[SCEV] Make exact taken count calculation more optimistic
Currently, `getExact` fails if it sees two exit counts in different blocks. There is
no solid reason to do so, given that we only calculate exact non-taken count
for exiting blocks that dominate latch. Using this fact, we can simply take min
out of all exits of all blocks to get the exact taken count.
This patch makes the calculation more optimistic with enforcing our assumption
with asserts. It allows us to calculate exact backedge taken count in trivial loops
like
for (int i = 0; i < 100; i++) {
if (i > 50) break;
. . .
}
Differential Revision: https://reviews.llvm.org/D44676
Reviewed By: fhahn
llvm-svn: 328611
diff --git a/llvm/test/Analysis/ScalarEvolution/exact_iter_count.ll b/llvm/test/Analysis/ScalarEvolution/exact_iter_count.ll
new file mode 100644
index 0000000..ba0bc1f
--- /dev/null
+++ b/llvm/test/Analysis/ScalarEvolution/exact_iter_count.ll
@@ -0,0 +1,27 @@
+; RUN: opt < %s -scalar-evolution -analyze | FileCheck %s
+
+; One side exit dominating the latch, exact backedge taken count is known.
+define void @test_01() {
+
+; CHECK-LABEL: Determining loop execution counts for: @test_01
+; CHECK-NEXT: Loop %loop: <multiple exits> backedge-taken count is 50
+
+entry:
+ br label %loop
+
+loop:
+ %iv = phi i32 [ 0, %entry ], [ %iv.next, %backedge ]
+ %side.cond = icmp slt i32 %iv, 50
+ br i1 %side.cond, label %backedge, label %side.exit
+
+backedge:
+ %iv.next = add i32 %iv, 1
+ %loop.cond = icmp slt i32 %iv, 100
+ br i1 %loop.cond, label %loop, label %exit
+
+exit:
+ ret void
+
+side.exit:
+ ret void
+}