[llvm] [AArch64] Cap upper-bound unrolling of loops with uncomputable trip counts (PR #205102)

[David Sherwood via llvm-commits]

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@@ -6042,6 +6042,19 @@ void AArch64TTIImpl::getUnrollingPreferences(
 
   UP.UpperBound = true;
 
+  // A loop can have a small maximum trip count while SCEV still cannot
+  // form an exact backedge count - typically a data-dependent exit, e.g.
+  // shifting a value until it reaches zero. Unrolling such a loop trades one
+  // well-predicted backedge for a chain of rarely-taken exit branches, so hold
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david-arm wrote:

Surely this is only true for some loops (presumably including the one you care about), but in the general case it may not be. In your `varint_len` example below what if v=1? If we unroll such a loop then every early exit would be taken, so it would be a chain of always-taken exit branches instead? i.e.

if ((1 >> 7) == 0) break;
if ((1 >> 14) == 0) break;
etc.

This comment suggests we are making assumptions about the input data and using that as the motivation for the change.

https://github.com/llvm/llvm-project/pull/205102