[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, 6 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
index 2f1a759870c..20d687fbcc8 100644
--- a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -553,10 +553,10 @@ static bool canProfitablyUnrollMultiExitLoop(
 bool llvm::UnrollRuntimeLoopRemainder(Loop *L, unsigned Count,
                                       bool AllowExpensiveTripCount,
                                       bool UseEpilogRemainder,
-                                      bool UnrollRemainder, LoopInfo *LI,
-                                      ScalarEvolution *SE, DominatorTree *DT,
-                                      AssumptionCache *AC, bool PreserveLCSSA,
-                                      Loop **ResultLoop) {
+                                      bool UnrollRemainder, bool ForgetAllSCEV,
+                                      LoopInfo *LI, ScalarEvolution *SE,
+                                      DominatorTree *DT, AssumptionCache *AC,
+                                      bool PreserveLCSSA, Loop **ResultLoop) {
   LLVM_DEBUG(dbgs() << "Trying runtime unrolling on Loop: \n");
   LLVM_DEBUG(L->dump());
   LLVM_DEBUG(UseEpilogRemainder ? dbgs() << "Using epilog remainder.\n"
@@ -953,8 +953,8 @@ bool llvm::UnrollRuntimeLoopRemainder(Loop *L, unsigned Count,
                    /*Force*/ false, /*AllowRuntime*/ false,
                    /*AllowExpensiveTripCount*/ false, /*PreserveCondBr*/ true,
                    /*PreserveOnlyFirst*/ false, /*TripMultiple*/ 1,
-                   /*PeelCount*/ 0, /*UnrollRemainder*/ false, LI, SE, DT, AC,
-                   /*ORE*/ nullptr, PreserveLCSSA);
+                   /*PeelCount*/ 0, /*UnrollRemainder*/ false, ForgetAllSCEV,
+                   LI, SE, DT, AC, /*ORE*/ nullptr, PreserveLCSSA);
   }
 
   if (ResultLoop && UnrollResult != LoopUnrollResult::FullyUnrolled)