[llvm] LoopVectorize: Set branch_weight for conditional branches (PR #72450)

[David Li via llvm-commits]

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@@ -396,6 +398,19 @@ static cl::opt<bool> UseWiderVFIfCallVariantsPresent(
     cl::Hidden,
     cl::desc("Try wider VFs if they enable the use of vector variants"));
 
+// Likelyhood of bypassing the vectorized loop because assumptions about SCEV
+// variables not overflowing do not hold. See `emitSCEVChecks`.
+static constexpr uint32_t SCEVCheckBypassWeights[] = {1, 127};
+// Likelyhood of bypassing the vectorized loop because pointers overlap. See
+// `emitMemRuntimeChecks`.
+static constexpr uint32_t MemCheckBypassWeights[] = {1, 127};
+// Likelyhood of bypassing the vectorized loop because there are zero trips left
+// after prolog. See `emitIterationCountCheck`.
+static constexpr uint32_t MinItersBypassWeights[] = {1, 127};
+// Likelyhood of bypassing the vectorized loop because of zero trips necessary.
+// See `emitMinimumVectorEpilogueIterCountCheck`.
+static constexpr uint32_t EpilogueMinItersBypassWeights[] = {1, 127};
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david-xl wrote:

How are the default values selected? 127 seems too large as default e.g.for the epilogue minBy pass.  The average trip count of the remainder loop is 0.5*mainUF*mainVF, how likely it is larger than EVF*EUF?

https://github.com/llvm/llvm-project/pull/72450