[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, 12 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
index 0d62a81..fa8a0ab 100644
--- a/llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -335,8 +335,9 @@ LoopUnrollResult llvm::UnrollLoop(
     Loop *L, unsigned Count, unsigned TripCount, bool Force, bool AllowRuntime,
     bool AllowExpensiveTripCount, bool PreserveCondBr, bool PreserveOnlyFirst,
     unsigned TripMultiple, unsigned PeelCount, bool UnrollRemainder,
-    LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT, AssumptionCache *AC,
-    OptimizationRemarkEmitter *ORE, bool PreserveLCSSA, Loop **RemainderLoop) {
+    bool ForgetAllSCEV, LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT,
+    AssumptionCache *AC, OptimizationRemarkEmitter *ORE, bool PreserveLCSSA,
+    Loop **RemainderLoop) {
 
   BasicBlock *Preheader = L->getLoopPreheader();
   if (!Preheader) {
@@ -469,8 +470,9 @@ LoopUnrollResult llvm::UnrollLoop(
 
   if (RuntimeTripCount && TripMultiple % Count != 0 &&
       !UnrollRuntimeLoopRemainder(L, Count, AllowExpensiveTripCount,
-                                  EpilogProfitability, UnrollRemainder, LI, SE,
-                                  DT, AC, PreserveLCSSA, RemainderLoop)) {
+                                  EpilogProfitability, UnrollRemainder,
+                                  ForgetAllSCEV, LI, SE, DT, AC, PreserveLCSSA,
+                                  RemainderLoop)) {
     if (Force)
       RuntimeTripCount = false;
     else {
@@ -554,8 +556,12 @@ LoopUnrollResult llvm::UnrollLoop(
   // and if something changes inside them then any of outer loops may also
   // change. When we forget outermost loop, we also forget all contained loops
   // and this is what we need here.
-  if (SE)
-    SE->forgetTopmostLoop(L);
+  if (SE) {
+    if (ForgetAllSCEV)
+      SE->forgetAllLoops();
+    else
+      SE->forgetTopmostLoop(L);
+  }
 
   bool ContinueOnTrue = L->contains(BI->getSuccessor(0));
   BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);