[InstCombine] restrict icmp fold with 2 sdiv exact operands (PR32949)

[spatel]

Summary

This is the InstCombine counterpart to D32954. I added some comments about the code duplication in:
rL302436

Alive-based verification:
http://rise4fun.com/Alive/dPw

This is a 2nd fix for the problem reported in:
https://bugs.llvm.org/show_bug.cgi?id=32949

[spatel]

Ping.

[majnemer]

LGTM

[davide]

This is the same as the other no? If so, LGTM.

[spatel]

In D32970#755263, @davide wrote:

This is the same as the other no? If so, LGTM.

Yep - the logic is unfortunately duplicated in InstCombine and InstSimplify, so we need 2 patches to solve the bug completely. Since this is a miscompile, I thought it was better to get the fixes in and then think about refactoring to eliminate that duplication.

llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp

case Instruction::Mul:
               APInt::getLowBitsSet(AP.getBitWidth(),
                                    AP.getBitWidth() - AP.countTrailingZeros()));
           Value *And1 = Builder->CreateAnd(BO0->getOperand(0), Mask);
           Value *And2 = Builder->CreateAnd(BO1->getOperand(0), Mask);
           return new ICmpInst(Pred, And1, And2);
         }
       }
       break;
+
     case Instruction::UDiv:
     case Instruction::LShr:
-      if (I.isSigned())
+      if (I.isSigned() || !BO0->isExact() || !BO1->isExact())
         break;
-      LLVM_FALLTHROUGH;
+      return new ICmpInst(Pred, BO0->getOperand(0), BO1->getOperand(0));
+
     case Instruction::SDiv:
+      if (!I.isEquality() || !BO0->isExact() || !BO1->isExact())
+        break;
+      return new ICmpInst(Pred, BO0->getOperand(0), BO1->getOperand(0));
+
     case Instruction::AShr:
       if (!BO0->isExact() || !BO1->isExact())
         break;
       return new ICmpInst(Pred, BO0->getOperand(0), BO1->getOperand(0));
+
     case Instruction::Shl: {
       bool NUW = BO0->hasNoUnsignedWrap() && BO1->hasNoUnsignedWrap();
       bool NSW = BO0->hasNoSignedWrap() && BO1->hasNoSignedWrap();
       if (!NUW && !NSW)
         break;
       if (!NSW && I.isSigned())
         break;
       return new ICmpInst(Pred, BO0->getOperand(0), BO1->getOperand(0));

llvm/trunk/test/Transforms/InstCombine/icmp.ll

 ; CHECK-NEXT:    ret i1 [[C]]
 ;
   %A = sdiv exact i32 %X, %Z
   %B = sdiv exact i32 %Y, %Z
   %C = icmp eq i32 %A, %B
   ret i1 %C
 }
 
-; FIXME: The above transform only works for equality predicates.
+; The above transform only works for equality predicates.
 
 define i1 @PR32949(i32 %X, i32 %Y, i32 %Z) {
 ; CHECK-LABEL: @PR32949(
-; CHECK-NEXT:    [[C:%.*]] = icmp sgt i32 %X, %Y
+; CHECK-NEXT:    [[A:%.*]] = sdiv exact i32 %X, %Z
+; CHECK-NEXT:    [[B:%.*]] = sdiv exact i32 %Y, %Z
+; CHECK-NEXT:    [[C:%.*]] = icmp sgt i32 [[A]], [[B]]
 ; CHECK-NEXT:    ret i1 [[C]]
 ;
   %A = sdiv exact i32 %X, %Z
   %B = sdiv exact i32 %Y, %Z
   %C = icmp sgt i32 %A, %B
   ret i1 %C
 }