[llvm] [LV] Rewrite UDiv A, B -> UDiv A, UMax(B, 1) in trip counts if needed. (PR #92177)

[Eli Friedman via llvm-commits]

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@@ -527,6 +528,76 @@ exit:
   ret i64 %p
 }
 
+define i64 @multi_exit_4_exit_count_with_udiv_by_frozen_value_in_latch(ptr %dst, i64 %N) {
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efriedma-quic wrote:

I'm not sure there's really a useful distinction between an abnormal exit inside the loop, vs. an abnormal exit in the loop preheader.  Either way, if you want to hoist the trip count past such an operation, you need to take additional measures beyond what SCEV normally provides.  You need safe-udiv for all udiv operations, and if you want to use the value for anything, you need to freeze it.

For simple vectorization, we should be able to assume the loop exits normally, and we want to use that information to avoid unnecessary freeze operations.  So we need to distinguish which freeze/safe-div operations are necessary.  The way we currently figure out which operands we need to freeze is via umin_seq; I think it makes sense to figure out which operands need safe-div the same way.

https://github.com/llvm/llvm-project/pull/92177