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

[spatel]

Summary

These folds were introduced with https://reviews.llvm.org/rL127064 as part of solving:
https://bugs.llvm.org/show_bug.cgi?id=9343

As shown here:
http://rise4fun.com/Alive/C8
...however, the sdiv exact case needs a stronger predicate.

I opted for duplicated code instead of adding another fallthrough because I think that's easier to read (and edit in case we need/want to restrict/loosen the predicates any more).

This should fix:
https://bugs.llvm.org/show_bug.cgi?id=32949
https://bugs.llvm.org/show_bug.cgi?id=32948

[spatel]

Ping.

[majnemer]

LGTM

[davide]

LGTM

lib/Analysis/InstructionSimplify.cpp

static Value *simplifyICmpWithBinOp(CmpInst::Predicate Pred, Value *LHS,
 
   if (MaxRecurse && LBO && RBO && LBO->getOpcode() == RBO->getOpcode() &&
       LBO->getOperand(1) == RBO->getOperand(1)) {
     switch (LBO->getOpcode()) {
     default:
       break;
     case Instruction::UDiv:
     case Instruction::LShr:
-      if (ICmpInst::isSigned(Pred))
-        break;
-      LLVM_FALLTHROUGH;
+      if (ICmpInst::isSigned(Pred) || !LBO->isExact() || !RBO->isExact())
+        break;
+      if (Value *V = SimplifyICmpInst(Pred, LBO->getOperand(0),
+                                      RBO->getOperand(0), Q, MaxRecurse - 1))
+          return V;
     case Instruction::SDiv:
+      if (!ICmpInst::isEquality(Pred) || !LBO->isExact() || !RBO->isExact())
+        break;
+      if (Value *V = SimplifyICmpInst(Pred, LBO->getOperand(0),
+                                      RBO->getOperand(0), Q, MaxRecurse - 1))
+        return V;
+      break;
     case Instruction::AShr:
       if (!LBO->isExact() || !RBO->isExact())
         break;
       if (Value *V = SimplifyICmpInst(Pred, LBO->getOperand(0),
                                       RBO->getOperand(0), Q, MaxRecurse - 1))
         return V;
       break;
     case Instruction::Shl: {

test/Transforms/InstSimplify/compare.ll

 ; CHECK-NEXT:    ret i1 false
 ;
   %A = sdiv exact i32 10, %Z
   %B = sdiv exact i32 20, %Z
   %C = icmp eq i32 %A, %B
   ret i1 %C
 }
 
-; FIXME: But not other preds: PR32949 - https://bugs.llvm.org/show_bug.cgi?id=32949
+; But not other preds: PR32949 - https://bugs.llvm.org/show_bug.cgi?id=32949
 
 define i1 @sdiv_exact_not_equality(i32 %Z) {
 ; CHECK-LABEL: @sdiv_exact_not_equality(
-; CHECK-NEXT:    ret i1 true
+; CHECK-NEXT:    [[A:%.*]] = sdiv exact i32 10, %Z
+; CHECK-NEXT:    [[B:%.*]] = sdiv exact i32 20, %Z
+; CHECK-NEXT:    [[C:%.*]] = icmp ult i32 [[A]], [[B]]
+; CHECK-NEXT:    ret i1 [[C]]
 ;
   %A = sdiv exact i32 10, %Z
   %B = sdiv exact i32 20, %Z
   %C = icmp ult i32 %A, %B
   ret i1 %C
 }
 
 define i1 @udiv3(i32 %X, i32 %Y) {