[llvm] [RISCV] Update SpacemiT X60 scheduling latencies based on hardware measurements (PR #144730)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-risc-v

Author: Mikhail R. Gadelha (mikhailramalho)

<details>
<summary>Changes</summary>

This patch updates the RISC-V SpacemiT X60 scheduling model with latency values collected from the X60 hardware. The previous values were empirically derived but were slightly off.

  Changes:
  - LoadLatency (baseline for load instructions): 5 --> 3 cycles
  - Memory operations: unified at 4 cycles
  - Atomic loads/stores: 5 --> 8 cycles
  - Atomic RMW operations: 5 --> 12 cycles

Hardware-measured values provide more accurate instruction scheduling for the in-order X60 core. Testing shows NFC across benchmarks except for 523.xalancbmk_r (known to be noisy).

https://lnt.lukelau.me/db_default/v4/nts/663?compare_to=657

---
Full diff: https://github.com/llvm/llvm-project/pull/144730.diff


1 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVSchedSpacemitX60.td (+8-7) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVSchedSpacemitX60.td b/llvm/lib/Target/RISCV/RISCVSchedSpacemitX60.td
index 8948694c420a0..9059d5a4e497b 100644
--- a/llvm/lib/Target/RISCV/RISCVSchedSpacemitX60.td
+++ b/llvm/lib/Target/RISCV/RISCVSchedSpacemitX60.td
@@ -16,7 +16,7 @@
 def SpacemitX60Model : SchedMachineModel {
   let IssueWidth        = 2; // dual-issue
   let MicroOpBufferSize = 0; // in-order
-  let LoadLatency       = 5; // worse case: >= 3
+  let LoadLatency       = 3; // worse case: >= 3
   let MispredictPenalty = 9; // nine-stage
 
   let CompleteModel = 0;
@@ -114,7 +114,7 @@ def : WriteRes<WriteBEXT, [SMX60_IEU]>;
 def : WriteRes<WriteBEXTI, [SMX60_IEU]>;
 
 // Memory/Atomic memory
-let Latency = 3 in {
+let Latency = 4 in {
   def : WriteRes<WriteSTB, [SMX60_LS]>;
   def : WriteRes<WriteSTH, [SMX60_LS]>;
   def : WriteRes<WriteSTW, [SMX60_LS]>;
@@ -122,11 +122,7 @@ let Latency = 3 in {
   def : WriteRes<WriteFST16, [SMX60_LS]>;
   def : WriteRes<WriteFST32, [SMX60_LS]>;
   def : WriteRes<WriteFST64, [SMX60_LS]>;
-  def : WriteRes<WriteAtomicSTW, [SMX60_LS]>;
-  def : WriteRes<WriteAtomicSTD, [SMX60_LS]>;
-}
 
-let Latency = 5 in {
   def : WriteRes<WriteLDB, [SMX60_LS]>;
   def : WriteRes<WriteLDH, [SMX60_LS]>;
   def : WriteRes<WriteLDW, [SMX60_LS]>;
@@ -137,9 +133,14 @@ let Latency = 5 in {
 }
 
 // Atomics
-let Latency = 5 in {
+let Latency = 8 in {
+  def : WriteRes<WriteAtomicSTW, [SMX60_LS]>;
+  def : WriteRes<WriteAtomicSTD, [SMX60_LS]>;
   def : WriteRes<WriteAtomicLDW, [SMX60_LS]>;
   def : WriteRes<WriteAtomicLDD, [SMX60_LS]>;
+}
+
+let Latency = 12 in {
   def : WriteRes<WriteAtomicW, [SMX60_LS]>;
   def : WriteRes<WriteAtomicD, [SMX60_LS]>;
 }

``````````

</details>


https://github.com/llvm/llvm-project/pull/144730