[llvm] [TableGen] Print MVT name in the isel table when it doesn't require a VBR. (PR #175128)

[Craig Topper via llvm-commits]

https://github.com/topperc created https://github.com/llvm/llvm-project/pull/175128

If the MVT is small enough, we can emit the enum name instead of
printing its encoding.

Stacked on #175127

>From 54e6cb796d9f7b200516c1d54fc1cf3652811eff Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper at sifive.com>
Date: Thu, 8 Jan 2026 23:26:58 -0800
Subject: [PATCH 1/2] [TableGen] Add a helper function for emitting an MVT in
 the isel table. NFC

---
 llvm/utils/TableGen/DAGISelMatcherEmitter.cpp | 49 ++++++-------------
 1 file changed, 16 insertions(+), 33 deletions(-)

diff --git a/llvm/utils/TableGen/DAGISelMatcherEmitter.cpp b/llvm/utils/TableGen/DAGISelMatcherEmitter.cpp
index 7877a1f36424f..afad7818d18da 100644
--- a/llvm/utils/TableGen/DAGISelMatcherEmitter.cpp
+++ b/llvm/utils/TableGen/DAGISelMatcherEmitter.cpp
@@ -424,6 +424,12 @@ void MatcherTableEmitter::EmitPatternMatchTable(raw_ostream &OS) {
   EndEmitFunction(OS);
 }
 
+static unsigned emitMVT(MVT VT, raw_ostream &OS) {
+  if (!OmitComments)
+    OS << "/*" << getEnumName(VT) << "*/";
+  return EmitVBRValue(VT.SimpleTy, OS);
+}
+
 /// EmitMatcher - Emit bytes for the specified matcher and return
 /// the number of bytes emitted.
 unsigned MatcherTableEmitter::EmitMatcher(const Matcher *N,
@@ -631,12 +637,8 @@ unsigned MatcherTableEmitter::EmitMatcher(const Matcher *N,
       OS << ' ';
       if (const SwitchOpcodeMatcher *SOM = dyn_cast<SwitchOpcodeMatcher>(N))
         OS << "TARGET_VAL(" << SOM->getCaseOpcode(i).getEnumName() << "),";
-      else {
-        if (!OmitComments)
-          OS << "/*" << getEnumName(cast<SwitchTypeMatcher>(N)->getCaseType(i))
-             << "*/";
-        EmitVBRValue(cast<SwitchTypeMatcher>(N)->getCaseType(i).SimpleTy, OS);
-      }
+      else
+        emitMVT(cast<SwitchTypeMatcher>(N)->getCaseType(i), OS);
       if (!OmitComments)
         OS << " // ->" << CurrentIdx + ChildSize;
       OS << '\n';
@@ -669,18 +671,13 @@ unsigned MatcherTableEmitter::EmitMatcher(const Matcher *N,
         return 1;
       default:
         OS << "OPC_CheckType, ";
-        if (!OmitComments)
-          OS << "/*" << getEnumName(VT) << "*/";
-        unsigned NumBytes = EmitVBRValue(VT.SimpleTy, OS);
+        unsigned NumBytes = emitMVT(VT, OS);
         OS << "\n";
         return NumBytes + 1;
       }
     }
     OS << "OPC_CheckTypeRes, " << cast<CheckTypeMatcher>(N)->getResNo() << ", ";
-    if (!OmitComments)
-      OS << "/*" << getEnumName(cast<CheckTypeMatcher>(N)->getType()) << "*/";
-    unsigned NumBytes =
-        EmitVBRValue(cast<CheckTypeMatcher>(N)->getType().SimpleTy, OS);
+    unsigned NumBytes = emitMVT(cast<CheckTypeMatcher>(N)->getType(), OS);
     OS << "\n";
     return NumBytes + 2;
   }
@@ -696,9 +693,7 @@ unsigned MatcherTableEmitter::EmitMatcher(const Matcher *N,
     default:
       OS << "OPC_CheckChild" << cast<CheckChildTypeMatcher>(N)->getChildNo()
          << "Type, ";
-      if (!OmitComments)
-        OS << "/*" << getEnumName(VT) << "*/";
-      unsigned NumBytes = EmitVBRValue(VT.SimpleTy, OS);
+      unsigned NumBytes = emitMVT(VT, OS);
       OS << "\n";
       return NumBytes + 1;
     }
@@ -731,11 +726,7 @@ unsigned MatcherTableEmitter::EmitMatcher(const Matcher *N,
 
   case Matcher::CheckValueType: {
     OS << "OPC_CheckValueType, ";
-    if (!OmitComments)
-      OS << "/*" << getEnumName(cast<CheckValueTypeMatcher>(N)->getVT())
-         << "*/";
-    unsigned NumBytes =
-        EmitVBRValue(cast<CheckValueTypeMatcher>(N)->getVT().SimpleTy, OS);
+    unsigned NumBytes = emitMVT(cast<CheckValueTypeMatcher>(N)->getVT(), OS);
     OS << "\n";
     return NumBytes + 1;
   }
@@ -807,9 +798,7 @@ unsigned MatcherTableEmitter::EmitMatcher(const Matcher *N,
       break;
     default:
       OS << "OPC_EmitInteger, ";
-      if (!OmitComments)
-        OS << "/*" << getEnumName(VT) << "*/";
-      TypeBytes = EmitVBRValue(VT.SimpleTy, OS);
+      TypeBytes = emitMVT(VT, OS);
       OS << ' ';
       break;
     }
@@ -840,9 +829,7 @@ unsigned MatcherTableEmitter::EmitMatcher(const Matcher *N,
     // use EmitRegister2.
     if (Reg && Reg->EnumValue > 255) {
       OS << "OPC_EmitRegister2, ";
-      if (!OmitComments)
-        OS << "/*" << getEnumName(VT) << "*/";
-      OpBytes = EmitVBRValue(VT.SimpleTy, OS);
+      OpBytes = emitMVT(VT, OS);
       OS << "TARGET_VAL(" << getQualifiedName(Reg->TheDef) << "),\n";
       return OpBytes + 3;
     }
@@ -854,9 +841,7 @@ unsigned MatcherTableEmitter::EmitMatcher(const Matcher *N,
       break;
     default:
       OS << "OPC_EmitRegister, ";
-      if (!OmitComments)
-        OS << "/*" << getEnumName(VT) << "*/";
-      OpBytes = EmitVBRValue(VT.SimpleTy, OS) + 1;
+      OpBytes = emitMVT(VT, OS) + 1;
       break;
     }
     if (Reg) {
@@ -1020,9 +1005,7 @@ unsigned MatcherTableEmitter::EmitMatcher(const Matcher *N,
     unsigned NumTypeBytes = 0;
     for (unsigned i = 0, e = EN->getNumVTs(); i != e; ++i) {
       OS << ' ';
-      if (!OmitComments)
-        OS << "/*" << getEnumName(EN->getVT(i)) << "*/";
-      NumTypeBytes += EmitVBRValue(EN->getVT(i).SimpleTy, OS);
+      NumTypeBytes += emitMVT(EN->getVT(i), OS);
     }
 
     OS << ' ' << EN->getNumOperands();

>From 4e7d3fb4b8426a08a5ab61b6528af73f44e5619b Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper at sifive.com>
Date: Thu, 8 Jan 2026 23:36:59 -0800
Subject: [PATCH 2/2] [TableGen] Print MVT name in the isel table when it
 doesn't require a VBR.

If the MVT is small enough, we can emit the enum name instead of
printing its encoding.
---
 llvm/utils/TableGen/DAGISelMatcherEmitter.cpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/llvm/utils/TableGen/DAGISelMatcherEmitter.cpp b/llvm/utils/TableGen/DAGISelMatcherEmitter.cpp
index afad7818d18da..6eaac318d3e41 100644
--- a/llvm/utils/TableGen/DAGISelMatcherEmitter.cpp
+++ b/llvm/utils/TableGen/DAGISelMatcherEmitter.cpp
@@ -425,6 +425,12 @@ void MatcherTableEmitter::EmitPatternMatchTable(raw_ostream &OS) {
 }
 
 static unsigned emitMVT(MVT VT, raw_ostream &OS) {
+  // Print the MVT directly if it doesn't require a VBR.
+  if (VT.SimpleTy <= 127) {
+    OS << getEnumName(VT) << ',';
+    return 1;
+  }
+
   if (!OmitComments)
     OS << "/*" << getEnumName(VT) << "*/";
   return EmitVBRValue(VT.SimpleTy, OS);