[PATCH] D26806: [IRCE] Avoid loop optimizations on pre and post loops

[Anna Thomas via llvm-commits]

anna created this revision.
anna added reviewers: sanjoy, reames.
anna added a subscriber: llvm-commits.

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).

I have tried this on some of the test cases in IRCE to make sure that the
metadata is added to pre/post loops. Will add test case once confirmed that this
is the right path forward.


https://reviews.llvm.org/D26806

Files:
  lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp


Index: lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
===================================================================
--- lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
+++ lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
@@ -399,6 +399,37 @@
                                                   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.
+  SmallVector<Metadata *, 6> MDs;
+  // Reserve first location for self reference to the LoopID metadata node.
+  MDs.push_back(nullptr);
+  LLVMContext &Context = L.getHeader()->getContext();
+  MDNode *DisableUnroll = MDNode::get(
+      Context, {MDString::get(Context, "llvm.loop.unroll.disable")});
+  MDs.push_back(DisableUnroll);
+  Metadata *FalseVal =
+      ConstantAsMetadata::get(ConstantInt::get(Type::getInt1Ty(Context), 0));
+  MDNode *DisableVectorize = MDNode::get(
+      Context,
+      {MDString::get(Context, "llvm.loop.vectorize.enable"), FalseVal});
+  MDs.push_back(DisableVectorize);
+  MDNode *DisableLICMVersioning = MDNode::get(
+      Context, {MDString::get(Context, "llvm.loop.licm_versioning.disable")});
+  MDs.push_back(DisableLICMVersioning);
+  MDNode *DisableDistribution= MDNode::get(
+      Context,
+      {MDString::get(Context, "llvm.loop.distribute.enable"), FalseVal});
+  MDs.push_back(DisableDistribution);
+  MDNode *NewLoopID = MDNode::get(Context, MDs);
+  // 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,
@@ -1309,13 +1340,19 @@
         &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()) {
     auto *L = createClonedLoopStructure(
         &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);


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