[InstCombine] Fold icmp eq/ne (udiv i32 A, B), 0 -> icmp ugt/ule B, A

[mcrosier]

Summary

Please take a look,
Chad

[mcrosier]

Add full context.

[majnemer]

lib/Transforms/InstCombine/InstCombineCompares.cpp

1549

Why do you need to do this ConstantInt::get call? Shouldn't A and CI2 have the same type?

[majnemer]

lib/Transforms/InstCombine/InstCombineCompares.cpp

1543–1550

This transform is valid regardless of what value CI2 holds. We should require it to be a ConstantInt.

test/Transforms/InstCombine/compare-udiv.ll

3–9

This transform is valid, regardless of what value the numerator is. We shouldn't require a constant here.

11–17

It'd be good to have a version of this test where the numerator is -1.

[mcrosier]

Address David's comments:
-Handle both constant and non-constant numerator.
-Add test with -1 numerator.

[majnemer]

LGTM

[mcrosier]

Committed in r268960. I did have to move the optimizations after visitICmpInstWithInstAndIntCst() in order to not regress udiv_icmp1 in exact.ll. Also, I added an additional test case, udiv_icmp2, to provide more context and to improve coverage.

lib/Transforms/InstCombine/InstCombineCompares.cpp

Instruction *InstCombiner::FoldICmpCstShlCst(ICmpInst &I, Value *Op, Value *A,
 
   if (Shift > 0 && AP2.shl(Shift) == AP1)
     return getICmp(I.ICMP_EQ, A, ConstantInt::get(A->getType(), Shift));
 
   // Shifting const2 will never be equal to const1.
   return getConstant(false);
 }
 
+Instruction *InstCombiner::FoldICmpCstUDivCst(ICmpInst &I, Value *Op,
+                                              ConstantInt *CI1,
+                                              ConstantInt *CI2, Value *A) {
+  // (icmp eq/ne (udiv i32 CI2, A), 0) -> (icmp ugt/ule A, CI2)
+  if (I.isEquality() && CI1->isZero()) {
+    ICmpInst::Predicate Pred = I.getPredicate() == ICmpInst::ICMP_EQ
+                                   ? ICmpInst::ICMP_UGT
+                                   : ICmpInst::ICMP_ULE;
+    return new ICmpInst(Pred, A,
+                        ConstantInt::get(A->getType(), CI2->getValue()));
+  }
+  // FIXME: Handle (icmp ugt (udiv i32 CI2, A), CI1) and
+  // (icmp ult (udiv i32 CI2, A), CI1).
+  return nullptr;
+}
+
 /// visitICmpInstWithInstAndIntCst - Handle "icmp (instr, intcst)".
 ///
 Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
                                                           Instruction *LHSI,
                                                           ConstantInt *RHS) {
   const APInt &RHSV = RHS->getValue();
 
   switch (LHSI->getOpcode()) {
if (I.isEquality()) {
         if (Instruction *Inst = FoldICmpCstShrCst(I, Op0, A, CI, CI2))
           return Inst;
       }
       if (match(Op0, m_Shl(m_ConstantInt(CI2), m_Value(A)))) {
         // (icmp eq/ne (shl const2, A), const1)
         if (Instruction *Inst = FoldICmpCstShlCst(I, Op0, A, CI, CI2))
           return Inst;
       }
+      if (match(Op0, m_UDiv(m_ConstantInt(CI2), m_Value(A)))) {
+        // (icmp eq/ne (udiv const2, A), const1)
+        if (Instruction *Inst = FoldICmpCstUDivCst(I, Op0, CI, CI2, A))
+          return Inst;
+      }
     }
-
     // If this comparison is a normal comparison, it demands all
     // bits, if it is a sign bit comparison, it only demands the sign bit.
     bool UnusedBit;
     isSignBit = isSignBitCheck(I.getPredicate(), CI, UnusedBit);
 
     // Canonicalize icmp instructions based on dominating conditions.
     BasicBlock *Parent = I.getParent();
     BasicBlock *Dom = Parent->getSinglePredecessor();

lib/Transforms/InstCombine/InstCombineInternal.h

public:
   Instruction *FoldICmpDivCst(ICmpInst &ICI, BinaryOperator *DivI,
                               ConstantInt *DivRHS);
   Instruction *FoldICmpShrCst(ICmpInst &ICI, BinaryOperator *DivI,
                               ConstantInt *DivRHS);
   Instruction *FoldICmpCstShrCst(ICmpInst &I, Value *Op, Value *A,
                                  ConstantInt *CI1, ConstantInt *CI2);
   Instruction *FoldICmpCstShlCst(ICmpInst &I, Value *Op, Value *A,
                                  ConstantInt *CI1, ConstantInt *CI2);
+  Instruction *FoldICmpCstUDivCst(ICmpInst &I, Value *Op, ConstantInt *CI1,
+                                  ConstantInt *CI2, Value *A);
   Instruction *FoldICmpAddOpCst(Instruction &ICI, Value *X, ConstantInt *CI,
                                 ICmpInst::Predicate Pred);
   Instruction *FoldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
                            ICmpInst::Predicate Cond, Instruction &I);
   Instruction *FoldAllocaCmp(ICmpInst &ICI, AllocaInst *Alloca, Value *Other);
   Instruction *FoldShiftByConstant(Value *Op0, Constant *Op1,
                                    BinaryOperator &I);
   Instruction *commonCastTransforms(CastInst &CI);

test/Transforms/InstCombine/compare-udiv.ll

+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+; CHECK-LABEL: @test1
+; CHECK: %cmp1 = icmp ugt i32 %a, 64
+define i1 @test1(i32 %a) {
+  %div = udiv i32 64, %a
+  %cmp1 = icmp eq i32 %div, 0
+  ret i1 %cmp1
+}
+
+; CHECK-LABEL: @test2
+; CHECK: %cmp1 = icmp ult i32 %a, 65
+define i1 @test2(i32 %a) {
+  %div = udiv i32 64, %a
+  %cmp1 = icmp ne i32 %div, 0
+  ret i1 %cmp1
+}