[llvm] r325148 - [InstCombine] Don't fold select(C, Z, binop(select(C, X, Y), W)) -> select(C, Z, binop(Y, W)) if the binop is rem or div.

[Craig Topper via llvm-commits]

Author: ctopper
Date: Wed Feb 14 10:08:33 2018
New Revision: 325148

URL: http://llvm.org/viewvc/llvm-project?rev=325148&view=rev
Log:
[InstCombine] Don't fold select(C, Z, binop(select(C, X, Y), W)) -> select(C, Z, binop(Y, W)) if the binop is rem or div.

The select may have been preventing a division by zero or INT_MIN/-1 so removing it might not be safe.

Fixes PR36362.

Differential Revision: https://reviews.llvm.org/D43276

Added:
    llvm/trunk/test/Transforms/InstCombine/pr36362.ll
Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp?rev=325148&r1=325147&r2=325148&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineSelect.cpp Wed Feb 14 10:08:33 2018
@@ -1760,11 +1760,25 @@ Instruction *InstCombiner::visitSelectIn
     }
   }
 
+  auto canMergeSelectThroughBinop = [](BinaryOperator *BO) {
+    // The select might be preventing a division by 0.
+    switch (BO->getOpcode()) {
+    default:
+      return true;
+    case Instruction::SRem:
+    case Instruction::URem:
+    case Instruction::SDiv:
+    case Instruction::UDiv:
+      return false;
+    }
+  };
+
   // Try to simplify a binop sandwiched between 2 selects with the same
   // condition.
   // select(C, binop(select(C, X, Y), W), Z) -> select(C, binop(X, W), Z)
   BinaryOperator *TrueBO;
-  if (match(TrueVal, m_OneUse(m_BinOp(TrueBO)))) {
+  if (match(TrueVal, m_OneUse(m_BinOp(TrueBO))) &&
+      canMergeSelectThroughBinop(TrueBO)) {
     if (auto *TrueBOSI = dyn_cast<SelectInst>(TrueBO->getOperand(0))) {
       if (TrueBOSI->getCondition() == CondVal) {
         TrueBO->setOperand(0, TrueBOSI->getTrueValue());
@@ -1783,7 +1797,8 @@ Instruction *InstCombiner::visitSelectIn
 
   // select(C, Z, binop(select(C, X, Y), W)) -> select(C, Z, binop(Y, W))
   BinaryOperator *FalseBO;
-  if (match(FalseVal, m_OneUse(m_BinOp(FalseBO)))) {
+  if (match(FalseVal, m_OneUse(m_BinOp(FalseBO))) &&
+      canMergeSelectThroughBinop(FalseBO)) {
     if (auto *FalseBOSI = dyn_cast<SelectInst>(FalseBO->getOperand(0))) {
       if (FalseBOSI->getCondition() == CondVal) {
         FalseBO->setOperand(0, FalseBOSI->getFalseValue());

Added: llvm/trunk/test/Transforms/InstCombine/pr36362.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/pr36362.ll?rev=325148&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/pr36362.ll (added)
+++ llvm/trunk/test/Transforms/InstCombine/pr36362.ll Wed Feb 14 10:08:33 2018
@@ -0,0 +1,17 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+;RUN: opt -instcombine -S %s | FileCheck %s
+
+; We shouldn't remove the select before the srem
+define i32 @foo(i1 %a, i32 %b, i32 %c) {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:    [[SEL1:%.*]] = select i1 [[A:%.*]], i32 [[B:%.*]], i32 -1
+; CHECK-NEXT:    [[REM:%.*]] = srem i32 [[C:%.*]], [[SEL1]]
+; CHECK-NEXT:    [[SEL2:%.*]] = select i1 [[A]], i32 [[REM]], i32 0
+; CHECK-NEXT:    ret i32 [[SEL2]]
+;
+  %sel1 = select i1 %a, i32 %b, i32 -1
+  %rem = srem i32 %c, %sel1
+  %sel2 = select i1 %a, i32 %rem, i32 0
+  ret i32 %sel2
+}
+