[llvm] [AArch64] Neoverse V2 FeatureDisableLatencySchedHeuristic (PR #140897)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-aarch64

Author: Sjoerd Meijer (sjoerdmeijer)

<details>
<summary>Changes</summary>

This adds FeatureDisableLatencySchedHeuristic to the Neoverse V2 core tuning description. This gives us a 20% improvement on a key workload, some other minor improvements here and there, and no real regressions; nothing outside the noise levels.

Earlier attempts to solve this problems included disabling the MI scheduler entirely (#<!-- -->127784), and #<!-- -->139557 was about a heuristic to not schedule hand-written vector code. This solution is preferred because it avoids another heuristic and achieves what we want, and for what is worth, there is a lot of precedent for setting this feature.

Thanks to:
- Ricardo Jesus for pointing out this subtarget feature, and
- Cameron McInally for the extensive performance testing.

---
Full diff: https://github.com/llvm/llvm-project/pull/140897.diff


1 Files Affected:

- (modified) llvm/lib/Target/AArch64/AArch64Processors.td (+2-1) 


``````````diff
diff --git a/llvm/lib/Target/AArch64/AArch64Processors.td b/llvm/lib/Target/AArch64/AArch64Processors.td
index a5f0f6a2eb150..e7a3527202f6a 100644
--- a/llvm/lib/Target/AArch64/AArch64Processors.td
+++ b/llvm/lib/Target/AArch64/AArch64Processors.td
@@ -561,7 +561,8 @@ def TuneNeoverseV2 : SubtargetFeature<"neoversev2", "ARMProcFamily", "NeoverseV2
                                       FeatureEnableSelectOptimize,
                                       FeatureUseFixedOverScalableIfEqualCost,
                                       FeatureAvoidLDAPUR,
-                                      FeaturePredictableSelectIsExpensive]>;
+                                      FeaturePredictableSelectIsExpensive,
+                                      FeatureDisableLatencySchedHeuristic]>;
 
 def TuneNeoverseV3 : SubtargetFeature<"neoversev3", "ARMProcFamily", "NeoverseV3",
                                       "Neoverse V3 ARM processors", [

``````````

</details>


https://github.com/llvm/llvm-project/pull/140897