpr in if conditions (#130294)

This happens a lot with `if constexpr` with a condition based on a
template param. In those cases, the condition is a ConstantExpr with a
value already set, so we can use that and ignore the other branch.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -5167,6 +5167,12 @@ bool Compiler<Emitter>::visitReturnStmt(const ReturnStmt *RS) {
 }
 
 template <class Emitter> bool Compiler<Emitter>::visitIfStmt(const IfStmt *IS) {
+  auto visitChildStmt = [&](const Stmt *S) -> bool {
+    LocalScope<Emitter> SScope(this);
+    if (!visitStmt(S))
+      return false;
+    return SScope.destroyLocals();
+  };
   if (auto *CondInit = IS->getInit())
     if (!visitStmt(CondInit))
       return false;
@@ -5175,7 +5181,22 @@ template <class Emitter> bool Compiler<Emitter>::visitIfStmt(const IfStmt *IS) {
     if (!visitDeclStmt(CondDecl))
       return false;
 
-  // Compile condition.
+  // Save ourselves compiling some code and the jumps, etc. if the condition is
+  // stataically known to be either true or false. We could look at more cases
+  // here, but I think all the ones that actually happen are using a
+  // ConstantExpr.
+  if (const auto *CE = dyn_cast_if_present<ConstantExpr>(IS->getCond());
+      CE && CE->hasAPValueResult() &&
+      CE->getResultAPValueKind() == APValue::ValueKind::Int) {
+    APSInt Value = CE->getResultAsAPSInt();
+    if (Value.getBoolValue())
+      return visitChildStmt(IS->getThen());
+    else if (const Stmt *Else = IS->getElse())
+      return visitChildStmt(Else);
+    return true;
+  }
+
+  // Otherwise, compile the condition.
   if (IS->isNonNegatedConsteval()) {
     if (!this->emitIsConstantContext(IS))
       return false;
@@ -5194,35 +5215,20 @@ template <class Emitter> bool Compiler<Emitter>::visitIfStmt(const IfStmt *IS) {
     LabelTy LabelEnd = this->getLabel();
     if (!this->jumpFalse(LabelElse))
       return false;
-    {
-      LocalScope<Emitter> ThenScope(this);
-      if (!visitStmt(IS->getThen()))
-        return false;
-      if (!ThenScope.destroyLocals())
-        return false;
-    }
+    if (!visitChildStmt(IS->getThen()))
+      return false;
     if (!this->jump(LabelEnd))
       return false;
     this->emitLabel(LabelElse);
-    {
-      LocalScope<Emitter> ElseScope(this);
-      if (!visitStmt(Else))
-        return false;
-      if (!ElseScope.destroyLocals())
-        return false;
-    }
+    if (!visitChildStmt(Else))
+      return false;
     this->emitLabel(LabelEnd);
   } else {
     LabelTy LabelEnd = this->getLabel();
     if (!this->jumpFalse(LabelEnd))
       return false;
-    {
-      LocalScope<Emitter> ThenScope(this);
-      if (!visitStmt(IS->getThen()))
-        return false;
-      if (!ThenScope.destroyLocals())
-        return false;
-    }
+    if (!visitChildStmt(IS->getThen()))
+      return false;
     this->emitLabel(LabelEnd);
   }