[MCA] Avoid repeated hash lookups (NFC)

[kazutakahirata]

No description provided.

[llvmbot]

@llvm/pr-subscribers-tools-llvm-mca

Author: Kazu Hirata (kazutakahirata)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/129192.diff

1 Files Affected:

• (modified) llvm/lib/MCA/InstrBuilder.cpp (+2-4)

diff --git a/llvm/lib/MCA/InstrBuilder.cpp b/llvm/lib/MCA/InstrBuilder.cpp
index 2cb1908695308..2bac99b6309af 100644
--- a/llvm/lib/MCA/InstrBuilder.cpp
+++ b/llvm/lib/MCA/InstrBuilder.cpp
@@ -634,16 +634,14 @@ InstrBuilder::createInstrDescImpl(const MCInst &MCI,
   bool IsVariadic = MCDesc.isVariadic();
   if ((ID->IsRecyclable = !IsVariadic && !IsVariant)) {
     auto DKey = std::make_pair(MCI.getOpcode(), SchedClassID);
-    Descriptors[DKey] = std::move(ID);
-    return *Descriptors[DKey];
+    return *(Descriptors[DKey] = std::move(ID));
   }
 
   auto VDKey = std::make_pair(hashMCInst(MCI), SchedClassID);
   assert(
       !VariantDescriptors.contains(VDKey) &&
       "Expected VariantDescriptors to not already have a value for this key.");
-  VariantDescriptors[VDKey] = std::move(ID);
-  return *VariantDescriptors[VDKey];
+  return *(VariantDescriptors[VDKey] = std::move(ID));
 }
 
 Expected<const InstrDesc &>

[adibiagio]

Apologies if I am a bit late on this review.

Have you considered using insert_or_assign() ?
https://llvm.org/doxygen/DenseMap_8h_source.html#l00306

[adibiagio]

Using insert_or_assign() should solve the issue in a nicer way. That is, assuming that my understanding of how insert_or_assign() works is correct :) .

[kazutakahirata]

I could use insert_or_assign, but that would be a bit mouthful though:

    return *(Descriptors[DKey] = std::move(ID));
    return *(Descriptors.insert_or_assign(DKey, std::move(ID)).first->second);

[adibiagio]

Mmm yeah, it forces you to do that annoying first->second to get the iterator from the pair.
Nevermind then. Thanks for the fix!