Introduce DebugCounter into DCE pass

[zhizhouy]

Summary

This patch introduces DebugCounter into DCE pass at per-transformation level.

It will provide an option skip first n or stop after n transformations for the whole DCE pass.

This will make debug easier for the pass, also providing chance to do transformation level bisecting.

[zhizhouy]

Add REQUIRES to test.

[zhizhouy]

ping for reviewing... thanks!

lib/Transforms/Scalar/DCE.cpp

 #include "llvm/Transforms/Scalar/DCE.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/IR/InstIterator.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/Pass.h"
+#include "llvm/Support/DebugCounter.h"
 #include "llvm/Transforms/Scalar.h"
 using namespace llvm;
 
 #define DEBUG_TYPE "dce"
 
 STATISTIC(DIEEliminated, "Number of insts removed by DIE pass");
 STATISTIC(DCEEliminated, "Number of insts removed");
+DEBUG_COUNTER(DCECounter, "dce-transform",
+              "Controls which instructions are eliminated");
 
 namespace {
   //===--------------------------------------------------------------------===//
   // DeadInstElimination pass implementation
   //
   struct DeadInstElimination : public BasicBlockPass {
     static char ID; // Pass identification, replacement for typeid
     DeadInstElimination() : BasicBlockPass(ID) {
       initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry());
     }
     bool runOnBasicBlock(BasicBlock &BB) override {
       if (skipBasicBlock(BB))
         return false;
       auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
       TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr;
       bool Changed = false;
       for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) {
         Instruction *Inst = &*DI++;
         if (isInstructionTriviallyDead(Inst, TLI)) {
+          if (!DebugCounter::shouldExecute(DCECounter))
+            continue;
           salvageDebugInfo(*Inst);
           Inst->eraseFromParent();
           Changed = true;
           ++DIEEliminated;
         }
       }
       return Changed;
     }

 Pass *llvm::createDeadInstEliminationPass() {
   return new DeadInstElimination();
 }
 
 static bool DCEInstruction(Instruction *I,
                            SmallSetVector<Instruction *, 16> &WorkList,
                            const TargetLibraryInfo *TLI) {
   if (isInstructionTriviallyDead(I, TLI)) {
+    if (!DebugCounter::shouldExecute(DCECounter))
+      return false;
+
     salvageDebugInfo(*I);
 
     // Null out all of the instruction's operands to see if any operand becomes
     // dead as we go.
     for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
       Value *OpV = I->getOperand(i);
       I->setOperand(i, nullptr);
 

test/Other/debugcounter-dce.ll

+; REQUIRES: asserts
+; RUN: opt -dce -S -debug-counter=dce-transform-skip=1,dce-transform-count=2  < %s | FileCheck %s
+;; Test that, with debug counters on, we will skip the first DCE opportunity, perform next 2,
+;; and ignore all the others left.
+
+; CHECK-LABEL: @test
+; CHECK-NEXT: %add1 = add i32 1, 2
+; CHECK-NEXT: %sub1 = sub i32 %add1, 1
+; CHECK-NEXT: %add2 = add i32 1, 2
+; CHECK-NEXT: %add3 = add i32 1, 2
+; CHECK-NEXT: ret void
+define void @test() {
+  %add1 = add i32 1, 2
+  %sub1 = sub i32 %add1, 1
+  %add2 = add i32 1, 2
+  %sub2 = sub i32 %add2, 1
+  %add3 = add i32 1, 2
+  %sub3 = sub i32 %add3, 1
+  ret void
+}