[PATCH] D122155: Add warning when eval-method is set in the presence of value unsafe floating-point calculations.

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added inline comments.


================
Comment at: clang/include/clang/Basic/DiagnosticFrontendKinds.td:50-53
+def warn_eval_method_setting_via_option_in_value_unsafe_context : Warning<
+    "setting the eval method via '-ffp-eval-method' has not effect when numeric "
+    "results of floating-point calculations aren't value-safe.">,
+    InGroup<IncompatibleFPOpts>;
----------------
aaron.ballman wrote:
> zahiraam wrote:
> > aaron.ballman wrote:
> > > andrew.w.kaylor wrote:
> > > > zahiraam wrote:
> > > > > aaron.ballman wrote:
> > > > > > Unless you have a strong reason for this to be a warning, this seems like a situation we should diagnose as an error with a much clearer message.
> > > > > May  be @andrew.w.kaylor would weigh in on this?
> > > > I was going to say that for the command line option we could just issue a warning saying that the later option overrides the earlier, but it's a bit complicated to sort out what that would mean if the eval method follows a fast-math option and it might not always be what the user intended. So, I guess I'd agree that it should be an error.
> > > > 
> > > > For the case with pragmas, the model I'd follow is the mixing of #pragma float_control(except, on) with a fast-math mode or #pragma float_control(precise, off) with a non-ignore exception mode. In both those cases we issue an error.
> > > > For the case with pragmas, the model I'd follow is the mixing of #pragma float_control(except, on) with a fast-math mode or #pragma float_control(precise, off) with a non-ignore exception mode. In both those cases we issue an error.
> > > 
> > > Good catch, I think that's a good approach as well.
> > I think i  will have the issue with the order of appearance of the options on the command line. 
> > // RUN: -freciprocal-math -mreassociate   -ffp-eval-method=source 
> > and
> > // RUN: -mreassociate -ffp-eval-method=source 
> > 
> > will depend on which order I will test for LangOpts.ApproxFunc/AllowFPReasson/AllowRecip being used or not?
> > 
> > The run lines above might give the same diagnostic. Unless I do something really complicated to check the order of the options on the command line?
> > I think i will have the issue with the order of appearance of the options on the command line.
> 
> You shouldn't -- you should be able to test the language options after the command line was fully parsed. See `FixupInvocation()` in `CompilerInvocation.cpp`.
I still prefer the suggested wording I had originally: `"'-ffp-eval-method' cannot be used with '%0'"`; I think it's a good generalization but still sufficiently informative. WDYT?


================
Comment at: clang/include/clang/Basic/DiagnosticSemaKinds.td:6480
+  "eval method setting via '%0' cannot be used with "
+  "'pragma_clang_fp_eval_reassociate'">, InGroup<Pragmas>;
+def warn_pragma_clang_fp_eval_method_used_with_fapprox_func : Warning<
----------------
I'm not certain what `pragma_clang_fp_eval_reassociate` is?


================
Comment at: clang/include/clang/Basic/DiagnosticSemaKinds.td:6481-6489
+def warn_pragma_clang_fp_eval_method_used_with_fapprox_func : Warning<
+  "'#pragma clang fp eval_method' cannot be used with 'fapprox_func'">,
+  InGroup<Pragmas>;
+def warn_pragma_clang_fp_eval_method_used_with_mreassociate : Warning<
+  "'#pragma clang fp eval_method' cannot be used with 'mreassociate'">,
+  InGroup<Pragmas>;
+def warn_pragma_clang_fp_eval_method_used_with_freciprocal : Warning<
----------------
These should be combined into one diagnostic, which I believe we wanted to be an error instead of a warning. Also, because this will trigger for pragma use OR command line argument use, I think we need to be more generalize about what it cannot be used with. e.g., `'#pragma clang fp eval_method' cannot be used when %select{approximate functions|reassociation|reciprocal whatever}0 is enabled` or something along those lines (I'm hoping @andrew.w.kaylor can help figure out what the best terminology is here, as I'm not super familiar with those floating-point features).


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D122155/new/

https://reviews.llvm.org/D122155