[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, 18 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
index 9e241aa335a..7491fe06841 100644
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -158,6 +158,12 @@ cl::opt<bool> EnableOrderFileInstrumentation(
     "enable-order-file-instrumentation", cl::init(false), cl::Hidden,
     cl::desc("Enable order file instrumentation (default = off)"));
 
+cl::opt<bool> ForgetSCEVInLoopUnroll(
+    "forget-scev-loop-unroll", cl::init(false), cl::Hidden,
+    cl::desc("Forget everything in SCEV when doing LoopUnroll, instead of just"
+             " the current top-most loop. This is somtimes preferred to reduce"
+             " compile time."));
+
 PassManagerBuilder::PassManagerBuilder() {
     OptLevel = 2;
     SizeLevel = 0;
@@ -169,6 +175,7 @@ PassManagerBuilder::PassManagerBuilder() {
     RerollLoops = RunLoopRerolling;
     NewGVN = RunNewGVN;
     DisableGVNLoadPRE = false;
+    ForgetAllSCEVInLoopUnroll = ForgetSCEVInLoopUnroll;
     VerifyInput = false;
     VerifyOutput = false;
     MergeFunctions = false;
@@ -386,8 +393,9 @@ void PassManagerBuilder::addFunctionSimplificationPasses(
   if (EnableLoopInterchange)
     MPM.add(createLoopInterchangePass()); // Interchange loops
 
-  MPM.add(createSimpleLoopUnrollPass(OptLevel,
-                                     DisableUnrollLoops)); // Unroll small loops
+  // Unroll small loops
+  MPM.add(createSimpleLoopUnrollPass(OptLevel, DisableUnrollLoops,
+                                     ForgetAllSCEVInLoopUnroll));
   addExtensionsToPM(EP_LoopOptimizerEnd, MPM);
   // This ends the loop pass pipelines.
 
@@ -724,8 +732,9 @@ void PassManagerBuilder::populateModulePassManager(
     MPM.add(createLoopUnrollAndJamPass(OptLevel));
   }
 
-  MPM.add(createLoopUnrollPass(OptLevel,
-                               DisableUnrollLoops)); // Unroll small loops
+  // Unroll small loops
+  MPM.add(createLoopUnrollPass(OptLevel, DisableUnrollLoops,
+                               ForgetAllSCEVInLoopUnroll));
 
   if (!DisableUnrollLoops) {
     // LoopUnroll may generate some redundency to cleanup.
@@ -919,11 +928,13 @@ void PassManagerBuilder::addLTOOptimizationPasses(legacy::PassManagerBase &PM) {
   if (EnableLoopInterchange)
     PM.add(createLoopInterchangePass());
 
-  PM.add(createSimpleLoopUnrollPass(OptLevel,
-                                    DisableUnrollLoops)); // Unroll small loops
+  // Unroll small loops
+  PM.add(createSimpleLoopUnrollPass(OptLevel, DisableUnrollLoops,
+                                    ForgetAllSCEVInLoopUnroll));
   PM.add(createLoopVectorizePass(true, !LoopVectorize));
   // The vectorizer may have significantly shortened a loop body; unroll again.
-  PM.add(createLoopUnrollPass(OptLevel, DisableUnrollLoops));
+  PM.add(createLoopUnrollPass(OptLevel, DisableUnrollLoops,
+                              ForgetAllSCEVInLoopUnroll));
 
   PM.add(createWarnMissedTransformationsPass());