[WIP] Try to fix more SCEV stuff

Author: resistor

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -2418,6 +2418,14 @@ bool RISCVTTIImpl::isLegalBaseRegForLSR(const SCEV *S) const {
           return false;
       }
     }
+    if (const auto *Add = dyn_cast<SCEVAddExpr>(S)) {
+      const auto *Offset = dyn_cast<SCEVConstant>(Add->getOperand(0));
+      if (Offset && Offset->getValue()->isNegative())
+        return false;
+      Offset = dyn_cast<SCEVConstant>(Add->getOperand(1));
+      if (Offset && Offset->getValue()->isNegative())
+        return false;
+    }
   }
   return BasicTTIImplBase::isLegalBaseRegForLSR(S);
 }

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

@@ -1502,6 +1502,14 @@ void Cost::RateFormula(const Formula &F,
   if (isLoser())
     return;
   assert(F.isCanonical(*L) && "Cost is accurate only for canonical formula");
+
+  for (const SCEV *Reg : Regs) {
+    if (!TTI->isLegalBaseRegForLSR(Reg)) {
+      Lose();
+      return;
+    }
+  }
+
   // Tally up the registers.
   unsigned PrevAddRecCost = C.AddRecCost;
   unsigned PrevNumRegs = C.NumRegs;