[clang][cli] Factor out call to EXTRACTOR in generateCC1CommandLine (NFC)

[dang]

Rebase on top of latest changes

[Bigcheese]

clang/lib/Frontend/CompilerInvocation.cpp

3931

Will this ever have an issue with lifetime? I can see various values for EXTRACTOR causing issues here. https://abseil.io/tips/107

It would be good to at least document somewhere the restrictions on EXTRACTOR.

[jansvoboda11]

Taking over this patch as Daniel is no longer involved.

[jansvoboda11]

Rebase and add documenting possible reference lifetime extension issue

[jansvoboda11]

Addressed review feedback. WDYT @Bigcheese?

clang/lib/Frontend/CompilerInvocation.cpp

3931

Mentioned the reference lifetime extension in a comment near extractor definitions.

[jansvoboda11]

Rebase

[dexonsmith]

clang/lib/Frontend/CompilerInvocation.cpp

3931

It might be safer to refactor as:

// Capture the extracted value as a lambda argument to avoid potential
// lifetime extension issues.
[&](const auto &Extracted) {
  if (ALWAYS_EMIT || Extracted != (DEFAULT_VALUE))
    DENORMALIZER(Args, SPELLING, SA, TABLE_INDEX, Extracted);
}(EXTRACTOR(this->KEYPATH));

3931

IIUC, then we can do this more simply as:

bool Extracted = EXTRACTOR(this->KEYPATH);

That might clarify why we don't have lifetime extension concerns here.

[dexonsmith]

clang/lib/Frontend/CompilerInvocation.cpp

3931

Might be even better to avoid the generic lambda:

// Capture the extracted value as a lambda argument to avoid potential
// lifetime extension issues.
using ExtractedType =
    std::remove_const_t<std::remove_reference_t<
        decltype(EXTRACTOR(this->KEYPATH))>>
[&](const ExtractedType &Extracted) {
  if (ALWAYS_EMIT || Extracted != (DEFAULT_VALUE))
    DENORMALIZER(Args, SPELLING, SA, TABLE_INDEX, Extracted);
}(EXTRACTOR(this->KEYPATH));

(since generic vs. non-generic could affect compile-time of CompilerInvocation.cpp given how many instances there will be).

[dexonsmith]

Marking as "requested changes" to get this off my queue, but if my lambda suggestion doesn't work well for some reason (maybe it significantly increases compile-time), I'm open to sticking to the documentation @Bigcheese suggested.

[jansvoboda11]

clang/lib/Frontend/CompilerInvocation.cpp

3931

Thanks for the suggestions @dexonsmith. I'm having trouble writing a test case where the lambda workaround produces a different result than const auto & variable.
@Bigcheese, could you show a concrete example of an extractor that causes issues so I can test it out?

3931

Yes, we could simplify this for booleans.

[Bigcheese]

clang/lib/Frontend/CompilerInvocation.cpp

3931

I think I was confused about when this can happen. The const auto & will never cause a conversion that would prevent lifetime extension. The only place this would break is if the copy constructor of the type is rather broken, which isn't a reasonable case to support.

[Bigcheese]

clang/lib/Frontend/CompilerInvocation.cpp

3931

Now I remember what the issue was, it's if the EXTRACTOR itself creates an owning temporary and converts back to a reference type: https://godbolt.org/z/xsvh4f

This is kind of weird to do, the EXTRACTOR would need to take an owning type with an implicit conversion from the type of KEYPATH, and then return a reference type. I'm not sure how realistic that situation is, but it seems fine to defend against with Duncan's suggestion.

[jansvoboda11]

Implement review feedback

[jansvoboda11]

Remove lifetime comment

[jansvoboda11]

Undo whitespace change

[dexonsmith]

LGTM!

clang/lib/Frontend/CompilerInvocation.cpp

 }
 
 void CompilerInvocation::generateCC1CommandLine(
     SmallVectorImpl<const char *> &Args, StringAllocator SA) const {
 #define OPTION_WITH_MARSHALLING(                                               \
     PREFIX_TYPE, NAME, ID, KIND, GROUP, ALIAS, ALIASARGS, FLAGS, PARAM,        \
     HELPTEXT, METAVAR, VALUES, SPELLING, ALWAYS_EMIT, KEYPATH, DEFAULT_VALUE,  \
     TYPE, NORMALIZER, DENORMALIZER, MERGER, EXTRACTOR, TABLE_INDEX)            \
-  if (((FLAGS)&options::CC1Option) &&                                          \
-      (ALWAYS_EMIT || EXTRACTOR(this->KEYPATH) != DEFAULT_VALUE)) {            \
-    DENORMALIZER(Args, SPELLING, SA, TABLE_INDEX, EXTRACTOR(this->KEYPATH));   \
+  if ((FLAGS)&options::CC1Option) {                                            \
+    const auto &Extracted = EXTRACTOR(this->KEYPATH);                          \
+    if (ALWAYS_EMIT || Extracted != DEFAULT_VALUE)                             \
+      DENORMALIZER(Args, SPELLING, SA, TABLE_INDEX, Extracted);                \
   }
 
 #include "clang/Driver/Options.inc"
 #undef OPTION_WITH_MARSHALLING
 }
 
 namespace clang {