[llvm] [SelectionDAG] Provide context for vector count / type mismatch (PR #175433)

[Trevor Gross via llvm-commits]

https://github.com/tgross35 updated https://github.com/llvm/llvm-project/pull/175433

>From 35d4a8f35b2ceeecfc63c2ca390a5b64e2d6d843 Mon Sep 17 00:00:00 2001
From: Trevor Gross <tg at trevorgross.com>
Date: Sun, 11 Jan 2026 01:26:43 -0500
Subject: [PATCH] [SelectionDAG] Provide context for vector count / type
 mismatch

This now gives better details about what the mismatch is, e.g.:
---
 .../llvm/CodeGenTypes/MachineValueType.h      |  3 +++
 .../SelectionDAG/SelectionDAGBuilder.cpp      | 25 +++++++++++++------
 llvm/lib/CodeGen/ValueTypes.cpp               | 14 ++++++-----
 3 files changed, 29 insertions(+), 13 deletions(-)

diff --git a/llvm/include/llvm/CodeGenTypes/MachineValueType.h b/llvm/include/llvm/CodeGenTypes/MachineValueType.h
index 08a9c85a213e0..64687d515d21a 100644
--- a/llvm/include/llvm/CodeGenTypes/MachineValueType.h
+++ b/llvm/include/llvm/CodeGenTypes/MachineValueType.h
@@ -64,6 +64,9 @@ namespace llvm {
     bool operator>=(const MVT& S) const { return SimpleTy >= S.SimpleTy; }
     bool operator<=(const MVT& S) const { return SimpleTy <= S.SimpleTy; }
 
+    /// Return the value type as a string, e.g. `i32`.
+    std::string getString() const;
+
     /// Support for debugging, callable in GDB: VT.dump()
     LLVM_ABI void dump() const;
 
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index bff4799963fc2..355b279798d86 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -95,6 +95,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/InstructionCost.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/raw_ostream.h"
@@ -330,6 +331,21 @@ static void diagnosePossiblyInvalidConstraint(LLVMContext &Ctx, const Value *V,
   return Ctx.emitError(I, ErrMsg);
 }
 
+/// Emit a fatal error if the broken dow registers don't match part type
+// or count.
+static void ensureMatchedVecRegParts(unsigned NumRegs, unsigned NumParts,
+                                     MVT RegisterVT, MVT PartVT) {
+  if (NumRegs != NumParts || RegisterVT != PartVT)
+    report_fatal_error(Twine("Part count doesn't match vector breakdown! ")
+                           .concat(Twine(NumRegs))
+                           .concat(" registers, ")
+                           .concat(Twine(NumParts))
+                           .concat(" parts, ")
+                           .concat(Twine(RegisterVT.getString()))
+                           .concat(" RegisterVT, ")
+                           .concat(Twine(PartVT.getString()))
+                           .concat(" PartVT"));
+}
 /// getCopyFromPartsVector - Create a value that contains the specified legal
 /// parts combined into the value they represent.  If the parts combine to a
 /// type larger than ValueVT then AssertOp can be used to specify whether the
@@ -364,9 +380,7 @@ static SDValue getCopyFromPartsVector(SelectionDAG &DAG, const SDLoc &DL,
                                      NumIntermediates, RegisterVT);
     }
 
-    assert(NumRegs == NumParts && "Part count doesn't match vector breakdown!");
-    NumParts = NumRegs; // Silence a compiler warning.
-    assert(RegisterVT == PartVT && "Part type doesn't match vector breakdown!");
+    ensureMatchedVecRegParts(NumRegs, NumParts, RegisterVT, PartVT);
     assert(RegisterVT.getSizeInBits() ==
            Parts[0].getSimpleValueType().getSizeInBits() &&
            "Part type sizes don't match!");
@@ -769,10 +783,7 @@ static void getCopyToPartsVector(SelectionDAG &DAG, const SDLoc &DL,
                                    NumIntermediates, RegisterVT);
   }
 
-  assert(NumRegs == NumParts && "Part count doesn't match vector breakdown!");
-  NumParts = NumRegs; // Silence a compiler warning.
-  assert(RegisterVT == PartVT && "Part type doesn't match vector breakdown!");
-
+  ensureMatchedVecRegParts(NumRegs, NumParts, RegisterVT, PartVT);
   assert(IntermediateVT.isScalableVector() == ValueVT.isScalableVector() &&
          "Mixing scalable and fixed vectors when copying in parts");
 
diff --git a/llvm/lib/CodeGen/ValueTypes.cpp b/llvm/lib/CodeGen/ValueTypes.cpp
index 0743c92c5b95b..f3e4eaa2ef00f 100644
--- a/llvm/lib/CodeGen/ValueTypes.cpp
+++ b/llvm/lib/CodeGen/ValueTypes.cpp
@@ -333,6 +333,13 @@ const fltSemantics &EVT::getFltSemantics() const {
   return getScalarType().getSimpleVT().getFltSemantics();
 }
 
+std::string MVT::getString() const {
+  if (SimpleTy == INVALID_SIMPLE_VALUE_TYPE)
+    return "invalid";
+
+  return EVT(*this).getEVTString();
+}
+
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 void MVT::dump() const {
   print(dbgs());
@@ -340,9 +347,4 @@ void MVT::dump() const {
 }
 #endif
 
-void MVT::print(raw_ostream &OS) const {
-  if (SimpleTy == INVALID_SIMPLE_VALUE_TYPE)
-    OS << "invalid";
-  else
-    OS << EVT(*this).getEVTString();
-}
+void MVT::print(raw_ostream &OS) const { OS << getString(); }