[SCEV] Use fact that B >u 0 for A <u B in applyLoopGuards.

If LHS <u RHS holds, RHS should be guaranteed to be > 0. By using
using 'umax(RHS, 1) -1' instead of 'RHS - 1' the results in
applyLoopGuards can be improved in some cases.

Note that the TODO for the tests mentioned the max BTC being 11, but
unless I am missing something 10 should be correct.

https://alive2.llvm.org/ce/z/44nP7F

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D126503

Author: fhahn

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -15052,10 +15052,14 @@ const SCEV *ScalarEvolution::applyLoopGuards(const SCEV *Expr, const Loop *L) {
 
     const SCEV *RewrittenRHS = nullptr;
     switch (Predicate) {
-    case CmpInst::ICMP_ULT:
+    case CmpInst::ICMP_ULT: {
+      if (RHS->getType()->isPointerTy())
+        break;
+      const SCEV *One = getOne(RHS->getType());
       RewrittenRHS =
-          getUMinExpr(RewrittenLHS, getMinusSCEV(RHS, getOne(RHS->getType())));
+          getUMinExpr(RewrittenLHS, getMinusSCEV(getUMaxExpr(RHS, One), One));
       break;
+    }
     case CmpInst::ICMP_SLT:
       RewrittenRHS =
           getSMinExpr(RewrittenLHS, getMinusSCEV(RHS, getOne(RHS->getType())));

llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll

@@ -308,19 +308,18 @@ exit:
   ret void
 }
 
-; TODO: Currently we miss getting the tightest constant max backedge-taken count (11).
 define void @test_multiple_var_guards_order1(ptr nocapture %a, i64 %i, i64 %N) {
 ; CHECK-LABEL: 'test_multiple_var_guards_order1'
 ; CHECK-NEXT:  Classifying expressions for: @test_multiple_var_guards_order1
 ; CHECK-NEXT:    %iv = phi i64 [ %iv.next, %loop ], [ 0, %guardbb ]
-; CHECK-NEXT:    --> {0,+,1}<nuw><nsw><%loop> U: [0,-9223372036854775808) S: [0,-9223372036854775808) Exits: %i LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {0,+,1}<nuw><nsw><%loop> U: [0,11) S: [0,11) Exits: %i LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %idx = getelementptr inbounds i32, ptr %a, i64 %iv
 ; CHECK-NEXT:    --> {%a,+,4}<nuw><%loop> U: full-set S: full-set Exits: ((4 * %i) + %a) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.next = add nuw nsw i64 %iv, 1
-; CHECK-NEXT:    --> {1,+,1}<nuw><%loop> U: [1,0) S: [1,0) Exits: (1 + %i) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {1,+,1}<nuw><nsw><%loop> U: [1,12) S: [1,12) Exits: (1 + %i) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @test_multiple_var_guards_order1
 ; CHECK-NEXT:  Loop %loop: backedge-taken count is %i
-; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is -1
+; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is 10
 ; CHECK-NEXT:  Loop %loop: symbolic max backedge-taken count is %i
 ; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is %i
 ; CHECK-NEXT:   Predicates:
@@ -346,19 +345,18 @@ exit:
   ret void
 }
 
-; TODO: Currently we miss getting the tightest constant max backedge-taken count (11).
 define void @test_multiple_var_guards_order2(ptr nocapture %a, i64 %i, i64 %N) {
 ; CHECK-LABEL: 'test_multiple_var_guards_order2'
 ; CHECK-NEXT:  Classifying expressions for: @test_multiple_var_guards_order2
 ; CHECK-NEXT:    %iv = phi i64 [ %iv.next, %loop ], [ 0, %guardbb ]
-; CHECK-NEXT:    --> {0,+,1}<nuw><nsw><%loop> U: [0,-9223372036854775808) S: [0,-9223372036854775808) Exits: %i LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {0,+,1}<nuw><nsw><%loop> U: [0,11) S: [0,11) Exits: %i LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %idx = getelementptr inbounds i32, ptr %a, i64 %iv
 ; CHECK-NEXT:    --> {%a,+,4}<nuw><%loop> U: full-set S: full-set Exits: ((4 * %i) + %a) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.next = add nuw nsw i64 %iv, 1
-; CHECK-NEXT:    --> {1,+,1}<nuw><%loop> U: [1,0) S: [1,0) Exits: (1 + %i) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {1,+,1}<nuw><nsw><%loop> U: [1,12) S: [1,12) Exits: (1 + %i) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @test_multiple_var_guards_order2
 ; CHECK-NEXT:  Loop %loop: backedge-taken count is %i
-; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is -1
+; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is 10
 ; CHECK-NEXT:  Loop %loop: symbolic max backedge-taken count is %i
 ; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is %i
 ; CHECK-NEXT:   Predicates:
@@ -393,10 +391,10 @@ define void @test_multiple_var_guards_cycle(ptr nocapture %a, i64 %i, i64 %N) {
 ; CHECK-NEXT:    %idx = getelementptr inbounds i32, ptr %a, i64 %iv
 ; CHECK-NEXT:    --> {%a,+,4}<nuw><%loop> U: full-set S: full-set Exits: ((4 * %N) + %a) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.next = add nuw nsw i64 %iv, 1
-; CHECK-NEXT:    --> {1,+,1}<nuw><%loop> U: [1,0) S: [1,0) Exits: (1 + %N) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {1,+,1}<nuw><%loop> U: [1,-1) S: [1,-1) Exits: (1 + %N) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @test_multiple_var_guards_cycle
 ; CHECK-NEXT:  Loop %loop: backedge-taken count is %N
-; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is -1
+; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is -3
 ; CHECK-NEXT:  Loop %loop: symbolic max backedge-taken count is %N
 ; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is %N
 ; CHECK-NEXT:   Predicates: