[PM/CC1] Add -f[no-]split-cold-code CC1 option to toggle splitting

This patch adds -f[no-]split-cold-code CC1 options to clang. This allows
the splitting pass to be toggled on/off. The current method of passing
`-mllvm -hot-cold-split=true` to clang isn't ideal as it may not compose
correctly (say, with `-O0` or `-Oz`).

To implement the -fsplit-cold-code option, an attribute is applied to
functions to indicate that they may be considered for splitting. This
removes some complexity from the old/new PM pipeline builders, and
behaves as expected when LTO is enabled.

Co-authored by: Saleem Abdulrasool <compnerd@compnerd.org>
Differential Revision: https://reviews.llvm.org/D57265
Reviewed By: Aditya Kumar, Vedant Kumar
Reviewers: Teresa Johnson, Aditya Kumar, Fedor Sergeev, Philip Pfaffe, Vedant Kumar

1 files changed, 11 insertions, 0 deletions

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index a4c56cc..87d64f1 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1469,6 +1469,17 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
 
   Opts.PassByValueIsNoAlias = Args.hasArg(OPT_fpass_by_value_is_noalias);
 
+  // -f[no-]split-cold-code
+  // This may only be enabled when optimizing, and when small code size
+  // increases are tolerable.
+  Opts.SplitColdCode =
+      (Opts.OptimizationLevel > 0) && (Opts.OptimizeSize != 2) &&
+      Args.hasFlag(OPT_fsplit_cold_code, OPT_fno_split_cold_code, false);
+  if (Arg *A = Args.getLastArg(OPT_fsplit_cold_code))
+    if (!Opts.SplitColdCode)
+      Diags.Report(diag::warn_fe_ignored_opt_split_cold_code)
+          << A->getAsString(Args) << (Opts.OptimizeSize != 2);
+
   return Success;
 }