[llvm] [RISCV] Avoid redundant SchedRead on _TIED VPseudos (PR #113940)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Min-Yih Hsu (mshockwave)

<details>
<summary>Changes</summary>

_TIED and _MASK_TIED pseudos have one less operand compared to other pseudos, thus we shouldn't attach the same number of SchedRead for these instructions.

----

I don't think we have a way to (explicitly) check scheduling classes. So I only test this patch with existing tests.

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Full diff: https://github.com/llvm/llvm-project/pull/113940.diff


1 Files Affected:

- (modified) llvm/lib/Target/RISCV/RISCVInstrInfoV.td (+15-2) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfoV.td b/llvm/lib/Target/RISCV/RISCVInstrInfoV.td
index 4e8619c5ec2392..e63a23899152fa 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfoV.td
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfoV.td
@@ -104,13 +104,26 @@ class SchedCommon<list<SchedWrite> writes, list<SchedRead> reads,
                   string mx = "WorstCase", int sew = 0, bit forceMasked = 0,
                   bit forcePassthruRead = 0> : Sched<[]> {
   defvar isMasked = !ne(!find(NAME, "_MASK"), -1);
+  defvar isTied = !ne(!find(NAME, "_TIED"), -1);
   defvar isMaskedOrForceMasked = !or(forceMasked, isMasked);
+  defvar isTiedMasked = !and(isMaskedOrForceMasked, isTied);
   defvar passthruRead = !if(!or(!eq(mx, "WorstCase"), !eq(sew, 0)),
                             !cast<SchedRead>("ReadVPassthru_" # mx),
                             !cast<SchedRead>("ReadVPassthru_" # mx # "_E" #sew));
-  defvar needsPassthruRead = !or(isMaskedOrForceMasked, forcePassthruRead);
+  // We don't need passthru operand if it's already _TIED without mask.
+  defvar needsForcePassthruRead = !and(forcePassthruRead, !not(isTied));
+  defvar needsPassthruRead = !or(isMaskedOrForceMasked, needsForcePassthruRead);
+  // If this is a _TIED + masked operation, $rs2 (i.e. the first operand) is
+  // merged with the mask.
+  // NOTE: the following if statement is written in such a weird way because
+  // TableGen's `!if` doesn't have a proper short-circuit behavior, so if the
+  // predicate of this `!if` cannot be resolved right away, `!tail(reads)` will
+  // still be resolved right away even when `reads` is empty, which leads to
+  // an assertion failure.
+  defvar readsWithTiedMask =
+      !if(isTiedMasked, !if(!not(!empty(reads)), !tail(reads), reads), reads);
   defvar readsWithMask =
-      !if(isMaskedOrForceMasked, !listconcat(reads, [ReadVMask]), reads);
+      !if(isMaskedOrForceMasked, !listconcat(readsWithTiedMask, [ReadVMask]), reads);
   defvar allReads =
       !if(needsPassthruRead, !listconcat([passthruRead], readsWithMask), reads);
   let SchedRW = !listconcat(writes, allReads);

``````````

</details>


https://github.com/llvm/llvm-project/pull/113940