[PATCH] [SLSR] garbage-collect unused instructions

[Jingyue Wu]

Hi broune, meheff,

After we rewrite a candidate, the instructions used by the old form may
become unused. This patch cleans up these unused instructions so that we
needn't run DCE after SLSR.

http://reviews.llvm.org/D9101

Files:
  lib/Transforms/Scalar/StraightLineStrengthReduce.cpp
  test/Transforms/StraightLineStrengthReduce/X86/no-slsr.ll
  test/Transforms/StraightLineStrengthReduce/slsr-add.ll
  test/Transforms/StraightLineStrengthReduce/slsr-gep.ll
  test/Transforms/StraightLineStrengthReduce/slsr-mul.ll

Index: lib/Transforms/Scalar/StraightLineStrengthReduce.cpp
===================================================================
--- lib/Transforms/Scalar/StraightLineStrengthReduce.cpp
+++ lib/Transforms/Scalar/StraightLineStrengthReduce.cpp
@@ -68,6 +68,7 @@
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/Local.h"
 
 using namespace llvm;
 using namespace PatternMatch;
@@ -599,9 +600,14 @@
   switch (C.CandidateKind) {
   case Candidate::Add:
   case Candidate::Mul:
+    // C = Basis + Bump
     if (BinaryOperator::isNeg(Bump)) {
+      // If Bump is a neg instruction, emit C = Basis - (-Bump).
       Reduced =
           Builder.CreateSub(Basis.Ins, BinaryOperator::getNegArgument(Bump));
+      // We only use the negative argument of Bump, and Bump itself may be
+      // trivially dead.
+      RecursivelyDeleteTriviallyDeadInstructions(Bump);
     } else {
       Reduced = Builder.CreateAdd(Basis.Ins, Bump);
     }
@@ -637,7 +643,6 @@
   };
   Reduced->takeName(C.Ins);
   C.Ins->replaceAllUsesWith(Reduced);
-  C.Ins->dropAllReferences();
   // Unlink C.Ins so that we can skip other candidates also corresponding to
   // C.Ins. The actual deletion is postponed to the end of runOnFunction.
   C.Ins->removeFromParent();
@@ -670,8 +675,13 @@
   }
 
   // Delete all unlink instructions.
-  for (auto I : UnlinkedInstructions) {
-    delete I;
+  for (auto *UnlinkedInst : UnlinkedInstructions) {
+    for (unsigned I = 0, E = UnlinkedInst->getNumOperands(); I != E; ++I) {
+      Value *Op = UnlinkedInst->getOperand(I);
+      UnlinkedInst->setOperand(I, nullptr);
+      RecursivelyDeleteTriviallyDeadInstructions(Op);
+    }
+    delete UnlinkedInst;
   }
   bool Ret = !UnlinkedInstructions.empty();
   UnlinkedInstructions.clear();
Index: test/Transforms/StraightLineStrengthReduce/X86/no-slsr.ll
===================================================================
--- test/Transforms/StraightLineStrengthReduce/X86/no-slsr.ll
+++ test/Transforms/StraightLineStrengthReduce/X86/no-slsr.ll
@@ -1,4 +1,4 @@
-; RUN: opt < %s -slsr -gvn -dce -S | FileCheck %s
+; RUN: opt < %s -slsr -gvn -S | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
Index: test/Transforms/StraightLineStrengthReduce/slsr-add.ll
===================================================================
--- test/Transforms/StraightLineStrengthReduce/slsr-add.ll
+++ test/Transforms/StraightLineStrengthReduce/slsr-add.ll
@@ -1,4 +1,4 @@
-; RUN: opt < %s -slsr -gvn -dce -S | FileCheck %s
+; RUN: opt < %s -slsr -gvn -S | FileCheck %s
 
 target datalayout = "e-i64:64-v16:16-v32:32-n16:32:64"
 
Index: test/Transforms/StraightLineStrengthReduce/slsr-gep.ll
===================================================================
--- test/Transforms/StraightLineStrengthReduce/slsr-gep.ll
+++ test/Transforms/StraightLineStrengthReduce/slsr-gep.ll
@@ -1,4 +1,4 @@
-; RUN: opt < %s -slsr -gvn -dce -S | FileCheck %s
+; RUN: opt < %s -slsr -gvn -S | FileCheck %s
 
 target datalayout = "e-i64:64-v16:16-v32:32-n16:32:64"
 
Index: test/Transforms/StraightLineStrengthReduce/slsr-mul.ll
===================================================================
--- test/Transforms/StraightLineStrengthReduce/slsr-mul.ll
+++ test/Transforms/StraightLineStrengthReduce/slsr-mul.ll
@@ -1,4 +1,4 @@
-; RUN: opt < %s -slsr -gvn -dce -S | FileCheck %s
+; RUN: opt < %s -slsr -gvn -S | FileCheck %s
 
 target datalayout = "e-i64:64-v16:16-v32:32-n16:32:64"

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