From 89f224177014760e34ff75adbfc8bb9ea43e01b2 Mon Sep 17 00:00:00 2001
From: Philip Reames <listmail@philipreames.com>
Date: Tue, 20 Mar 2018 17:09:21 +0000
Subject: Add an analysis printer for must execute reasoning

Many of our loop passes make use of so called "must execute" or "guaranteed to execute" facts to prove the legality of code motion. The basic notion is that we know (by assumption) an instruction didn't fault at it's original location, so if the location we move it to is strictly post dominated by the original, then we can't have introduced a new fault.

At the moment, the testing for this logic is somewhat adhoc and done mostly through LICM. Since I'm working on that code, I want to improve the testing. This patch is the first step in that direction. It doesn't actually test the variant used by the loop passes - I need to move that to the Analysis library first - but instead exercises an alternate implementation used by SCEV. (I plan on merging both implementations.)

Note: I'll be replacing the printing logic within this with an annotation based version in the near future.  Anna suggested this in review, and it seems like a strictly better format.

Differential Revision: https://reviews.llvm.org/D44524

llvm-svn: 328004
---
 llvm/lib/Analysis/MustExecute.cpp | 102 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 102 insertions(+)
 create mode 100644 llvm/lib/Analysis/MustExecute.cpp

(limited to 'llvm/lib/Analysis/MustExecute.cpp')

diff --git a/llvm/lib/Analysis/MustExecute.cpp b/llvm/lib/Analysis/MustExecute.cpp
new file mode 100644
index 00000000000..3d98a61c0ed
--- /dev/null
+++ b/llvm/lib/Analysis/MustExecute.cpp
@@ -0,0 +1,102 @@
+//===- MustExecute.cpp - Printer for isGuaranteedToExecute ----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/Passes.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/InstIterator.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/LoopUtils.h"
+using namespace llvm;
+
+namespace {
+  struct MustExecutePrinter : public FunctionPass {
+    DenseMap<Value*, SmallVector<Loop*, 4> > MustExec;
+    SmallVector<Value *, 4> Ordering;
+
+    static char ID; // Pass identification, replacement for typeid
+    MustExecutePrinter() : FunctionPass(ID) {
+      initializeMustExecutePrinterPass(*PassRegistry::getPassRegistry());
+    }
+    void getAnalysisUsage(AnalysisUsage &AU) const override {
+      AU.setPreservesAll();
+      AU.addRequired<DominatorTreeWrapperPass>();
+      AU.addRequired<LoopInfoWrapperPass>();
+    }
+    bool runOnFunction(Function &F) override;
+    void print(raw_ostream &OS, const Module * = nullptr) const override;
+    void releaseMemory() override {
+      MustExec.clear();
+      Ordering.clear();
+    }
+  };
+}
+
+char MustExecutePrinter::ID = 0;
+INITIALIZE_PASS_BEGIN(MustExecutePrinter, "print-mustexecute",
+                      "Instructions which execute on loop entry", false, true)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_END(MustExecutePrinter, "print-mustexecute",
+                    "Instructions which execute on loop entry", false, true)
+
+FunctionPass *llvm::createMustExecutePrinter() {
+  return new MustExecutePrinter();
+}
+
+bool isMustExecuteIn(Instruction &I, Loop *L, DominatorTree *DT) {
+  // TODO: move loop specific code to analysis
+  //LoopSafetyInfo LSI;
+  //computeLoopSafetyInfo(&LSI, L);
+  //return isGuaranteedToExecute(I, DT, L, &LSI);
+  return isGuaranteedToExecuteForEveryIteration(&I, L);
+}
+
+bool MustExecutePrinter::runOnFunction(Function &F) {
+  auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+  auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+  for (auto &I: instructions(F)) {
+    Loop *L = LI.getLoopFor(I.getParent());
+    while (L) {
+      if (isMustExecuteIn(I, L, &DT)) {
+        if (!MustExec.count(&I))
+          Ordering.push_back(&I);
+        MustExec[&I].push_back(L);
+      }
+      L = L->getParentLoop();
+    };
+  }
+  return false;
+}
+
+void MustExecutePrinter::print(raw_ostream &OS, const Module *M) const {
+  OS << "The following are guaranteed to execute (for the respective loops):\n";
+  for (Value *V: Ordering) {
+    V->printAsOperand(OS);
+    auto NumLoops = MustExec.lookup(V).size();
+    if (NumLoops > 1)
+      OS << "\t(mustexec in " << NumLoops << " loops: ";
+    else
+      OS << "\t(mustexec in: ";
+    
+    bool first = true;
+    for (const Loop *L : MustExec.lookup(V)) {
+      if (!first)
+        OS << ", ";
+      first = false;
+      OS << L->getHeader()->getName();
+    }
+    OS << ")\n";
+  }
+  OS << "\n";
+}
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