Add -freroll-loops to enable loop rerolling

This adds -freroll-loops (and -fno-reroll-loops in the usual way) to enable
loop rerolling as part of the optimization pass manager. This transformation
can enable vectorization, reduce code size (or both).

Briefly, loop rerolling can transform a loop like this:

for (int i = 0; i < 3200; i += 5) {
  a[i]     += alpha * b[i];
  a[i + 1] += alpha * b[i + 1];
  a[i + 2] += alpha * b[i + 2];
  a[i + 3] += alpha * b[i + 3];
  a[i + 4] += alpha * b[i + 4];
}

into this:

for (int i = 0; i < 3200; ++i) {
  a[i] += alpha * b[i];
}

Loop rerolling is currently disabled by default at all optimization levels.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194967 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 1 insertions, 0 deletions

diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 581d56d462..94a3a33235 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -355,6 +355,7 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   Opts.UnrollLoops =
       Args.hasFlag(OPT_funroll_loops, OPT_fno_unroll_loops,
                    (Opts.OptimizationLevel > 1 && !Opts.OptimizeSize));
+  Opts.RerollLoops = Args.hasArg(OPT_freroll_loops);
 
   Opts.Autolink = !Args.hasArg(OPT_fno_autolink);
   Opts.SampleProfileFile = Args.getLastArgValue(OPT_fprofile_sample_use_EQ);