DAGCombiner: Turn truncate of a bitcasted vector to an extract

On AMDGPU where operations i64 operations are often bitcasted to v2i32
and back, this pattern shows up regularly where it breaks some
expected combines on i64, such as load width reducing.

This fixes some test failures in a future commit when i64 loads
are changed to promote.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262397 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -7176,6 +7176,22 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
     }
   }
 
+  // Fold truncate of a bitcast of a vector to an extract of the low vector
+  // element.
+  //
+  // e.g. trunc (i64 (bitcast v2i32:x)) -> extract_vector_elt v2i32:x, 0
+  if (N0.getOpcode() == ISD::BITCAST && !VT.isVector()) {
+    SDValue VecSrc = N0.getOperand(0);
+    EVT SrcVT = VecSrc.getValueType();
+    if (SrcVT.isVector() && SrcVT.getScalarType() == VT) {
+      SDLoc SL(N);
+
+      EVT IdxVT = TLI.getVectorIdxTy(DAG.getDataLayout());
+      return DAG.getNode(ISD::EXTRACT_VECTOR_ELT, SL, VT,
+                         VecSrc, DAG.getConstant(0, SL, IdxVT));
+    }
+  }
+
   // Simplify the operands using demanded-bits information.
   if (!VT.isVector() &&
       SimplifyDemandedBits(SDValue(N, 0)))

test/CodeGen/AMDGPU/half.ll

@@ -396,12 +396,11 @@ define void @global_extload_v2f16_to_v2f64(<2 x double> addrspace(1)* %out, <2 x
 ; GCN-LABEL: {{^}}global_extload_v3f16_to_v3f64:
 
 ; GCN: buffer_load_dwordx2 [[LOAD:v\[[0-9]+:[0-9]+\]]]
-; GCN: v_lshrrev_b32_e32 {{v[0-9]+}}, 16, {{v[0-9]+}}
-
 ; GCN: v_cvt_f32_f16_e32
 ; GCN: v_cvt_f32_f16_e32
+; GCN-DAG: v_lshrrev_b32_e32 {{v[0-9]+}}, 16, {{v[0-9]+}}
 ; GCN: v_cvt_f32_f16_e32
-; GCN-NOT: v_cvt_f32_f16_e32
+; GCN-NOT: v_cvt_f32_f16
 
 ; GCN: v_cvt_f64_f32_e32
 ; GCN: v_cvt_f64_f32_e32

test/CodeGen/AMDGPU/trunc-bitcast-vector.ll

@@ -0,0 +1,93 @@
+; RUN: llc -march=amdgcn -verify-machineinstrs < %s | FileCheck %s
+; RUN: llc -march=amdgcn -mcpu=tonga -verify-machineinstrs < %s | FileCheck %s
+
+; CHECK-LABEL: {{^}}trunc_i64_bitcast_v2i32:
+; CHECK: buffer_load_dword v
+; CHECK: buffer_store_dword v
+define void @trunc_i64_bitcast_v2i32(i32 addrspace(1)* %out, <2 x i32> addrspace(1)* %in) {
+  %ld = load <2 x i32>, <2 x i32> addrspace(1)* %in
+  %bc = bitcast <2 x i32> %ld to i64
+  %trunc = trunc i64 %bc to i32
+  store i32 %trunc, i32 addrspace(1)* %out
+  ret void
+}
+
+; CHECK-LABEL: {{^}}trunc_i96_bitcast_v3i32:
+; CHECK: buffer_load_dword v
+; CHECK: buffer_store_dword v
+define void @trunc_i96_bitcast_v3i32(i32 addrspace(1)* %out, <3 x i32> addrspace(1)* %in) {
+  %ld = load <3 x i32>, <3 x i32> addrspace(1)* %in
+  %bc = bitcast <3 x i32> %ld to i96
+  %trunc = trunc i96 %bc to i32
+  store i32 %trunc, i32 addrspace(1)* %out
+  ret void
+}
+
+; CHECK-LABEL: {{^}}trunc_i128_bitcast_v4i32:
+; CHECK: buffer_load_dword v
+; CHECK: buffer_store_dword v
+define void @trunc_i128_bitcast_v4i32(i32 addrspace(1)* %out, <4 x i32> addrspace(1)* %in) {
+  %ld = load <4 x i32>, <4 x i32> addrspace(1)* %in
+  %bc = bitcast <4 x i32> %ld to i128
+  %trunc = trunc i128 %bc to i32
+  store i32 %trunc, i32 addrspace(1)* %out
+  ret void
+}
+
+; Don't want load width reduced in this case.
+; CHECK-LABEL: {{^}}trunc_i16_bitcast_v2i16:
+; CHECK: buffer_load_dword [[VAL:v[0-9]+]]
+; CHECK: buffer_store_short [[VAL]]
+define void @trunc_i16_bitcast_v2i16(i16 addrspace(1)* %out, <2 x i16> addrspace(1)* %in) {
+  %ld = load <2 x i16>, <2 x i16> addrspace(1)* %in
+  %bc = bitcast <2 x i16> %ld to i32
+  %trunc = trunc i32 %bc to i16
+  store i16 %trunc, i16 addrspace(1)* %out
+  ret void
+}
+
+; FIXME: Don't want load width reduced here.
+; CHECK-LABEL: {{^}}trunc_i16_bitcast_v4i16:
+; CHECK: buffer_load_ushort [[VAL:v[0-9]+]]
+; CHECK: buffer_store_short [[VAL]]
+define void @trunc_i16_bitcast_v4i16(i16 addrspace(1)* %out, <4 x i16> addrspace(1)* %in) {
+  %ld = load <4 x i16>, <4 x i16> addrspace(1)* %in
+  %bc = bitcast <4 x i16> %ld to i64
+  %trunc = trunc i64 %bc to i16
+  store i16 %trunc, i16 addrspace(1)* %out
+  ret void
+}
+
+; FIXME: Don't want load width reduced in this case.
+; CHECK-LABEL: {{^}}trunc_i8_bitcast_v2i8:
+; CHECK: buffer_load_ubyte [[VAL:v[0-9]+]]
+; CHECK: buffer_store_byte [[VAL]]
+define void @trunc_i8_bitcast_v2i8(i8 addrspace(1)* %out, <2 x i8> addrspace(1)* %in) {
+  %ld = load <2 x i8>, <2 x i8> addrspace(1)* %in
+  %bc = bitcast <2 x i8> %ld to i16
+  %trunc = trunc i16 %bc to i8
+  store i8 %trunc, i8 addrspace(1)* %out
+  ret void
+}
+
+; CHECK-LABEL: {{^}}trunc_i32_bitcast_v4i8:
+; CHECK: buffer_load_dword [[VAL:v[0-9]+]]
+; CHECK: buffer_store_byte [[VAL]]
+define void @trunc_i32_bitcast_v4i8(i8 addrspace(1)* %out, <4 x i8> addrspace(1)* %in) {
+  %ld = load <4 x i8>, <4 x i8> addrspace(1)* %in
+  %bc = bitcast <4 x i8> %ld to i32
+  %trunc = trunc i32 %bc to i8
+  store i8 %trunc, i8 addrspace(1)* %out
+  ret void
+}
+
+; CHECK-LABEL: {{^}}trunc_i24_bitcast_v3i8:
+; CHECK: buffer_load_dword [[VAL:v[0-9]+]]
+; CHECK: buffer_store_byte [[VAL]]
+define void @trunc_i24_bitcast_v3i8(i8 addrspace(1)* %out, <3 x i8> addrspace(1)* %in) {
+  %ld = load <3 x i8>, <3 x i8> addrspace(1)* %in
+  %bc = bitcast <3 x i8> %ld to i24
+  %trunc = trunc i24 %bc to i8
+  store i8 %trunc, i8 addrspace(1)* %out
+  ret void
+}