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[AArch64] Avoid partial register deps on insertelt of load into lane 0.

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This improves upon r246462: that prevented FMOVs from being emitted
for the cross-class INSERT_SUBREGs by disabling the formation of
INSERT_SUBREGs of LOAD.  But the ld1.s that we started selecting
caused us to introduce partial dependencies on the vector register.

Avoid that by using SCALAR_TO_VECTOR: it's a first-class citizen that
is folded away by many patterns, including the scalar LDRS that we
want in this case.

Credit goes to Adam for finding the issue!

llvm-svn: 299482
This commit is contained in:
Ahmed Bougacha 2017-04-04 22:55:53 +00:00
parent bbceead2ad
commit 4b7c6d9d15
2 changed files with 10 additions and 16 deletions
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16
lib/Target/AArch64/AArch64ISelLowering.cpp
View File

@ -6587,19 +6587,13 @@ FailedModImm:
SDValue Op0 = Op.getOperand(0);
unsigned ElemSize = VT.getScalarSizeInBits();
unsigned i = 0;
// For 32 and 64 bit types, use INSERT_SUBREG for lane zero to
// For 32 and 64 bit types, use SCALAR_TO_VECTOR for lane zero to
// a) Avoid a RMW dependency on the full vector register, and
// b) Allow the register coalescer to fold away the copy if the
// value is already in an S or D register.
// Do not do this for UNDEF/LOAD nodes because we have better patterns
// for those avoiding the SCALAR_TO_VECTOR/BUILD_VECTOR.
if (!Op0.isUndef() && Op0.getOpcode() != ISD::LOAD &&
(ElemSize == 32 || ElemSize == 64)) {
unsigned SubIdx = ElemSize == 32 ? AArch64::ssub : AArch64::dsub;
MachineSDNode *N =
DAG.getMachineNode(TargetOpcode::INSERT_SUBREG, dl, VT, Vec, Op0,
DAG.getTargetConstant(SubIdx, dl, MVT::i32));
Vec = SDValue(N, 0);
// value is already in an S or D register, and we're forced to emit an
// INSERT_SUBREG that we can't fold anywhere.
if (!Op0.isUndef() && (ElemSize == 32 || ElemSize == 64)) {
Vec = DAG.getNode(ISD::SCALAR_TO_VECTOR, dl, VT, Op0);
++i;
}
for (; i < NumElts; ++i) {

10
test/CodeGen/AArch64/arm64-indexed-vector-ldst.ll
View File

@ -6216,11 +6216,11 @@ define <4 x i16> @test_v4i16_post_reg_ld1lane_forced_narrow(i16* %bar, i16** %pt
declare <2 x i32> @llvm.ctpop.v2i32(<2 x i32>)
; CHECK-LABEL: test_ld1lane_build:
; CHECK-DAG: ld1.s { [[REG0:v[0-9]+]] }[0], [x0]
; CHECK-DAG: ld1.s { [[REG0:v[0-9]+]] }[1], [x1]
; CHECK-DAG: ld1.s { [[REG1:v[0-9]+]] }[0], [x2]
; CHECK-DAG: ld1.s { [[REG1:v[0-9]+]] }[1], [x3]
; CHECK: sub.2s v[[REGNUM2:[0-9]+]], [[REG0]], [[REG1]]
; CHECK-DAG: ldr s[[REGNUM0:[0-9]+]], [x0]
; CHECK-DAG: ld1.s { v[[REGNUM0:[0-9]+]] }[1], [x1]
; CHECK-DAG: ldr s[[REGNUM1:[0-9]+]], [x2]
; CHECK-DAG: ld1.s { v[[REGNUM1:[0-9]+]] }[1], [x3]
; CHECK: sub.2s v[[REGNUM2:[0-9]+]], v[[REGNUM0]], v[[REGNUM1]]
; CHECK-NEXT: str d[[REGNUM2]], [x4]
; CHECK-NEXT: ret
define void @test_ld1lane_build(i32* %ptr0, i32* %ptr1, i32* %ptr2, i32* %ptr3, <2 x i32>* %out) {