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[SCCP] Handle llvm.experimental.get.vector.length calls #169527

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[SCCP] Handle llvm.experimental.get.vector.length calls #169527

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732969a
Precommit tests
lukel97 Nov 25, 2025
0fa4cb6
[SCCP] Handle llvm.experimental.get.vector.length calls
lukel97 Nov 25, 2025
67801c5
Use larger of the two bitwidths, matches how SelectionDAGBuilder expands
lukel97 Nov 25, 2025
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32 changes: 32 additions & 0 deletions llvm/lib/Transforms/Utils/SCCPSolver.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2098,6 +2098,38 @@ void SCCPInstVisitor::handleCallResult(CallBase &CB) {
return (void)mergeInValue(ValueState[II], II,
ValueLatticeElement::getRange(Result));
}
if (II->getIntrinsicID() == Intrinsic::experimental_get_vector_length) {
Value *CountArg = II->getArgOperand(0);
Value *VF = II->getArgOperand(1);
bool Scalable = cast<ConstantInt>(II->getArgOperand(2))->isOne();

// Computation happens in the larger type.
unsigned BitWidth = std::max(CountArg->getType()->getScalarSizeInBits(),
VF->getType()->getScalarSizeInBits());

ConstantRange Count = getValueState(CountArg)
.asConstantRange(CountArg->getType(), false)
.zextOrTrunc(BitWidth);
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@dtcxzyw dtcxzyw Nov 25, 2025

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i32 @llvm.experimental.get.vector.length.i64(i64 2**33, i32 4, i1 false) will be folded to zero.

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@lukel97 lukel97 Nov 25, 2025

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Thanks, changed it to use the larger of the two types in 67801c5. I think that matches how SelectionDAGBuilder.cpp expands the intrinsic if the target doesn't natively support it.

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@dtcxzyw dtcxzyw Nov 27, 2025

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Suggested change
.zextOrTrunc(BitWidth);
.zext(BitWidth);

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@lukel97 lukel97 Nov 27, 2025

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ConstantRange::zeroExtend unfortunately asserts that the new BitWidth needs to be larger, not just equal to in size. So I used zextOrTrunc which returns the input when OldBitWidth == NewBitWidth.

Should we relax the assertion? It seems to be from the early days in 2004. Allowing equal bitwidths would make it more consistent with IRBuilder::CreateZExt

ConstantRange MaxLanes = getValueState(VF)
.asConstantRange(VF->getType(), false)
.zextOrTrunc(BitWidth);
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@dtcxzyw dtcxzyw Nov 27, 2025

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Suggested change
.zextOrTrunc(BitWidth);
.zext(BitWidth);

if (Scalable)
MaxLanes =
MaxLanes.multiply(getVScaleRange(II->getFunction(), BitWidth));

// The result is always less than both Count and MaxLanes.
ConstantRange Result(
APInt::getZero(BitWidth),
APIntOps::umin(Count.getUpper(), MaxLanes.getUpper()));

// If Count <= MaxLanes, getvectorlength(Count, MaxLanes) = Count
if (Count.icmp(CmpInst::ICMP_ULE, MaxLanes))
Result = Count;

Result = Result.zextOrTrunc(II->getType()->getScalarSizeInBits());
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@bababuck bababuck Nov 27, 2025

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I think this Trunc causes an issue in cases where:

  • Count is of a type > i32 (e.g. i64)
  • Vscale is such that Vscale * Count > 2^32
  • Count is > 2^32
define i32 @trunc() vscale_range(4, 4) {
  %x = call i32 @llvm.experimental.get.vector.length(i64 4294967296, i32 2147483647, i1 true)
  ret i32 %x
}

after running SCCP (built from 67801c5):

define i32 @trunc() #0 {
  ret i32 0
}

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@lukel97 lukel97 Nov 27, 2025

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I think in this case the result is poison from the definition in the LangRef:

If the result value does not fit in the result type, then the result is a poison value.

MaxLanes is 0x1FFFFFFFC and Count is 0x100000000, and because Count <= MaxLanes the result is Count. But 0x100000000 is larger than 32 bits so it returns poison, so I think the transform should be valid

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@dtcxzyw dtcxzyw Nov 27, 2025

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MaxLanes * VScale should not overflow.

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@dtcxzyw dtcxzyw Nov 27, 2025

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Suggested change
Result = Result.zextOrTrunc(II->getType()->getScalarSizeInBits());
Result = Result.trunc(II->getType()->getScalarSizeInBits());

return (void)mergeInValue(ValueState[II], II,
ValueLatticeElement::getRange(Result));
}

if (ConstantRange::isIntrinsicSupported(II->getIntrinsicID())) {
// Compute result range for intrinsics supported by ConstantRange.
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147 changes: 147 additions & 0 deletions llvm/test/Transforms/SCCP/get_vector_length-intrinsic.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,147 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
; RUN: opt < %s -p sccp -S | FileCheck %s

define i1 @result_le_count() {
; CHECK-LABEL: define i1 @result_le_count() {
; CHECK-NEXT: ret i1 true
;
%x = call i32 @llvm.experimental.get.vector.length(i32 3, i32 4, i1 false)
%res = icmp ule i32 %x, 3
ret i1 %res
}

define i1 @result_le_max_lanes(i32 %count) {
; CHECK-LABEL: define i1 @result_le_max_lanes(
; CHECK-SAME: i32 [[COUNT:%.*]]) {
; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 [[COUNT]], i32 3, i1 false)
; CHECK-NEXT: ret i1 true
;
%x = call i32 @llvm.experimental.get.vector.length(i32 %count, i32 3, i1 false)
%res = icmp ule i32 %x, 3
ret i1 %res
}

define i1 @result_le_max_lanes_scalable(i32 %count) vscale_range(2, 4) {
; CHECK-LABEL: define i1 @result_le_max_lanes_scalable(
; CHECK-SAME: i32 [[COUNT:%.*]]) #[[ATTR0:[0-9]+]] {
; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 [[COUNT]], i32 4, i1 true)
; CHECK-NEXT: ret i1 true
;
%x = call i32 @llvm.experimental.get.vector.length(i32 %count, i32 4, i1 true)
%res = icmp ule i32 %x, 16
ret i1 %res
}

define i32 @count_le_max_lanes() {
; CHECK-LABEL: define i32 @count_le_max_lanes() {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
; CHECK-NEXT: br label %[[EXIT:.*]]
; CHECK: [[EXIT]]:
; CHECK-NEXT: ret i32 4
;
entry:
br label %loop

loop:
%iv = phi i32 [4, %entry], [%iv.next, %loop]
%x = call i32 @llvm.experimental.get.vector.length(i32 %iv, i32 4, i1 false)
%iv.next = sub i32 %iv, %x
%ec = icmp eq i32 %iv.next, 0
br i1 %ec, label %exit, label %loop

exit:
ret i32 %x
}

; Can't simplify because %iv isn't <= max lanes.
define i32 @count_not_le_max_lanes() {
; CHECK-LABEL: define range(i32 0, 5) i32 @count_not_le_max_lanes() {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 6, %[[ENTRY]] ], [ [[IV_NEXT:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 [[IV]], i32 4, i1 false)
; CHECK-NEXT: [[IV_NEXT]] = sub i32 [[IV]], [[X]]
; CHECK-NEXT: [[EC:%.*]] = icmp eq i32 [[IV_NEXT]], 0
; CHECK-NEXT: br i1 [[EC]], label %[[EXIT:.*]], label %[[LOOP]]
; CHECK: [[EXIT]]:
; CHECK-NEXT: ret i32 [[X]]
;
entry:
br label %loop

loop:
%iv = phi i32 [6, %entry], [%iv.next, %loop]
%x = call i32 @llvm.experimental.get.vector.length(i32 %iv, i32 4, i1 false)
%iv.next = sub i32 %iv, %x
%ec = icmp eq i32 %iv.next, 0
br i1 %ec, label %exit, label %loop

exit:
ret i32 %x
}

define i32 @count_le_max_lanes_scalable_known() vscale_range(4, 8) {
; CHECK-LABEL: define i32 @count_le_max_lanes_scalable_known(
; CHECK-SAME: ) #[[ATTR1:[0-9]+]] {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
; CHECK-NEXT: br label %[[EXIT:.*]]
; CHECK: [[EXIT]]:
; CHECK-NEXT: ret i32 16
;
entry:
br label %loop

loop:
%iv = phi i32 [16, %entry], [%iv.next, %loop]
%x = call i32 @llvm.experimental.get.vector.length(i32 %iv, i32 4, i1 true)
%iv.next = sub i32 %iv, %x
%ec = icmp eq i32 %iv.next, 0
br i1 %ec, label %exit, label %loop

exit:
ret i32 %x
}

; Can't simplify because %iv isn't guaranteed <= max lanes.
define i32 @count_le_max_lanes_scalable_unknown() {
; CHECK-LABEL: define range(i32 0, -1) i32 @count_le_max_lanes_scalable_unknown() {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 16, %[[ENTRY]] ], [ [[IV_NEXT:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 [[IV]], i32 4, i1 true)
; CHECK-NEXT: [[IV_NEXT]] = sub i32 [[IV]], [[X]]
; CHECK-NEXT: [[EC:%.*]] = icmp eq i32 [[IV_NEXT]], 0
; CHECK-NEXT: br i1 [[EC]], label %[[EXIT:.*]], label %[[LOOP]]
; CHECK: [[EXIT]]:
; CHECK-NEXT: ret i32 [[X]]
;
entry:
br label %loop

loop:
%iv = phi i32 [16, %entry], [%iv.next, %loop]
%x = call i32 @llvm.experimental.get.vector.length(i32 %iv, i32 4, i1 true)
%iv.next = sub i32 %iv, %x
%ec = icmp eq i32 %iv.next, 0
br i1 %ec, label %exit, label %loop

exit:
ret i32 %x
}

define i1 @result_le_overflow() {
; CHECK-LABEL: define i1 @result_le_overflow() {
; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i64(i64 4294967296, i32 4, i1 false)
; CHECK-NEXT: [[RES:%.*]] = icmp ule i32 [[X]], 3
; CHECK-NEXT: ret i1 [[RES]]
;
%x = call i32 @llvm.experimental.get.vector.length(i64 u0x100000000, i32 4, i1 false)
%res = icmp ule i32 %x, 3
ret i1 %res
}