[llvm] Prevent infinite loop in InstCombine of select statements

[tpopp]

Summary

This fixes an issue where the RHS and LHS the comparison operation
creating the predicate were swapped back and forth forever.

[nikic]

LGTM

A test case would be nice of course, but with this kind of worklist order issue (select folded before icmp) it's always tricky.

[xbolva00]

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

1129

llvm patternmatch has m_ImmConstant :) check it

[tpopp]

I apologize, but I was not having any luck at creating a test case that wasn't constant folded away.

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

1129

Thank you for the pointer.

[lebedev.ri]

In D94934#2506157, @tpopp wrote:

I apologize, but I was not having any luck at creating a test case that wasn't constant folded away.

Why create it? You already have a reproducer, right? Just reduce it.

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

if (Value *V = SimplifyWithOpReplaced(TrueVal, CmpLHS, CmpRHS, SQ,
                                           /* AllowRefinement */ true))
       return replaceOperand(Sel, Swapped ? 2 : 1, V);
 
     // Even if TrueVal does not simplify, we can directly replace a use of
     // CmpLHS with CmpRHS, as long as the instruction is not used anywhere
     // else. Only do this if CmpRHS is a constant, as profitability is not
     // clear for other cases.
     // FIXME: The replacement could be performed recursively.
-    if (isa<Constant>(CmpRHS) && !isa<ConstantExpr>(CmpRHS))
+    if (match(CmpRHS, m_ImmConstant()) && !match(CmpLHS, m_ImmConstant()))
       if (auto *I = dyn_cast<Instruction>(TrueVal))
         if (I->hasOneUse())
           for (Use &U : I->operands())
             if (U == CmpLHS) {
               replaceUse(U, CmpRHS);
               return &Sel;
             }
   }
   if (TrueVal != CmpRHS &&
       isGuaranteedNotToBeUndefOrPoison(CmpLHS, SQ.AC, &Sel, &DT))
     if (Value *V = SimplifyWithOpReplaced(TrueVal, CmpRHS, CmpLHS, SQ,