[ScalarEvolution] Make isKnownNonZero handle more cases.

[efriedma]

Summary

Using an unsigned range instead of signed ranges is a bit more precise.

[reames]

LGTM

Nice catch!

llvm/lib/Analysis/ScalarEvolution.cpp

bool ScalarEvolution::isKnownNonNegative(const SCEV *S) {
   return !getSignedRangeMin(S).isNegative();
 }
 
 bool ScalarEvolution::isKnownNonPositive(const SCEV *S) {
   return !getSignedRangeMax(S).isStrictlyPositive();
 }
 
 bool ScalarEvolution::isKnownNonZero(const SCEV *S) {
-  return isKnownNegative(S) || isKnownPositive(S);
+  return getUnsignedRangeMin(S) != 0;
 }
 
 std::pair<const SCEV *, const SCEV *>
 ScalarEvolution::SplitIntoInitAndPostInc(const Loop *L, const SCEV *S) {
   // Compute SCEV on entry of loop L.
   const SCEV *Start = SCEVInitRewriter::rewrite(S, L, *this);
   if (Start == getCouldNotCompute())
     return { Start, Start };

llvm/test/Analysis/ScalarEvolution/trip-count9.ll

loop:
   br i1 %t, label %loop, label %exit
 exit:
   ret void
 }
 
 define void @even_start1(i4 %n) {
 ; CHECK-LABEL: 'even_start1'
 ; CHECK-NEXT:  Determining loop execution counts for: @even_start1
-; CHECK-NEXT:  Loop %loop: backedge-taken count is (-2 + (2 smax (2 * %n)))<nsw>
+; CHECK-NEXT:  Loop %loop: backedge-taken count is (-2 + (2 * %n))
 ; CHECK-NEXT:  Loop %loop: max backedge-taken count is 4
-; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is (-2 + (2 smax (2 * %n)))<nsw>
+; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is (-2 + (2 * %n))
 ; CHECK-NEXT:   Predicates:
 ; CHECK:       Loop %loop: Trip multiple is 1
 ;
 entry:
   %m = shl i4 %n, 1
   %s = icmp sgt i4 %m, 0
   br i1 %s, label %loop, label %exit
 loop:
   %i = phi i4 [ 1, %entry ], [ %i.next, %loop ]
   %i.next = add i4 %i, 1
   %t = icmp slt i4 %i.next, %m
   br i1 %t, label %loop, label %exit
 exit:
   ret void
 }
 
 define void @even_start1_step2(i4 %n) {
 ; CHECK-LABEL: 'even_start1_step2'
 ; CHECK-NEXT:  Determining loop execution counts for: @even_start1_step2
-; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-2 + (3 smax (2 * %n)))<nsw> /u 2)
+; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-2 + (2 * %n)) /u 2)
 ; CHECK-NEXT:  Loop %loop: max backedge-taken count is 2
-; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((-2 + (3 smax (2 * %n)))<nsw> /u 2)
+; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((-2 + (2 * %n)) /u 2)
 ; CHECK-NEXT:   Predicates:
 ; CHECK:       Loop %loop: Trip multiple is 1
 ;
 entry:
   %m = shl i4 %n, 1
   %s = icmp sgt i4 %m, 0
   br i1 %s, label %loop, label %exit
 loop:
loop:
   br i1 %t, label %loop, label %exit
 exit:
   ret void
 }
 
 define void @even_nsw_start1(i4 %n) {
 ; CHECK-LABEL: 'even_nsw_start1'
 ; CHECK-NEXT:  Determining loop execution counts for: @even_nsw_start1
-; CHECK-NEXT:  Loop %loop: backedge-taken count is (-2 + (2 smax (2 * %n)))<nsw>
+; CHECK-NEXT:  Loop %loop: backedge-taken count is (-2 + (2 * %n))
 ; CHECK-NEXT:  Loop %loop: max backedge-taken count is 4
-; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is (-2 + (2 smax (2 * %n)))<nsw>
+; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is (-2 + (2 * %n))
 ; CHECK-NEXT:   Predicates:
 ; CHECK:       Loop %loop: Trip multiple is 1
 ;
 entry:
   %m = shl i4 %n, 1
   %s = icmp sgt i4 %m, 0
   br i1 %s, label %loop, label %exit
 loop:
   %i = phi i4 [ 1, %entry ], [ %i.next, %loop ]
   %i.next = add nsw i4 %i, 1
   %t = icmp slt i4 %i.next, %m
   br i1 %t, label %loop, label %exit
 exit:
   ret void
 }
 
 define void @even_nsw_start1_step2(i4 %n) {
 ; CHECK-LABEL: 'even_nsw_start1_step2'
 ; CHECK-NEXT:  Determining loop execution counts for: @even_nsw_start1_step2
-; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-2 + (3 smax (2 * %n)))<nsw> /u 2)
+; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-2 + (2 * %n)) /u 2)
 ; CHECK-NEXT:  Loop %loop: max backedge-taken count is 2
-; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((-2 + (3 smax (2 * %n)))<nsw> /u 2)
+; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((-2 + (2 * %n)) /u 2)
 ; CHECK-NEXT:   Predicates:
 ; CHECK:       Loop %loop: Trip multiple is 1
 ;
 entry:
   %m = shl i4 %n, 1
   %s = icmp sgt i4 %m, 0
   br i1 %s, label %loop, label %exit
 loop: