ScalarEvolution: Compute exit counts for loops with a power-of-2 step.

If we have a loop of the form
for (unsigned n = 0; n != (k & -32); n += 32) {}
then we know that n is always divisible by 32 and the loop must
terminate. Even if we have a condition where the loop counter will
overflow it'll always hold this invariant.

PR19183. Our loop vectorizer creates this pattern and it's also
occasionally formed by loop counters derived from pointers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204728 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 53 insertions, 0 deletions

diff --git a/test/Analysis/ScalarEvolution/trip-count-pow2.ll b/test/Analysis/ScalarEvolution/trip-count-pow2.ll
new file mode 100644
index 0000000000..2c5b72e49d
--- /dev/null
+++ b/test/Analysis/ScalarEvolution/trip-count-pow2.ll
@@ -0,0 +1,53 @@
+; RUN: opt < %s -scalar-evolution -analyze | FileCheck %s
+
+define void @test1(i32 %n) {
+entry:
+  %s = mul i32 %n, 96
+  br label %loop
+loop:
+  %i = phi i32 [ 0, %entry ], [ %i.next, %loop ]
+  %i.next = add i32 %i, 32
+  %t = icmp ne i32 %i.next, %s
+  br i1 %t, label %loop, label %exit
+exit:
+  ret void
+
+; CHECK-LABEL: @test1
+; CHECK: Loop %loop: backedge-taken count is ((-32 + (96 * %n)) /u 32)
+; CHECK: Loop %loop: max backedge-taken count is ((-32 + (96 * %n)) /u 32)
+}
+
+; PR19183
+define i32 @test2(i32 %n) {
+entry:
+  %s = and i32 %n, -32
+  br label %loop
+loop:
+  %i = phi i32 [ 0, %entry ], [ %i.next, %loop ]
+  %i.next = add i32 %i, 32
+  %t = icmp ne i32 %i.next, %s
+  br i1 %t, label %loop, label %exit
+exit:
+  ret i32 %i
+
+; CHECK-LABEL: @test2
+; CHECK: Loop %loop: backedge-taken count is ((-32 + (32 * (%n /u 32))) /u 32)
+; CHECK: Loop %loop: max backedge-taken count is ((-32 + (32 * (%n /u 32))) /u 32)
+}
+
+define void @test3(i32 %n) {
+entry:
+  %s = mul i32 %n, 96
+  br label %loop
+loop:
+  %i = phi i32 [ 0, %entry ], [ %i.next, %loop ]
+  %i.next = add i32 %i, 96
+  %t = icmp ne i32 %i.next, %s
+  br i1 %t, label %loop, label %exit
+exit:
+  ret void
+
+; CHECK-LABEL: @test3
+; CHECK: Loop %loop: Unpredictable backedge-taken count.
+; CHECK: Loop %loop: Unpredictable max backedge-taken count.
+}