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

[Peter Collingbourne via Phabricator via cfe-commits]

pcc accepted this revision.
pcc added a comment.
This revision is now accepted and ready to land.

LGTM



================
Comment at: include/clang/Basic/CodeGenOptions.def:120
+CODEGENOPT(EnableSplitLTOUnit, 1, 0) ///< Enable LTO unit splitting to support
+				     /// CFI and tranditional whole program
+				     /// devirtulization that require whole
----------------
traditional


================
Comment at: include/clang/Basic/CodeGenOptions.def:121
+				     /// CFI and tranditional whole program
+				     /// devirtulization that require whole
+				     /// program IR support.
----------------
devirtualization


================
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()) {
----------------
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");
```



Repository:
  rC Clang

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  https://reviews.llvm.org/D53891/new/

https://reviews.llvm.org/D53891