[SVE][CodeGenPrepare] Sink address calculations that match SVE gather/scatter addressing modes. #66996
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@llvm/pr-subscribers-backend-aarch64 @llvm/pr-subscribers-llvm-transforms ChangesSVE supports scalar+vector and scalar+extw(vector) addressing modes. This is problematic because it forces the code generator to legalise This patch sinks GEPs and extends for cases where one of the above NOTE: There are cases where it would be better to split the extend Full diff: https://github.com/llvm/llvm-project/pull/66996.diff 2 Files Affected:
diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index ad01a206c93fb39..8bbd5ab1818590d 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -14380,6 +14380,31 @@ static bool areOperandsOfVmullHighP64(Value *Op1, Value *Op2) {
return isOperandOfVmullHighP64(Op1) && isOperandOfVmullHighP64(Op2);
}
+static bool shouldSinkVectorOfPtrs(Value *Ptrs, SmallVectorImpl<Use *> &Ops) {
+ // Restrict ourselves to the form CodeGenPrepare typically constructs.
+ auto *GEP = dyn_cast<GetElementPtrInst>(Ptrs);
+ if (!GEP || GEP->getNumOperands() != 2)
+ return false;
+
+ Value *Base = GEP->getOperand(0);
+ Value *Offsets = GEP->getOperand(1);
+
+ // We only care about scalar_base+vector_offsets.
+ if (Base->getType()->isVectorTy() || !Offsets->getType()->isVectorTy())
+ return false;
+
+ // Sink extends that would allow us to use 32-bit offset vectors.
+ if (isa<SExtInst>(Offsets) || isa<ZExtInst>(Offsets)) {
+ auto *OffsetsInst = cast<Instruction>(Offsets);
+ if (OffsetsInst->getType()->getScalarSizeInBits() > 32 &&
+ OffsetsInst->getOperand(0)->getType()->getScalarSizeInBits() <= 32)
+ Ops.push_back(&GEP->getOperandUse(1));
+ }
+
+ // Sink the GEP.
+ return true;
+}
+
/// Check if sinking \p I's operands to I's basic block is profitable, because
/// the operands can be folded into a target instruction, e.g.
/// shufflevectors extracts and/or sext/zext can be folded into (u,s)subl(2).
@@ -14481,6 +14506,16 @@ bool AArch64TargetLowering::shouldSinkOperands(
Ops.push_back(&II->getArgOperandUse(0));
Ops.push_back(&II->getArgOperandUse(1));
return true;
+ case Intrinsic::masked_gather:
+ if (!shouldSinkVectorOfPtrs(II->getArgOperand(0), Ops))
+ return false;
+ Ops.push_back(&II->getArgOperandUse(0));
+ return true;
+ case Intrinsic::masked_scatter:
+ if (!shouldSinkVectorOfPtrs(II->getArgOperand(1), Ops))
+ return false;
+ Ops.push_back(&II->getArgOperandUse(1));
+ return true;
default:
return false;
}
diff --git a/llvm/test/Transforms/CodeGenPrepare/AArch64/sink-gather-scatter-addressing.ll b/llvm/test/Transforms/CodeGenPrepare/AArch64/sink-gather-scatter-addressing.ll
new file mode 100644
index 000000000000000..73322836d1b84a7
--- /dev/null
+++ b/llvm/test/Transforms/CodeGenPrepare/AArch64/sink-gather-scatter-addressing.ll
@@ -0,0 +1,231 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
+; RUN: opt -S --codegenprepare < %s | FileCheck %s
+
+target triple = "aarch64-unknown-linux-gnu"
+
+; Sink the GEP to make use of scalar+vector addressing modes.
+define <vscale x 4 x float> @gather_offsets_sink_gep(ptr %base, <vscale x 4 x i32> %indices, <vscale x 4 x i1> %mask, i1 %cond) {
+; CHECK-LABEL: define <vscale x 4 x float> @gather_offsets_sink_gep(
+; CHECK-SAME: ptr [[BASE:%.*]], <vscale x 4 x i32> [[INDICES:%.*]], <vscale x 4 x i1> [[MASK:%.*]], i1 [[COND:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 [[COND]], label [[COND_BLOCK:%.*]], label [[EXIT:%.*]]
+; CHECK: cond.block:
+; CHECK-NEXT: [[TMP0:%.*]] = getelementptr float, ptr [[BASE]], <vscale x 4 x i32> [[INDICES]]
+; CHECK-NEXT: [[LOAD:%.*]] = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32.nxv4p0(<vscale x 4 x ptr> [[TMP0]], i32 4, <vscale x 4 x i1> [[MASK]], <vscale x 4 x float> poison)
+; CHECK-NEXT: ret <vscale x 4 x float> [[LOAD]]
+; CHECK: exit:
+; CHECK-NEXT: ret <vscale x 4 x float> zeroinitializer
+;
+entry:
+ %ptrs = getelementptr float, ptr %base, <vscale x 4 x i32> %indices
+ br i1 %cond, label %cond.block, label %exit
+
+cond.block:
+ %load = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32(<vscale x 4 x ptr> %ptrs, i32 4, <vscale x 4 x i1> %mask, <vscale x 4 x float> poison)
+ br label %exit
+
+exit:
+ %ret = phi <vscale x 4 x float> [ zeroinitializer, %entry ], [ %load, %cond.block ]
+ ret <vscale x 4 x float> %ret
+}
+
+; Sink sext to make use of scalar+sxtw(vector) addressing modes.
+define <vscale x 4 x float> @gather_offsets_sink_sext(ptr %base, <vscale x 4 x i32> %indices, <vscale x 4 x i1> %mask, i1 %cond) {
+; CHECK-LABEL: define <vscale x 4 x float> @gather_offsets_sink_sext(
+; CHECK-SAME: ptr [[BASE:%.*]], <vscale x 4 x i32> [[INDICES:%.*]], <vscale x 4 x i1> [[MASK:%.*]], i1 [[COND:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 [[COND]], label [[COND_BLOCK:%.*]], label [[EXIT:%.*]]
+; CHECK: cond.block:
+; CHECK-NEXT: [[TMP0:%.*]] = sext <vscale x 4 x i32> [[INDICES]] to <vscale x 4 x i64>
+; CHECK-NEXT: [[PTRS:%.*]] = getelementptr float, ptr [[BASE]], <vscale x 4 x i64> [[TMP0]]
+; CHECK-NEXT: [[LOAD:%.*]] = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32.nxv4p0(<vscale x 4 x ptr> [[PTRS]], i32 4, <vscale x 4 x i1> [[MASK]], <vscale x 4 x float> poison)
+; CHECK-NEXT: ret <vscale x 4 x float> [[LOAD]]
+; CHECK: exit:
+; CHECK-NEXT: ret <vscale x 4 x float> zeroinitializer
+;
+entry:
+ %indices.sext = sext <vscale x 4 x i32> %indices to <vscale x 4 x i64>
+ br i1 %cond, label %cond.block, label %exit
+
+cond.block:
+ %ptrs = getelementptr float, ptr %base, <vscale x 4 x i64> %indices.sext
+ %load = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32(<vscale x 4 x ptr> %ptrs, i32 4, <vscale x 4 x i1> %mask, <vscale x 4 x float> poison)
+ br label %exit
+
+exit:
+ %ret = phi <vscale x 4 x float> [ zeroinitializer, %entry ], [ %load, %cond.block ]
+ ret <vscale x 4 x float> %ret
+}
+
+; As above but ensure both the GEP and sext is sunk.
+define <vscale x 4 x float> @gather_offsets_sink_sext_get(ptr %base, <vscale x 4 x i32> %indices, <vscale x 4 x i1> %mask, i1 %cond) {
+; CHECK-LABEL: define <vscale x 4 x float> @gather_offsets_sink_sext_get(
+; CHECK-SAME: ptr [[BASE:%.*]], <vscale x 4 x i32> [[INDICES:%.*]], <vscale x 4 x i1> [[MASK:%.*]], i1 [[COND:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 [[COND]], label [[COND_BLOCK:%.*]], label [[EXIT:%.*]]
+; CHECK: cond.block:
+; CHECK-NEXT: [[TMP0:%.*]] = sext <vscale x 4 x i32> [[INDICES]] to <vscale x 4 x i64>
+; CHECK-NEXT: [[TMP1:%.*]] = getelementptr float, ptr [[BASE]], <vscale x 4 x i64> [[TMP0]]
+; CHECK-NEXT: [[LOAD:%.*]] = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32.nxv4p0(<vscale x 4 x ptr> [[TMP1]], i32 4, <vscale x 4 x i1> [[MASK]], <vscale x 4 x float> poison)
+; CHECK-NEXT: ret <vscale x 4 x float> [[LOAD]]
+; CHECK: exit:
+; CHECK-NEXT: ret <vscale x 4 x float> zeroinitializer
+;
+entry:
+ %indices.sext = sext <vscale x 4 x i32> %indices to <vscale x 4 x i64>
+ %ptrs = getelementptr float, ptr %base, <vscale x 4 x i64> %indices.sext
+ br i1 %cond, label %cond.block, label %exit
+
+cond.block:
+ %load = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32(<vscale x 4 x ptr> %ptrs, i32 4, <vscale x 4 x i1> %mask, <vscale x 4 x float> poison)
+ br label %exit
+
+exit:
+ %ret = phi <vscale x 4 x float> [ zeroinitializer, %entry ], [ %load, %cond.block ]
+ ret <vscale x 4 x float> %ret
+}
+
+; Don't sink GEPs that cannot benefit from SVE's scalar+vector addressing modes.
+define <vscale x 4 x float> @gather_no_scalar_base(<vscale x 4 x ptr> %bases, <vscale x 4 x i32> %indices, <vscale x 4 x i1> %mask, i1 %cond) {
+; CHECK-LABEL: define <vscale x 4 x float> @gather_no_scalar_base(
+; CHECK-SAME: <vscale x 4 x ptr> [[BASES:%.*]], <vscale x 4 x i32> [[INDICES:%.*]], <vscale x 4 x i1> [[MASK:%.*]], i1 [[COND:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: [[PTRS:%.*]] = getelementptr float, <vscale x 4 x ptr> [[BASES]], <vscale x 4 x i32> [[INDICES]]
+; CHECK-NEXT: br i1 [[COND]], label [[COND_BLOCK:%.*]], label [[EXIT:%.*]]
+; CHECK: cond.block:
+; CHECK-NEXT: [[LOAD:%.*]] = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32.nxv4p0(<vscale x 4 x ptr> [[PTRS]], i32 4, <vscale x 4 x i1> [[MASK]], <vscale x 4 x float> poison)
+; CHECK-NEXT: ret <vscale x 4 x float> [[LOAD]]
+; CHECK: exit:
+; CHECK-NEXT: ret <vscale x 4 x float> zeroinitializer
+;
+entry:
+ %ptrs = getelementptr float, <vscale x 4 x ptr> %bases, <vscale x 4 x i32> %indices
+ br i1 %cond, label %cond.block, label %exit
+
+cond.block:
+ %load = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32(<vscale x 4 x ptr> %ptrs, i32 4, <vscale x 4 x i1> %mask, <vscale x 4 x float> poison)
+ br label %exit
+
+exit:
+ %ret = phi <vscale x 4 x float> [ zeroinitializer, %entry ], [ %load, %cond.block ]
+ ret <vscale x 4 x float> %ret
+}
+
+; Don't sink extends whose result type is already favourable for SVE's sxtw/uxtw addressing modes.
+; NOTE: We still want to sink the GEP.
+define <vscale x 4 x float> @gather_offset_type_too_small(ptr %base, <vscale x 4 x i8> %indices, <vscale x 4 x i1> %mask, i1 %cond) {
+; CHECK-LABEL: define <vscale x 4 x float> @gather_offset_type_too_small(
+; CHECK-SAME: ptr [[BASE:%.*]], <vscale x 4 x i8> [[INDICES:%.*]], <vscale x 4 x i1> [[MASK:%.*]], i1 [[COND:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: [[INDICES_SEXT:%.*]] = sext <vscale x 4 x i8> [[INDICES]] to <vscale x 4 x i32>
+; CHECK-NEXT: br i1 [[COND]], label [[COND_BLOCK:%.*]], label [[EXIT:%.*]]
+; CHECK: cond.block:
+; CHECK-NEXT: [[TMP0:%.*]] = getelementptr float, ptr [[BASE]], <vscale x 4 x i32> [[INDICES_SEXT]]
+; CHECK-NEXT: [[LOAD:%.*]] = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32.nxv4p0(<vscale x 4 x ptr> [[TMP0]], i32 4, <vscale x 4 x i1> [[MASK]], <vscale x 4 x float> poison)
+; CHECK-NEXT: ret <vscale x 4 x float> [[LOAD]]
+; CHECK: exit:
+; CHECK-NEXT: ret <vscale x 4 x float> zeroinitializer
+;
+entry:
+ %indices.sext = sext <vscale x 4 x i8> %indices to <vscale x 4 x i32>
+ %ptrs = getelementptr float, ptr %base, <vscale x 4 x i32> %indices.sext
+ br i1 %cond, label %cond.block, label %exit
+
+cond.block:
+ %load = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32(<vscale x 4 x ptr> %ptrs, i32 4, <vscale x 4 x i1> %mask, <vscale x 4 x float> poison)
+ br label %exit
+
+exit:
+ %ret = phi <vscale x 4 x float> [ zeroinitializer, %entry ], [ %load, %cond.block ]
+ ret <vscale x 4 x float> %ret
+}
+
+; Don't sink extends that cannot benefit from SVE's sxtw/uxtw addressing modes.
+; NOTE: We still want to sink the GEP.
+define <vscale x 4 x float> @gather_offset_type_too_big(ptr %base, <vscale x 4 x i48> %indices, <vscale x 4 x i1> %mask, i1 %cond) {
+; CHECK-LABEL: define <vscale x 4 x float> @gather_offset_type_too_big(
+; CHECK-SAME: ptr [[BASE:%.*]], <vscale x 4 x i48> [[INDICES:%.*]], <vscale x 4 x i1> [[MASK:%.*]], i1 [[COND:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: [[INDICES_SEXT:%.*]] = sext <vscale x 4 x i48> [[INDICES]] to <vscale x 4 x i64>
+; CHECK-NEXT: br i1 [[COND]], label [[COND_BLOCK:%.*]], label [[EXIT:%.*]]
+; CHECK: cond.block:
+; CHECK-NEXT: [[TMP0:%.*]] = getelementptr float, ptr [[BASE]], <vscale x 4 x i64> [[INDICES_SEXT]]
+; CHECK-NEXT: [[LOAD:%.*]] = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32.nxv4p0(<vscale x 4 x ptr> [[TMP0]], i32 4, <vscale x 4 x i1> [[MASK]], <vscale x 4 x float> poison)
+; CHECK-NEXT: ret <vscale x 4 x float> [[LOAD]]
+; CHECK: exit:
+; CHECK-NEXT: ret <vscale x 4 x float> zeroinitializer
+;
+entry:
+ %indices.sext = sext <vscale x 4 x i48> %indices to <vscale x 4 x i64>
+ %ptrs = getelementptr float, ptr %base, <vscale x 4 x i64> %indices.sext
+ br i1 %cond, label %cond.block, label %exit
+
+cond.block:
+ %load = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32(<vscale x 4 x ptr> %ptrs, i32 4, <vscale x 4 x i1> %mask, <vscale x 4 x float> poison)
+ br label %exit
+
+exit:
+ %ret = phi <vscale x 4 x float> [ zeroinitializer, %entry ], [ %load, %cond.block ]
+ ret <vscale x 4 x float> %ret
+}
+
+; Sink zext to make use of scalar+uxtw(vector) addressing modes.
+; TODO: There's an argument here to split the extend into i8->i32 and i32->i64,
+; which would be especially useful if the i8s are the result of a load because
+; it would maintain the use of sign-extending loads.
+define <vscale x 4 x float> @gather_offset_sink_zext(ptr %base, <vscale x 4 x i8> %indices, <vscale x 4 x i1> %mask, i1 %cond) {
+; CHECK-LABEL: define <vscale x 4 x float> @gather_offset_sink_zext(
+; CHECK-SAME: ptr [[BASE:%.*]], <vscale x 4 x i8> [[INDICES:%.*]], <vscale x 4 x i1> [[MASK:%.*]], i1 [[COND:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 [[COND]], label [[COND_BLOCK:%.*]], label [[EXIT:%.*]]
+; CHECK: cond.block:
+; CHECK-NEXT: [[TMP0:%.*]] = zext <vscale x 4 x i8> [[INDICES]] to <vscale x 4 x i64>
+; CHECK-NEXT: [[PTRS:%.*]] = getelementptr float, ptr [[BASE]], <vscale x 4 x i64> [[TMP0]]
+; CHECK-NEXT: [[LOAD:%.*]] = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32.nxv4p0(<vscale x 4 x ptr> [[PTRS]], i32 4, <vscale x 4 x i1> [[MASK]], <vscale x 4 x float> poison)
+; CHECK-NEXT: ret <vscale x 4 x float> [[LOAD]]
+; CHECK: exit:
+; CHECK-NEXT: ret <vscale x 4 x float> zeroinitializer
+;
+entry:
+ %indices.zext = zext <vscale x 4 x i8> %indices to <vscale x 4 x i64>
+ br i1 %cond, label %cond.block, label %exit
+
+cond.block:
+ %ptrs = getelementptr float, ptr %base, <vscale x 4 x i64> %indices.zext
+ %load = tail call <vscale x 4 x float> @llvm.masked.gather.nxv4f32(<vscale x 4 x ptr> %ptrs, i32 4, <vscale x 4 x i1> %mask, <vscale x 4 x float> poison)
+ br label %exit
+
+exit:
+ %ret = phi <vscale x 4 x float> [ zeroinitializer, %entry ], [ %load, %cond.block ]
+ ret <vscale x 4 x float> %ret
+}
+
+; Ensure we support scatters as well as gathers.
+define void @scatter_offsets_sink_sext_get(<vscale x 4 x float> %data, ptr %base, <vscale x 4 x i32> %indices, <vscale x 4 x i1> %mask, i1 %cond) {
+; CHECK-LABEL: define void @scatter_offsets_sink_sext_get(
+; CHECK-SAME: <vscale x 4 x float> [[DATA:%.*]], ptr [[BASE:%.*]], <vscale x 4 x i32> [[INDICES:%.*]], <vscale x 4 x i1> [[MASK:%.*]], i1 [[COND:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 [[COND]], label [[COND_BLOCK:%.*]], label [[EXIT:%.*]]
+; CHECK: cond.block:
+; CHECK-NEXT: [[TMP0:%.*]] = sext <vscale x 4 x i32> [[INDICES]] to <vscale x 4 x i64>
+; CHECK-NEXT: [[TMP1:%.*]] = getelementptr float, ptr [[BASE]], <vscale x 4 x i64> [[TMP0]]
+; CHECK-NEXT: tail call void @llvm.masked.scatter.nxv4f32.nxv4p0(<vscale x 4 x float> [[DATA]], <vscale x 4 x ptr> [[TMP1]], i32 4, <vscale x 4 x i1> [[MASK]])
+; CHECK-NEXT: ret void
+; CHECK: exit:
+; CHECK-NEXT: ret void
+;
+entry:
+ %indices.sext = sext <vscale x 4 x i32> %indices to <vscale x 4 x i64>
+ %ptrs = getelementptr float, ptr %base, <vscale x 4 x i64> %indices.sext
+ br i1 %cond, label %cond.block, label %exit
+
+cond.block:
+ tail call void @llvm.masked.scatter.nxv4f32(<vscale x 4 x float> %data, <vscale x 4 x ptr> %ptrs, i32 4, <vscale x 4 x i1> %mask)
+ br label %exit
+
+exit:
+ ret void
+}
+
+declare <vscale x 4 x float> @llvm.masked.gather.nxv4f32(<vscale x 4 x ptr>, i32, <vscale x 4 x i1>, <vscale x 4 x float>)
+declare void @llvm.masked.scatter.nxv4f32(<vscale x 4 x float>, <vscale x 4 x ptr>, i32, <vscale x 4 x i1>)
|
paulwalker-arm
requested review from
davemgreen,
huntergr-arm,
sdesmalen-arm and
david-arm
| // Sink extends that would allow us to use 32-bit offset vectors. | ||
| if (isa<SExtInst>(Offsets) || isa<ZExtInst>(Offsets)) { | ||
| auto *OffsetsInst = cast<Instruction>(Offsets); | ||
| if (OffsetsInst->getType()->getScalarSizeInBits() > 32 && |
There was a problem hiding this comment.
Is there a reason you did not write: OffsetsInst->getType()->getScalarSizeInBits() == 64 ?
There was a problem hiding this comment.
I just wanted to maximise coverage. For example, offsets like sext <vscale x 2 x i32> %0 to <vscale x 2 x i48> can also be handled more efficiently when the code generator can see the extend.
There was a problem hiding this comment.
Makes sense, that would be type legalised to <vscale x 2 x i64> and you'd still get the same benefit.
sdesmalen-arm
approved these changes
Oct 2, 2023
| // Sink extends that would allow us to use 32-bit offset vectors. | ||
| if (isa<SExtInst>(Offsets) || isa<ZExtInst>(Offsets)) { | ||
| auto *OffsetsInst = cast<Instruction>(Offsets); | ||
| if (OffsetsInst->getType()->getScalarSizeInBits() > 32 && |
There was a problem hiding this comment.
Makes sense, that would be type legalised to <vscale x 2 x i64> and you'd still get the same benefit.
…/scatter addressing modes. SVE supports scalar+vector and scalar+extw(vector) addressing modes. However, the masked gather/scatter intrinsics take a vector of addresses, which means address computations can be hoisted out of loops. The is especially true for things like offsets where the true size of offsets is lost by the time you get to code generation. This is problematic because it forces the code generator to legalise towards `<vscale x 2 x ty>` vectors that will not maximise bandwidth if the main block datatypes is in fact i32 or smaller. This patch sinks GEPs and extends for cases where one of the above addressing modes can be used. NOTE: There are cases where it would be better to split the extend in two with one half hoisted out of a loop and the other within the loop. Whilst true I think this switch of default is still better than before because the extra extends are an improvement over being forced to split a gather/scatter.
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SVE supports scalar+vector and scalar+extw(vector) addressing modes.
However, the masked gather/scatter intrinsics take a vector of
addresses, which means address computations can be hoisted out of
loops. The is especially true for things like offsets where the
true size of offsets is lost by the time you get to code generation.
This is problematic because it forces the code generator to legalise
towards
<vscale x 2 x ty>vectors that will not maximise bandwidthif the main block datatypes is in fact i32 or smaller.
This patch sinks GEPs and extends for cases where one of the above
addressing modes can be used.
NOTE: There are cases where it would be better to split the extend
in two with one half hoisted out of a loop and the other within the
loop. Whilst true I think this switch of default is still better
than before because the extra extends are an improvement over being
forced to split a gather/scatter.