[SCEV] ScalarEvolution::createSCEV(): Instruction::Or: drop completely bogus no-wrap flag detection

[lebedev.ri]

Summary

That's just really wrong. I don't see why it makes sense at all.

Good thing is, it wasn't miscompiling anything, because as long as
the operands of or had no common bits set, then the add
of these operands will never overflow: http://volta.cs.utah.edu:8080/z/gmt7Sy
IOW we need no detection, we are free to just set NUW+NSW.

But as rG39e3683534c83573da5c8b70c8adfb43948f601f shows,
even when the old code failed to "deduce" flags,
we'd eventually re-deduce them somewhere, later.

So let's just set them.

[efriedma]

If LHS is an AddRec, the result must be an AddRec because the add gets folded into the "Start". And that AddRec must have the same wrapping behavior: if it didn't, that would mean getMinTrailingZeros returned the wrong value. Therefore, setting the nowrap flags like this should be legal.

But sure, we can just specify the flags to getAddExpr itself, and let the SCEV construction code do the rest. LGTM

[lebedev.ri]

In D81246#2077373, @efriedma wrote:

If LHS is an AddRec, the result must be an AddRec because the add gets folded into the "Start". And that AddRec must have the same wrapping behavior: if it didn't, that would mean getMinTrailingZeros returned the wrong value. Therefore, setting the nowrap flags like this should be legal.

Ah, i see what the old code meant, thanks.

But sure, we can just specify the flags to getAddExpr itself, and let the SCEV construction code do the rest. LGTM

Thank you for the review!

llvm/lib/Analysis/ScalarEvolution.cpp

case Instruction::Or:
       // In order for this transformation to be safe, the LHS must be of the
       // form X*(2^n) and the Or constant must be less than 2^n.
       if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->RHS)) {
         const SCEV *LHS = getSCEV(BO->LHS);
         const APInt &CIVal = CI->getValue();
         if (GetMinTrailingZeros(LHS) >=
             (CIVal.getBitWidth() - CIVal.countLeadingZeros())) {
           // Build a plain add SCEV.
-          const SCEV *S = getAddExpr(LHS, getSCEV(CI));
-          // If the LHS of the add was an addrec and it has no-wrap flags,
-          // transfer the no-wrap flags, since an or won't introduce a wrap.
-          if (const SCEVAddRecExpr *NewAR = dyn_cast<SCEVAddRecExpr>(S)) {
-            const SCEVAddRecExpr *OldAR = cast<SCEVAddRecExpr>(LHS);
-            const_cast<SCEVAddRecExpr *>(NewAR)->setNoWrapFlags(
-                OldAR->getNoWrapFlags());
-          }
-          return S;
+          return getAddExpr(LHS, getSCEV(CI),
+                            (SCEV::NoWrapFlags)(SCEV::FlagNUW | SCEV::FlagNSW));
         }
       }
       break;
 
     case Instruction::Xor:
       if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->RHS)) {
         // If the RHS of xor is -1, then this is a not operation.
         if (CI->isMinusOne())