From 2deae0baf7c01bb87e4271dbe06537f585cf8545 Mon Sep 17 00:00:00 2001
From: ChaseYalon <149000633+ChaseYalon@users.noreply.github.com>
Date: Sat, 24 Jan 2026 10:13:45 -0500
Subject: [PATCH] X86: Fix VSCALE insert element crash in codegen (#177705)

When inserting elements into <vscale x N x i64> scalable vectors, the X86 backend now returns InstructionCost::getInvalid() instead of panicking.

Fixes #176823
---
 llvm/lib/Target/X86/X86TargetTransformInfo.cpp         |  4 ++++
 .../CostModel/X86/vscale-insertelement-crash.ll        | 10 ++++++++++
 2 files changed, 14 insertions(+)
 create mode 100644 llvm/test/Analysis/CostModel/X86/vscale-insertelement-crash.ll

diff --git a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
index 608727b74592..2a5e761a92aa 100644
--- a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
+++ b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
@@ -4815,6 +4815,10 @@ InstructionCost X86TTIImpl::getVectorInstrCost(unsigned Opcode, Type *Val,
    };
 
   assert(Val->isVectorTy() && "This must be a vector type");
+  auto *VT = cast<VectorType>(Val);
+  if (VT->isScalableTy())
+    return InstructionCost::getInvalid();
+
   Type *ScalarType = Val->getScalarType();
   InstructionCost RegisterFileMoveCost = 0;
 
diff --git a/llvm/test/Analysis/CostModel/X86/vscale-insertelement-crash.ll b/llvm/test/Analysis/CostModel/X86/vscale-insertelement-crash.ll
new file mode 100644
index 000000000000..77e68c582269
--- /dev/null
+++ b/llvm/test/Analysis/CostModel/X86/vscale-insertelement-crash.ll
@@ -0,0 +1,10 @@
+; RUN: opt -passes=print<cost-model> -disable-output < %s
+; This test triggers a crash in X86 TTI with scalable vectors
+
+target triple = "x86_64-unknown-linux-gnu"
+
+define <vscale x 1 x i64> @test(i64 %x) {
+entry:
+  %v = insertelement <vscale x 1 x i64> poison, i64 %x, i64 0
+  ret <vscale x 1 x i64> %v
+}