[llvm] r284013 - [CVP] Convert an AShr to a LShr if 1st operand is known to be nonnegative.

[Chad Rosier via llvm-commits]

Author: mcrosier
Date: Wed Oct 12 08:41:38 2016
New Revision: 284013

URL: http://llvm.org/viewvc/llvm-project?rev=284013&view=rev
Log:
[CVP] Convert an AShr to a LShr if 1st operand is known to be nonnegative.

An arithmetic shift can be safely changed to a logical shift if the first
operand is known positive. This allows ComputeKnownBits (and similar analysis)
to determine the sign bit of the shifted value in some cases. In turn, this
allows InstCombine to canonicalize a signed comparison (a > 0) into an equality
check (a != 0).

PR30577

Differential Revision: https://reviews.llvm.org/D25119

Added:
    llvm/trunk/test/Transforms/CorrelatedValuePropagation/ashr.ll
Modified:
    llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp?rev=284013&r1=284012&r2=284013&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp Wed Oct 12 08:41:38 2016
@@ -38,6 +38,7 @@ STATISTIC(NumCmps,      "Number of compa
 STATISTIC(NumReturns,   "Number of return values propagated");
 STATISTIC(NumDeadCases, "Number of switch cases removed");
 STATISTIC(NumSDivs,     "Number of sdiv converted to udiv");
+STATISTIC(NumAShrs,     "Number of ashr converted to lshr");
 STATISTIC(NumSRems,     "Number of srem converted to urem");
 
 static cl::opt<bool> DontProcessAdds("cvp-dont-process-adds", cl::init(false));
@@ -384,6 +385,25 @@ static bool processSDiv(BinaryOperator *
   return true;
 }
 
+static bool processAShr(BinaryOperator *SDI, LazyValueInfo *LVI) {
+  if (SDI->getType()->isVectorTy() || hasLocalDefs(SDI))
+    return false;
+
+  Constant *Zero = ConstantInt::get(SDI->getType(), 0);
+  if (LVI->getPredicateAt(ICmpInst::ICMP_SGE, SDI->getOperand(0), Zero, SDI) !=
+      LazyValueInfo::True)
+    return false;
+
+  ++NumAShrs;
+  auto *BO = BinaryOperator::CreateLShr(SDI->getOperand(0), SDI->getOperand(1),
+                                        SDI->getName(), SDI);
+  BO->setIsExact(SDI->isExact());
+  SDI->replaceAllUsesWith(BO);
+  SDI->eraseFromParent();
+
+  return true;
+}
+
 static bool processAdd(BinaryOperator *AddOp, LazyValueInfo *LVI) {
   typedef OverflowingBinaryOperator OBO;
 
@@ -495,6 +515,9 @@ static bool runImpl(Function &F, LazyVal
       case Instruction::SDiv:
         BBChanged |= processSDiv(cast<BinaryOperator>(II), LVI);
         break;
+      case Instruction::AShr:
+        BBChanged |= processAShr(cast<BinaryOperator>(II), LVI);
+        break;
       case Instruction::Add:
         BBChanged |= processAdd(cast<BinaryOperator>(II), LVI);
         break;

Added: llvm/trunk/test/Transforms/CorrelatedValuePropagation/ashr.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/CorrelatedValuePropagation/ashr.ll?rev=284013&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/CorrelatedValuePropagation/ashr.ll (added)
+++ llvm/trunk/test/Transforms/CorrelatedValuePropagation/ashr.ll Wed Oct 12 08:41:38 2016
@@ -0,0 +1,56 @@
+; RUN: opt < %s -correlated-propagation -S | FileCheck %s
+
+; CHECK-LABEL: @test1
+define void @test1(i32 %n) {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.body, %entry
+  %a = phi i32 [ %n, %entry ], [ %shr, %for.body ]
+  %cmp = icmp sgt i32 %a, 1
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+; CHECK: lshr i32 %a, 5
+  %shr = ashr i32 %a, 5
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+;; Negative test to show transform doesn't happen unless n > 0.
+; CHECK-LABEL: @test2
+define void @test2(i32 %n) {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.body, %entry
+  %a = phi i32 [ %n, %entry ], [ %shr, %for.body ]
+  %cmp = icmp sgt i32 %a, -2
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+; CHECK: ashr i32 %a, 2
+  %shr = ashr i32 %a, 2
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+;; Non looping test case.
+; CHECK-LABEL: @test3
+define void @test3(i32 %n) {
+entry:
+  %cmp = icmp sgt i32 %n, 0
+  br i1 %cmp, label %bb, label %exit
+
+bb:
+; CHECK: lshr exact i32 %n, 4
+  %shr = ashr exact i32 %n, 4
+  br label %exit
+
+exit:
+  ret void
+}