[SCEV] Add option to forget everything in SCEV.

Summary:
Create a method to forget everything in SCEV.
Add a cl::opt and PassManagerBuilder option to use this in LoopUnroll.

Motivation: Certain Halide applications spend a very long time compiling in forgetLoop, and prefer to forget everything and rebuild SCEV from scratch.
Sample difference in compile time reduction: 21.04 to 14.78 using current ToT release build.
Testcase showcasing this cannot be opensourced and is fairly large.

The option disabled by default, but it may be desirable to enable by
default. Evidence in favor (two difference runs on different days/ToT state):

File Before (s) After (s)
clang-9.bc 7267.91 6639.14
llvm-as.bc 194.12 194.12
llvm-dis.bc 62.50 62.50
opt.bc 1855.85 1857.53

File Before (s) After (s)
clang-9.bc 8588.70 7812.83
llvm-as.bc 196.20 194.78
llvm-dis.bc 61.55 61.97
opt.bc 1739.78 1886.26

Reviewers: sanjoy

Subscribers: mehdi_amini, jlebar, zzheng, javed.absar, dmgreen, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60144

llvm-svn: 358304

1 files changed, 22 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index dff80fbb24d..350eb3d8a7b 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -6789,6 +6789,28 @@ ScalarEvolution::getBackedgeTakenInfo(const Loop *L) {
   return BackedgeTakenCounts.find(L)->second = std::move(Result);
 }
 
+void ScalarEvolution::forgetAllLoops() {
+  // This method is intended to forget all info about loops. It should
+  // invalidate caches as if the following happened:
+  // - The trip counts of all loops have changed arbitrarily
+  // - Every llvm::Value has been updated in place to produce a different
+  // result.
+  BackedgeTakenCounts.clear();
+  PredicatedBackedgeTakenCounts.clear();
+  LoopPropertiesCache.clear();
+  ConstantEvolutionLoopExitValue.clear();
+  ValueExprMap.clear();
+  ValuesAtScopes.clear();
+  LoopDispositions.clear();
+  BlockDispositions.clear();
+  UnsignedRanges.clear();
+  SignedRanges.clear();
+  ExprValueMap.clear();
+  HasRecMap.clear();
+  MinTrailingZerosCache.clear();
+  PredicatedSCEVRewrites.clear();
+}
+
 void ScalarEvolution::forgetLoop(const Loop *L) {
   // Drop any stored trip count value.
   auto RemoveLoopFromBackedgeMap =