[PATCH] D83211: Factor out call to EXTRACTOR in generateCC1CommandLine

[Michael Spencer via Phabricator via cfe-commits]

Bigcheese accepted this revision.
Bigcheese added inline comments.


================
Comment at: clang/lib/Frontend/CompilerInvocation.cpp:3938
+  if ((FLAGS)&options::CC1Option) {                                            \
+    const auto &Extracted = EXTRACTOR(this->KEYPATH);                          \
+    if (ALWAYS_EMIT || Extracted != DEFAULT_VALUE)                             \
----------------
jansvoboda11 wrote:
> dexonsmith wrote:
> > dexonsmith wrote:
> > > jansvoboda11 wrote:
> > > > Bigcheese wrote:
> > > > > 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`.
> > > > Mentioned the reference lifetime extension in a comment near extractor definitions.
> > > 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));
> > > ```
> > > 
> > 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).
> 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?
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.


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  https://reviews.llvm.org/D83211/new/

https://reviews.llvm.org/D83211