[IRCE] Avoid loop optimizations on pre and post loops

Summary:
This patch will add loop metadata on the pre and post loops generated by IRCE.
Currently, we have metadata for disabling optimizations such as vectorization,
unrolling, loop distribution and LICM versioning (and confirmed that these
optimizations check for the metadata before proceeding with the transformation).

The pre and post loops generated by IRCE need not go through loop opts (since
these are slow paths).

Added two test cases as well.

Reviewers: sanjoy, reames

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26806

llvm-svn: 289588

1 files changed, 34 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp b/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
index 915e3d60c86..8e81541c233 100644
--- a/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
@@ -399,6 +399,34 @@ void InductiveRangeCheck::extractRangeChecksFromBranch(
                                                   Checks, Visited);
 }
 
+// Add metadata to the loop L to disable loop optimizations. Callers need to
+// confirm that optimizing loop L is not beneficial.
+static void DisableAllLoopOptsOnLoop(Loop &L) {
+  // We do not care about any existing loopID related metadata for L, since we
+  // are setting all loop metadata to false.
+  LLVMContext &Context = L.getHeader()->getContext();
+  // Reserve first location for self reference to the LoopID metadata node.
+  MDNode *Dummy = MDNode::get(Context, {});
+  MDNode *DisableUnroll = MDNode::get(
+      Context, {MDString::get(Context, "llvm.loop.unroll.disable")});
+  Metadata *FalseVal =
+      ConstantAsMetadata::get(ConstantInt::get(Type::getInt1Ty(Context), 0));
+  MDNode *DisableVectorize = MDNode::get(
+      Context,
+      {MDString::get(Context, "llvm.loop.vectorize.enable"), FalseVal});
+  MDNode *DisableLICMVersioning = MDNode::get(
+      Context, {MDString::get(Context, "llvm.loop.licm_versioning.disable")});
+  MDNode *DisableDistribution= MDNode::get(
+      Context,
+      {MDString::get(Context, "llvm.loop.distribute.enable"), FalseVal});
+  MDNode *NewLoopID =
+      MDNode::get(Context, {Dummy, DisableUnroll, DisableVectorize,
+                            DisableLICMVersioning, DisableDistribution});
+  // Set operand 0 to refer to the loop id itself.
+  NewLoopID->replaceOperandWith(0, NewLoopID);
+  L.setLoopID(NewLoopID);
+}
+
 namespace {
 
 // Keeps track of the structure of a loop.  This is similar to llvm::Loop,
@@ -1309,6 +1337,9 @@ bool LoopConstrainer::run() {
         &OriginalLoop, OriginalLoop.getParentLoop(), PreLoop.Map);
     formLCSSARecursively(*L, DT, &LI, &SE);
     simplifyLoop(L, &DT, &LI, &SE, nullptr, true);
+    // Pre loops are slow paths, we do not need to perform any loop
+    // optimizations on them.
+    DisableAllLoopOptsOnLoop(*L);
   }
 
   if (!PostLoop.Blocks.empty()) {
@@ -1316,6 +1347,9 @@ bool LoopConstrainer::run() {
         &OriginalLoop, OriginalLoop.getParentLoop(), PostLoop.Map);
     formLCSSARecursively(*L, DT, &LI, &SE);
     simplifyLoop(L, &DT, &LI, &SE, nullptr, true);
+    // Post loops are slow paths, we do not need to perform any loop
+    // optimizations on them.
+    DisableAllLoopOptsOnLoop(*L);
   }
 
   formLCSSARecursively(OriginalLoop, DT, &LI, &SE);