[llvm] [NFC][Analysis] Add more SCEV tests for ptr inductions (PR #108210)

[Eli Friedman via llvm-commits]

================
@@ -1595,6 +1595,145 @@ exit:
   ret i32 0
 }
 
+define void @ptr_induction_eq_1(ptr %a, ptr %b) {
+; CHECK-LABEL: 'ptr_induction_eq_1'
+; CHECK-NEXT:  Classifying expressions for: @ptr_induction_eq_1
+; CHECK-NEXT:    %ptr.iv = phi ptr [ %ptr.iv.next, %loop ], [ %a, %entry ]
+; CHECK-NEXT:    --> {%a,+,8}<nuw><%loop> U: full-set S: full-set Exits: ((8 * ((-8 + (-1 * (ptrtoint ptr %a to i64)) + (ptrtoint ptr %b to i64)) /u 8))<nuw> + %a) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    %ptr.iv.next = getelementptr inbounds i8, ptr %ptr.iv, i64 8
+; CHECK-NEXT:    --> {(8 + %a),+,8}<nuw><%loop> U: full-set S: full-set Exits: (8 + (8 * ((-8 + (-1 * (ptrtoint ptr %a to i64)) + (ptrtoint ptr %b to i64)) /u 8))<nuw> + %a) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:  Determining loop execution counts for: @ptr_induction_eq_1
+; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-8 + (-1 * (ptrtoint ptr %a to i64)) + (ptrtoint ptr %b to i64)) /u 8)
+; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is i64 2305843009213693951
+; CHECK-NEXT:  Loop %loop: symbolic max backedge-taken count is ((-8 + (-1 * (ptrtoint ptr %a to i64)) + (ptrtoint ptr %b to i64)) /u 8)
+; CHECK-NEXT:  Loop %loop: Trip multiple is 1
+;
+entry:
+  %cmp = icmp eq ptr %a, %b
+  br i1 %cmp, label %exit, label %loop
+
+loop:
+  %ptr.iv = phi ptr [ %ptr.iv.next, %loop ], [ %a, %entry ]
+  %ptr.iv.next = getelementptr inbounds i8, ptr %ptr.iv, i64 8
+  %exitcond = icmp eq ptr %ptr.iv.next, %b
+  br i1 %exitcond, label %exit, label %loop
+
+exit:
+  ret void
+}
+
+define void @ptr_induction_eq_2(ptr %a, i64 %n) {
+; CHECK-LABEL: 'ptr_induction_eq_2'
+; CHECK-NEXT:  Classifying expressions for: @ptr_induction_eq_2
+; CHECK-NEXT:    %b = getelementptr inbounds ptr, ptr %a, i64 %n
+; CHECK-NEXT:    --> ((8 * %n)<nsw> + %a) U: full-set S: full-set
+; CHECK-NEXT:    %ptr.iv = phi ptr [ %ptr.iv.next, %loop ], [ %a, %entry ]
+; CHECK-NEXT:    --> {%a,+,8}<nuw><%loop> U: full-set S: full-set Exits: ((8 * ((-8 + (8 * %n)<nsw>) /u 8))<nuw> + %a) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    %ptr.iv.next = getelementptr inbounds i8, ptr %ptr.iv, i64 8
+; CHECK-NEXT:    --> {(8 + %a),+,8}<nuw><%loop> U: full-set S: full-set Exits: (8 + (8 * ((-8 + (8 * %n)<nsw>) /u 8))<nuw> + %a) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:  Determining loop execution counts for: @ptr_induction_eq_2
+; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-8 + (8 * %n)<nsw>) /u 8)
+; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is i64 2305843009213693951
+; CHECK-NEXT:  Loop %loop: symbolic max backedge-taken count is ((-8 + (8 * %n)<nsw>) /u 8)
+; CHECK-NEXT:  Loop %loop: Trip multiple is 1
+;
+entry:
+  %b = getelementptr inbounds ptr, ptr %a, i64 %n
+  %cmp = icmp eq ptr %a, %b
+  br i1 %cmp, label %exit, label %loop
+
+loop:
+  %ptr.iv = phi ptr [ %ptr.iv.next, %loop ], [ %a, %entry ]
+  %ptr.iv.next = getelementptr inbounds i8, ptr %ptr.iv, i64 8
+  %exitcond = icmp eq ptr %ptr.iv.next, %b
+  br i1 %exitcond, label %exit, label %loop
+
+exit:
+  ret void
+}
+
+; TODO: It feels like we should be able to calculate the symbolic max
+; exit count for the loop.inc block here, in the same way as
+; ptr_induction_eq_1. The problem seems to be in howFarToZero when the
+; ControlsOnlyExit is set to false.
----------------
efriedma-quic wrote:

> The getelementptr operation does have inbounds on it - does that help in this case limit the trip count because it cannot wrap?

This helps us prove the loop is finite, but we can't produce a symbolic bound; at most, we could prove the max iteration count is 2^60 or something like that.  Which is marginally helpful.

---------

We could also generate a predicated backedge-taken count, which would be useful in contexts like vectorization.  Granted, we don't usually vectorize loops with multiple exits anyway, but I guess that's changing a bit.

https://github.com/llvm/llvm-project/pull/108210