[DirectX backend] change MinVectorRegisterBitWidth to 32.

This is to avoid vector-combine generate vector4 on float.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D132826

Author: python3kgae

llvm/lib/Target/DirectX/DirectXTargetTransformInfo.h

@@ -33,6 +33,7 @@ class DirectXTTIImpl : public BasicTTIImplBase<DirectXTTIImpl> {
   explicit DirectXTTIImpl(const DirectXTargetMachine *TM, const Function &F)
       : BaseT(TM, F.getParent()->getDataLayout()), ST(TM->getSubtargetImpl(F)),
         TLI(ST->getTargetLowering()) {}
+  unsigned getMinVectorRegisterBitWidth() const { return 32; }
 };
 } // namespace llvm
 

llvm/test/CodeGen/DirectX/min_vec_size.ll

@@ -0,0 +1,21 @@
+; RUN: opt -S -vector-combine < %s | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i1:32-i8:8-i16:16-i32:32-i64:64-f16:16-f32:32-f64:64-n8:16:32:64"
+target triple = "dxil-unknown-shadermodel6.7-library"
+
+; Make sure vec combine min vec size is 1 instead of 4 for float.
+; CHECK:@foo()
+; CHECK-NEXT:%[[LD:[0-9]+]] = load <1 x float>, ptr @a, align 8
+; CHECK-NEXT:%insert = shufflevector <1 x float> %[[LD]], <1 x float> poison, <2 x i32> <i32 0, i32 undef>
+; CHECK-NEXT:%shuffle = shufflevector <2 x float> %insert, <2 x float> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:ret <2 x float> %shuffle
+
+@a = external local_unnamed_addr constant float
+
+; Function Attrs: mustprogress nofree norecurse nosync nounwind readnone willreturn
+define noundef <2 x float> @foo() local_unnamed_addr {
+  %1 = load float, ptr @a, align 8
+  %insert = insertelement <2 x float> poison, float %1, i64 0
+  %shuffle = shufflevector <2 x float> %insert, <2 x float> poison, <2 x i32> zeroinitializer
+  ret <2 x float> %shuffle
+}