[PATCH] D53891: [LTO] Add option to enable LTOUnit splitting, and disable unless needed

[Peter Collingbourne via Phabricator via cfe-commits]

pcc added inline comments.


================
Comment at: lib/Driver/ToolChains/Clang.cpp:5112
+  bool EnableSplitLTOUnit = Args.hasFlag(
+      options::OPT_fsplit_lto_unit, options::OPT_fno_split_lto_unit, false);
+  if (EnableSplitLTOUnit || WholeProgramVTables || Sanitize.needsLTO()) {
----------------
pcc wrote:
> tejohnson wrote:
> > pcc wrote:
> > > Should this default to `WholeProgramVTables || Sanitize.needsLTO()` and emit an error if the user passes the (for now) unsupported combinations `-fno-split-lto-unit -fwhole-program-vtables` or `-fno-split-lto-unit -fsanitize=cfi`?
> > I think the code below needs to stay as is to allow -fsplit-lto-unit to also enable the splitting even when the other options aren't on (not sure when that would be used outside of testing, but I'm assuming we want a way to force that on). 
> > 
> > But yes I think it makes sense to emit an error on those combinations (when my follow on patches go in then we would remove the error with -fno-split-lto-unit -fwhole-program-vtables).
> `-fsplit-lto-unit` is required when linking translation units compiled with `-fsanitize=cfi` with translation units compiled without (the latter would need the flag).
> 
> May I suggest simplifying this code as follows:
> ```
> bool RequiresSplitLTOUnit = WholeProgramVTables || Sanitize.needsLTO();
> bool SplitLTOUnit =
>  Args.hasFlag(options::OPT_fwhole_program_vtables,
> 	      options::OPT_fno_whole_program_vtables, RequiresSplitLTOUnit);
> if (RequiresSplitLTOUnit && !SplitLTOUnit)
>    error;
> if (SplitLTOUnit)
>   CmdArgs.push_back("-fsplit-lto-unit");
> ```
> 
Sorry, this was meant to be `OPT_fsplit_lto_unit` and `OPT_fno_split_lto_unit`.


Repository:
  rC Clang

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D53891/new/

https://reviews.llvm.org/D53891