Per discussion with Dan G, inbounds geps *certainly* can have

unsigned overflow (e.g. "gep P, -1"), and while they can have
signed wrap in theoretical situations, modelling an AddRec as
not having signed wrap is going enough for any case we can 
think of today.  In the future if this isn't enough, we can
revisit this.  Modeling them as having NUW isn't causing any
known problems either FWIW.

llvm-svn: 125410

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -2785,10 +2785,21 @@ const SCEV *ScalarEvolution::createNodeForPHI(PHINode *PN) {
                   HasNSW = true;
               } else if (const GEPOperator *GEP = 
                             dyn_cast<GEPOperator>(BEValueV)) {
-                // If the increment is a GEP, then we know it won't perform an
-                // unsigned overflow, because the address space cannot be
+                // If the increment is a GEP, then we know it won't perform a
+                // signed overflow, because the address space cannot be
                 // wrapped around.
-                HasNUW |= GEP->isInBounds();
+                //
+                // NOTE: This isn't strictly true, because you could have an
+                // object straddling the 2G address boundary in a 32-bit address
+                // space (for example).  We really want to model this as a "has
+                // no signed/unsigned wrap" where the base pointer is treated as
+                // unsigned and the increment is known to not have signed
+                // wrapping.
+                //
+                // This is a highly theoretical concern though, and this is good
+                // enough for all cases we know of at this point. :)
+                //                
+                HasNSW |= GEP->isInBounds();
               }
 
               const SCEV *StartVal = getSCEV(StartValueV);

llvm/test/Analysis/ScalarEvolution/nsw.ll

@@ -62,11 +62,11 @@ for.body.lr.ph.i.i:                               ; preds = %entry
 for.body.i.i:                                     ; preds = %for.body.i.i, %for.body.lr.ph.i.i
   %__first.addr.02.i.i = phi i32* [ %begin, %for.body.lr.ph.i.i ], [ %ptrincdec.i.i, %for.body.i.i ]
 ; CHECK: %__first.addr.02.i.i
-; CHECK-NEXT: -->  {%begin,+,4}<nuw><%for.body.i.i>	
+; CHECK-NEXT: -->  {%begin,+,4}<nsw><%for.body.i.i>	
   store i32 0, i32* %__first.addr.02.i.i, align 4
   %ptrincdec.i.i = getelementptr inbounds i32* %__first.addr.02.i.i, i64 1
 ; CHECK: %ptrincdec.i.i
-; CHECK-NEXT: -->  {(4 + %begin),+,4}<nuw><%for.body.i.i>
+; CHECK-NEXT: -->  {(4 + %begin),+,4}<nsw><%for.body.i.i>
   %cmp.i.i = icmp eq i32* %ptrincdec.i.i, %end
   br i1 %cmp.i.i, label %for.cond.for.end_crit_edge.i.i, label %for.body.i.i