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[InstCombine] Fold @llvm.experimental.get.vector.length when cnt <= max_lanes #169293

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Merged
lukel97 merged 5 commits into from
Nov 27, 2025
Merged

[InstCombine] Fold @llvm.experimental.get.vector.length when cnt <= max_lanes #169293

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c61d4f2
Precommit tests
lukel97 Nov 24, 2025
4da2081
[InstCombine] Fold @llvm.experimental.get.vector.length when cnt <= m…
lukel97 Nov 24, 2025
2f71e8a
Use createZExtOrTrunc, use isOne()
lukel97 Nov 25, 2025
db24622
Perform in larger of two bitwidths
lukel97 Nov 26, 2025
68b9cb6
Merge branch 'main' into instcombine/getvectorlength
lukel97 Nov 27, 2025
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16 changes: 16 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4005,6 +4005,22 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
}
break;
}
case Intrinsic::experimental_get_vector_length: {
// get.vector.length(Cnt, MaxLanes) --> Cnt when Cnt <= MaxLanes
ConstantRange Cnt = computeConstantRangeIncludingKnownBits(
II->getArgOperand(0), false, SQ.getWithInstruction(II));
ConstantRange MaxLanes = cast<ConstantInt>(II->getArgOperand(1))
->getValue()
.zext(Cnt.getBitWidth());
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if (cast<ConstantInt>(II->getArgOperand(2))->isOne())
MaxLanes = MaxLanes.multiply(
getVScaleRange(II->getFunction(), Cnt.getBitWidth()));

if (Cnt.icmp(CmpInst::ICMP_ULE, MaxLanes))
return replaceInstUsesWith(
*II, Builder.CreateZExtOrTrunc(II->getArgOperand(0), II->getType()));
return nullptr;
}
default: {
// Handle target specific intrinsics
std::optional<Instruction *> V = targetInstCombineIntrinsic(*II);
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80 changes: 80 additions & 0 deletions llvm/test/Transforms/InstCombine/get_vector_length.ll
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@@ -0,0 +1,80 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
; RUN: opt < %s -passes=instcombine,verify -S | FileCheck %s

define i32 @cnt_known_lt() {
; CHECK-LABEL: define i32 @cnt_known_lt() {
; CHECK-NEXT: ret i32 1
;
%x = call i32 @llvm.experimental.get.vector.length(i32 1, i32 2, i1 false)
ret i32 %x
}

define i32 @cnt_not_known_lt() {
; CHECK-LABEL: define i32 @cnt_not_known_lt() {
; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 2, i32 1, i1 false)
; CHECK-NEXT: ret i32 [[X]]
;
%x = call i32 @llvm.experimental.get.vector.length(i32 2, i32 1, i1 false)
ret i32 %x
}

define i32 @cnt_known_lt_scalable() vscale_range(2, 4) {
; CHECK-LABEL: define i32 @cnt_known_lt_scalable(
; CHECK-SAME: ) #[[ATTR0:[0-9]+]] {
; CHECK-NEXT: ret i32 2
;
%x = call i32 @llvm.experimental.get.vector.length(i32 2, i32 1, i1 true)
ret i32 %x
}

define i32 @cnt_not_known_lt_scalable() {
; CHECK-LABEL: define i32 @cnt_not_known_lt_scalable() {
; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 2, i32 1, i1 true)
; CHECK-NEXT: ret i32 [[X]]
;
%x = call i32 @llvm.experimental.get.vector.length(i32 2, i32 1, i1 true)
ret i32 %x
}

define i32 @cnt_known_lt_runtime(i32 %x) {
; CHECK-LABEL: define i32 @cnt_known_lt_runtime(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[ICMP:%.*]] = icmp ult i32 [[X]], 4
; CHECK-NEXT: call void @llvm.assume(i1 [[ICMP]])
; CHECK-NEXT: ret i32 [[X]]
;
%icmp = icmp ule i32 %x, 3
call void @llvm.assume(i1 %icmp)
%y = call i32 @llvm.experimental.get.vector.length(i32 %x, i32 3, i1 false)
ret i32 %y
}

define i32 @cnt_known_lt_runtime_trunc(i64 %x) {
; CHECK-LABEL: define i32 @cnt_known_lt_runtime_trunc(
; CHECK-SAME: i64 [[X:%.*]]) {
; CHECK-NEXT: [[ICMP:%.*]] = icmp ult i64 [[X]], 4
; CHECK-NEXT: call void @llvm.assume(i1 [[ICMP]])
; CHECK-NEXT: [[Y:%.*]] = trunc nuw nsw i64 [[X]] to i32
; CHECK-NEXT: ret i32 [[Y]]
;
%icmp = icmp ule i64 %x, 3
call void @llvm.assume(i1 %icmp)
%y = call i32 @llvm.experimental.get.vector.length(i64 %x, i32 3, i1 false)
ret i32 %y
}

; FIXME: We should be able to deduce the constant range from AssumptionCache

@dtcxzyw dtcxzyw Nov 24, 2025

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Why doesn't this work?

llvm-project/llvm/lib/Analysis/ValueTracking.cpp

Lines 10303 to 10328 in 1580f4b

if (CtxI && AC) {
// Try to restrict the range based on information from assumptions.
for (auto &AssumeVH : AC->assumptionsFor(V)) {
if (!AssumeVH)
continue;
CallInst *I = cast<CallInst>(AssumeVH);
assert(I->getParent()->getParent() == CtxI->getParent()->getParent() &&
"Got assumption for the wrong function!");
assert(I->getIntrinsicID() == Intrinsic::assume &&
"must be an assume intrinsic");
if (!isValidAssumeForContext(I, CtxI, DT))
continue;
Value *Arg = I->getArgOperand(0);
ICmpInst *Cmp = dyn_cast<ICmpInst>(Arg);
// Currently we just use information from comparisons.
if (!Cmp || Cmp->getOperand(0) != V)
continue;
// TODO: Set "ForSigned" parameter via Cmp->isSigned()?
ConstantRange RHS =
computeConstantRange(Cmp->getOperand(1), /* ForSigned */ false,
UseInstrInfo, AC, I, DT, Depth + 1);
CR = CR.intersectWith(
ConstantRange::makeAllowedICmpRegion(Cmp->getPredicate(), RHS));
}
}

@lukel97 lukel97 Nov 24, 2025

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I think computeConstantRangeIncludingKnownBits doesn't pass in the AssumptionCache to computeConstantRange currently. I presume it's a simple fix in another PR?

@lukel97 lukel97 Nov 24, 2025

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Btw the reason for using computeConstantRangeIncludingKnownBits and not just computeConstantRange directly is because for the motivating case I think we need to be able handle a simple PHI recurrence, which only computeKnownBits seems to be able to handle at the moment.

@topperc topperc Nov 24, 2025 •
edited
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The get.vector.length being in the recurrence makes it no longer "simple", doesn't it?

@bababuck bababuck Nov 24, 2025

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From what I can tell that is correct, I didn't see anywhere in computeKnownBits where get.vector.length is handled currently. Here's a test for this that could get added to get_vector_length.ll:

define void @loop_get_len() {
; CHECK-LABEL: define void @loop_get_len() {
; CHECK-NEXT:  [[ENTRY:.*]]:
; CHECK-NEXT:    br label %[[LOOP:.*]]
; CHECK:       [[LOOP]]:
; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 16, %[[ENTRY]] ], [ [[REM:%.*]], %[[LOOP]] ]
; CHECK-NEXT:    [[GETLEN:%.*]] = tail call i32 @llvm.experimental.get.vector.length.i32(i32 [[PHI]], i32 16, i1 false)
; CHECK-NEXT:    [[REM]] = sub i32 [[PHI]], [[GETLEN]]
; CHECK-NEXT:    [[EXIT_COND:%.*]] = icmp eq i32 [[REM]], 0
; CHECK-NEXT:    br i1 [[EXIT_COND]], label %[[EXIT:.*]], label %[[LOOP]]
; CHECK:       [[EXIT]]:
; CHECK-NEXT:    ret void
;
entry:
  br label %loop
loop:
  %phi = phi i32 [ 16, %entry ], [ %rem, %loop ]
  %getlen = tail call i32 @llvm.experimental.get.vector.length.i32(i32 %phi, i32 16, i1 false)
  %rem = sub i32 %phi, %getlen
  %exit_cond = icmp eq i32 %rem, 0
  br i1 %exit_cond, label %exit, label %loop
exit:
  ret void
}

@lukel97 lukel97 Nov 25, 2025 •
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The get.vector.length being in the recurrence makes it no longer "simple", doesn't it?

matchSimpleRecurrence doesn't care what the step is so it can still handle get.vector.length in the recurrence. The only issue is that we currently don't deduce anything apart from the low zero bits in computeKnownBitsFromOperator.

From what I can tell that is correct, I didn't see anywhere in computeKnownBits where get.vector.length is handled currently

Yes, we should separately add support for that too.

But for the reduced example in #164762 (comment) I think we only need to teach computeKnownBitsFromOperator that sub recurrences with nuw should only decrease, so the number of leading zeros should increase. I gave this a super quick go and it seems to work, haven't checked if its generally correct yet:

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index dbceb8e55784..c2ba85d4bfac 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1857,6 +1857,15 @@ static void computeKnownBitsFromOperator(const Operator *I,
                                        Known3.countMinTrailingZeros()));
 
         auto *OverflowOp = dyn_cast<OverflowingBinaryOperator>(BO);
+
+        if (!OverflowOp)
+          break;
+
+        if (Opcode == Instruction::Sub && Q.IIQ.hasNoUnsignedWrap(OverflowOp)) {
+          Known.Zero.setHighBits(std::min(Known2.countMinLeadingZeros(),
+                                          Known3.countMinLeadingZeros()));
+        }
+

That diff above in combination with this patch is enough to flatten the loop.

Actually Known2 and Known3 weren't what I thought they were, I don't think the above is correct.

; rather than relying on KnownBits, which in this case only knows x <= 3.
define i32 @cnt_known_lt_runtime_assumption(i32 %x) {
; CHECK-LABEL: define i32 @cnt_known_lt_runtime_assumption(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[ICMP:%.*]] = icmp ult i32 [[X]], 3
; CHECK-NEXT: call void @llvm.assume(i1 [[ICMP]])
; CHECK-NEXT: [[Y:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 [[X]], i32 2, i1 false)
; CHECK-NEXT: ret i32 [[Y]]
;
%icmp = icmp ule i32 %x, 2
call void @llvm.assume(i1 %icmp)
%y = call i32 @llvm.experimental.get.vector.length(i32 %x, i32 2, i1 false)
ret i32 %y
}