[clang][Interp] Actually consider ConstantExpr result

[tbaeder]

Summary

Since we have visitAPValue now, we might as well use it here.

Not really possible to test this, so no test included.

[cor3ntin]

clang/lib/AST/Interp/ByteCodeExprGen.cpp

637–645

so if visitAPValue fails, we continue. Couldn't that lead to duplicated diagnostics? Shouldn't we simply return whatever visitAPValue returns unconditionally?

[tbaeder]

clang/lib/AST/Interp/ByteCodeExprGen.cpp

637–645

I did it this way because we don't handle all types of APValues in visitAPValue() (think lvalues with an lvalue path), so in those cases we need to visit the expression instead. If there is an APValue we're visting, we're not emitting any diagnostics I think.

[cor3ntin]

FYI there are build failures. you probably need to rebase (visitAPValue does not seem to actually exist)

clang/lib/AST/Interp/ByteCodeExprGen.cpp

637–645

Can you add a comment explaining that? I think it would help! thanks!

[tbaeder]

In D158502#4616174, @cor3ntin wrote:

FYI there are build failures. you probably need to rebase (visitAPValue does not seem to actually exist)

This depends on the __builtin_offsetof() patch.

[aaron.ballman]

LGTM

clang/lib/AST/Interp/ByteCodeExprGen.cpp

 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitSubstNonTypeTemplateParmExpr(
     const SubstNonTypeTemplateParmExpr *E) {
   return this->delegate(E->getReplacement());
 }
 
 template <class Emitter>
 bool ByteCodeExprGen<Emitter>::VisitConstantExpr(const ConstantExpr *E) {
-  // TODO: Check if the ConstantExpr already has a value set and if so,
-  //   use that instead of evaluating it again.
+  // Try to emit the APValue directly, without visiting the subexpr.
+  // This will only fail if we can't emit the APValue, so won't emit any
+  // diagnostics or any double values.
+  std::optional<PrimType> T = classify(E->getType());
+  if (T && E->hasAPValueResult() &&
+      this->visitAPValue(E->getAPValueResult(), *T, E))
+    return true;
+
   return this->delegate(E->getSubExpr());
 }
 
 static CharUnits AlignOfType(QualType T, const ASTContext &ASTCtx,
                              UnaryExprOrTypeTrait Kind) {
   bool AlignOfReturnsPreferred =
       ASTCtx.getLangOpts().getClangABICompat() <= LangOptions::ClangABI::Ver7;
 
   // C++ [expr.alignof]p3: