[llvm-branch-commits] [llvm] release/23.x: [Mips] Legalize vector UNDEF instead of expanding to zero BUILD_VECTOR (#211503) (PR #217519)

[Douglas Yung via llvm-branch-commits]

https://github.com/dyung updated https://github.com/llvm/llvm-project/pull/217519

>From 692fdd97958be7e6aaf19479dea1df479de72a89 Mon Sep 17 00:00:00 2001
From: yingopq <115543042+yingopq at users.noreply.github.com>
Date: Thu, 20 Aug 2026 10:59:05 +0800
Subject: [PATCH] [Mips] Legalize vector UNDEF instead of expanding to zero
 BUILD_VECTOR (#211503)

Currently, MIPS MSA expands ISD::UNDEF into a BUILD_VECTOR of all zeros
during legalization. This creates an infinite loop in DAGCombiner when
the following occurs:
1.Mips lower BUILD_VECTOR expands non-splat vectors into
INSERT_VECTOR_ELT with creating undef node
2.Then legalization expands UNDEF back to BUILD_VECTOR zero
3.Mips lower BUILD_VECTOR converts zero vector to BITCAST
4.DAGCombiner optimizes BITCAST(zero) to UNDEF
5.Back to step 2, infinite loop

Fix #210229.

(cherry picked from commit b1c24ea77d2aab84f9e8ccb9389bc4366c41e8dd)
---
 llvm/lib/Target/Mips/MipsSEISelLowering.cpp   |  1 +
 .../Mips/msa/buildvector-undef-loop.ll        | 37 +++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 llvm/test/CodeGen/Mips/msa/buildvector-undef-loop.ll

diff --git a/llvm/lib/Target/Mips/MipsSEISelLowering.cpp b/llvm/lib/Target/Mips/MipsSEISelLowering.cpp
index d5941fc8e0b33..628e8d4359f7c 100644
--- a/llvm/lib/Target/Mips/MipsSEISelLowering.cpp
+++ b/llvm/lib/Target/Mips/MipsSEISelLowering.cpp
@@ -447,6 +447,7 @@ addMSAFloatType(MVT::SimpleValueType Ty, const TargetRegisterClass *RC) {
   setOperationAction(ISD::EXTRACT_VECTOR_ELT, Ty, Legal);
   setOperationAction(ISD::INSERT_VECTOR_ELT, Ty, Legal);
   setOperationAction(ISD::BUILD_VECTOR, Ty, Custom);
+  setOperationAction(ISD::UNDEF, Ty, Legal);
 
   if (Ty != MVT::v8f16) {
     setOperationAction(ISD::FABS,  Ty, Legal);
diff --git a/llvm/test/CodeGen/Mips/msa/buildvector-undef-loop.ll b/llvm/test/CodeGen/Mips/msa/buildvector-undef-loop.ll
new file mode 100644
index 0000000000000..80fa4049d0f10
--- /dev/null
+++ b/llvm/test/CodeGen/Mips/msa/buildvector-undef-loop.ll
@@ -0,0 +1,37 @@
+; RUN: llc -mtriple=mips64el-linux-gnu64abi -mcpu=mips64r5 -mattr=+msa < %s \
+; RUN:   | FileCheck %s --check-prefixes=MIPS64R5
+
+ at v4f32 = global <4 x float> <float 0.0, float 0.0, float 0.0, float 0.0>
+
+define void @nonsplatvalue_v4f32(float %a, float %b, float %c, float %d) nounwind {
+; MIPS64R5-LABEL: nonsplatvalue_v4f32:
+; MIPS64R5:       # %bb.0:
+; MIPS64R5-NEXT:    # kill: def $f15 killed $f15 def $w15
+; MIPS64R5-NEXT:    # kill: def $f14 killed $f14 def $w14
+; MIPS64R5-NEXT:    # kill: def $f13 killed $f13 def $w13
+; MIPS64R5-NEXT:    # kill: def $f12 killed $f12 def $w12
+; MIPS64R5-NEXT:    insve.w $w0[0], $w12[0]
+; MIPS64R5-NEXT:    insve.w $w0[1], $w13[0]
+; MIPS64R5-NEXT:    insve.w $w0[2], $w14[0]
+; MIPS64R5-NEXT:    insve.w $w0[3], $w15[0]
+; MIPS64R5-NEXT:    fmax_a.w $w0, $w0, $w0
+; MIPS64R5-NEXT:    lui $1, %highest(v4f32)
+; MIPS64R5-NEXT:    daddiu $1, $1, %higher(v4f32)
+; MIPS64R5-NEXT:    dsll $1, $1, 16
+; MIPS64R5-NEXT:    daddiu $1, $1, %hi(v4f32)
+; MIPS64R5-NEXT:    dsll $1, $1, 16
+; MIPS64R5-NEXT:    daddiu $1, $1, %lo(v4f32)
+; MIPS64R5-NEXT:    jr $ra
+; MIPS64R5-NEXT:    st.w $w0, 0($1)
+  %v0 = insertelement <4 x float> poison, float %a, i64 0
+  %v1 = insertelement <4 x float> %v0, float %b, i32 1
+  %v2 = insertelement <4 x float> %v1, float %c, i32 2
+  %v3 = insertelement <4 x float> %v2, float %d, i32 3
+
+  %fabs = call <4 x float> @llvm.fabs.v4f32(<4 x float> %v3)
+  store volatile <4 x float> %fabs, ptr @v4f32
+
+  ret void
+}
+
+declare <4 x float> @llvm.fabs.v4f32(<4 x float>)