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[LoopUnroll] Introduce parallel accumulators when unrolling FP reductions. #166630

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Merged
merged 7 commits into from
Nov 27, 2025
Merged

[LoopUnroll] Introduce parallel accumulators when unrolling FP reductions. #166630

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214c03f
[LoopUnroll] Introduce parallel accumulators when unrolling FP reduct…
juliannagele Nov 5, 2025
7284232
fixup! [LoopUnroll] Introduce parallel accumulators when unrolling FP…
juliannagele Nov 20, 2025
3bdcf3c
Merge branch 'main' of https://github.com/llvm/llvm-project into unro…
juliannagele Nov 20, 2025
3e3db0b
!fixup Fix formatting
juliannagele Nov 20, 2025
9c401f3
Merge branch 'main' into unroll-fp-reductions
juliannagele Nov 20, 2025
da24236
Merge branch 'main' of https://github.com/llvm/llvm-project into unro…
juliannagele Nov 24, 2025
739c5e1
fixup! [LoopUnroll] Introduce parallel accumulators when unrolling FP…
juliannagele Nov 24, 2025
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12 changes: 9 additions & 3 deletions llvm/lib/Transforms/Utils/LoopUnroll.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1094,6 +1094,7 @@ llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
if (!RdxResult) {
RdxResult = PartialReductions.front();
IRBuilder Builder(ExitBlock, ExitBlock->getFirstNonPHIIt());
Builder.setFastMathFlags(Reductions.begin()->second.getFastMathFlags());
RecurKind RK = Reductions.begin()->second.getRecurrenceKind();
for (Instruction *RdxPart : drop_begin(PartialReductions)) {
RdxResult = Builder.CreateBinOp(
Expand Down Expand Up @@ -1256,14 +1257,19 @@ llvm::canParallelizeReductionWhenUnrolling(PHINode &Phi, Loop *L,
return std::nullopt;
RecurKind RK = RdxDesc.getRecurrenceKind();
// Skip unsupported reductions.
// TODO: Handle additional reductions, including FP and min-max
// reductions.
if (!RecurrenceDescriptor::isIntegerRecurrenceKind(RK) ||
// TODO: Handle additional reductions, including min-max reductions.
if (!(RecurrenceDescriptor::isIntegerRecurrenceKind(RK) ||
RecurrenceDescriptor::isFloatingPointRecurrenceKind(RK)) ||
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@fhahn fhahn Nov 17, 2025

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Suggested change
if (!(RecurrenceDescriptor::isIntegerRecurrenceKind(RK) ||
RecurrenceDescriptor::isFloatingPointRecurrenceKind(RK)) ||
if (

I think the condition is always true, given how isFloatingPointRecurrenceKind is implemented, as long as RK != None, which RecurrenceDescriptor::isReductionPHI should already ensure

bool RecurrenceDescriptor::isFloatingPointRecurrenceKind(RecurKind Kind) {                                <<<
  return (Kind != RecurKind::None) && !isIntegerRecurrenceKind(Kind);
}

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@juliannagele juliannagele Nov 20, 2025

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Makes sense, thanks -- done!

RecurrenceDescriptor::isAnyOfRecurrenceKind(RK) ||
RecurrenceDescriptor::isFindIVRecurrenceKind(RK) ||
RecurrenceDescriptor::isMinMaxRecurrenceKind(RK))
return std::nullopt;

if (RecurrenceDescriptor::isFloatingPointRecurrenceKind(RK)) {
if (!RdxDesc.getFastMathFlags().allowReassoc())
return std::nullopt;
}
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@fhahn fhahn Nov 6, 2025

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Do we need the check here? I think the recurrence descriptor analysis should take care of checking the required fast-math flags (in RecurrenceDescriptor::isRecurrenceInstr), and return RecurKind::None, if the required flags are missing.

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@juliannagele juliannagele Nov 7, 2025

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Hm, afaict isRecurrenceInstr doesn't return None but remembers the instruction that doesn't have reassoc

InstDesc(Kind == RecurKind::FAdd, I,
                    I->hasAllowReassoc() ? nullptr : I);

So I think we do need a check but we could simplify it to

  if (RdxDesc.hasExactFPMath())
      return std::nullopt;

where

  /// Returns true if the recurrence has floating-point math that requires
  /// precise (ordered) operations.
  bool hasExactFPMath() const { return ExactFPMathInst != nullptr; }


if (RdxDesc.IntermediateStore)
return std::nullopt;

Expand Down
21 changes: 14 additions & 7 deletions llvm/test/Transforms/LoopUnroll/partial-unroll-reductions.ll
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Expand Up @@ -319,27 +319,33 @@ define float @test_fadd_with_ressaoc(ptr %src, i64 %n, float %start) {
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[IV_NEXT_3:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[RDX:%.*]] = phi float [ [[START]], %[[ENTRY]] ], [ [[RDX_NEXT_3:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[RDX_1:%.*]] = phi float [ -0.000000e+00, %[[ENTRY]] ], [ [[RDX_NEXT_1:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[RDX_2:%.*]] = phi float [ -0.000000e+00, %[[ENTRY]] ], [ [[RDX_NEXT_2:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[RDX_3:%.*]] = phi float [ -0.000000e+00, %[[ENTRY]] ], [ [[RDX_NEXT_3:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[RDX:%.*]] = phi float [ [[START]], %[[ENTRY]] ], [ [[RDX_NEXT:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[IV_NEXT:%.*]] = add nuw nsw i64 [[IV]], 1
; CHECK-NEXT: [[GEP_SRC:%.*]] = getelementptr float, ptr [[SRC]], i64 [[IV]]
; CHECK-NEXT: [[L:%.*]] = load float, ptr [[GEP_SRC]], align 1
; CHECK-NEXT: [[RDX_NEXT:%.*]] = fadd float [[RDX]], [[L]]
; CHECK-NEXT: [[RDX_NEXT]] = fadd reassoc float [[RDX]], [[L]]
; CHECK-NEXT: [[IV_NEXT_1:%.*]] = add nuw nsw i64 [[IV]], 2
; CHECK-NEXT: [[GEP_SRC_1:%.*]] = getelementptr float, ptr [[SRC]], i64 [[IV_NEXT]]
; CHECK-NEXT: [[L_1:%.*]] = load float, ptr [[GEP_SRC_1]], align 1
; CHECK-NEXT: [[RDX_NEXT_1:%.*]] = fadd float [[RDX_NEXT]], [[L_1]]
; CHECK-NEXT: [[RDX_NEXT_1]] = fadd reassoc float [[RDX_1]], [[L_1]]
; CHECK-NEXT: [[IV_NEXT_2:%.*]] = add nuw nsw i64 [[IV]], 3
; CHECK-NEXT: [[GEP_SRC_2:%.*]] = getelementptr float, ptr [[SRC]], i64 [[IV_NEXT_1]]
; CHECK-NEXT: [[L_2:%.*]] = load float, ptr [[GEP_SRC_2]], align 1
; CHECK-NEXT: [[RDX_NEXT_2:%.*]] = fadd float [[RDX_NEXT_1]], [[L_2]]
; CHECK-NEXT: [[RDX_NEXT_2]] = fadd reassoc float [[RDX_2]], [[L_2]]
; CHECK-NEXT: [[IV_NEXT_3]] = add nuw nsw i64 [[IV]], 4
; CHECK-NEXT: [[GEP_SRC_24:%.*]] = getelementptr float, ptr [[SRC]], i64 [[IV_NEXT_2]]
; CHECK-NEXT: [[L_24:%.*]] = load float, ptr [[GEP_SRC_24]], align 1
; CHECK-NEXT: [[RDX_NEXT_3]] = fadd float [[RDX_NEXT_2]], [[L_24]]
; CHECK-NEXT: [[RDX_NEXT_3]] = fadd reassoc float [[RDX_3]], [[L_24]]
; CHECK-NEXT: [[EC_3:%.*]] = icmp ne i64 [[IV_NEXT_3]], 1000
; CHECK-NEXT: br i1 [[EC_3]], label %[[LOOP]], label %[[EXIT:.*]]
; CHECK: [[EXIT]]:
; CHECK-NEXT: [[RDX_NEXT_LCSSA:%.*]] = phi float [ [[RDX_NEXT_3]], %[[LOOP]] ]
; CHECK-NEXT: [[RDX_NEXT_LCSSA1:%.*]] = phi float [ [[RDX_NEXT_3]], %[[LOOP]] ]
; CHECK-NEXT: [[BIN_RDX:%.*]] = fadd reassoc float [[RDX_NEXT_1]], [[RDX_NEXT]]
; CHECK-NEXT: [[BIN_RDX1:%.*]] = fadd reassoc float [[RDX_NEXT_2]], [[BIN_RDX]]
; CHECK-NEXT: [[RDX_NEXT_LCSSA:%.*]] = fadd reassoc float [[RDX_NEXT_3]], [[BIN_RDX1]]
; CHECK-NEXT: ret float [[RDX_NEXT_LCSSA]]
;
entry:
Expand All @@ -351,13 +357,14 @@ loop:
%iv.next = add i64 %iv, 1
%gep.src = getelementptr float, ptr %src, i64 %iv
%l = load float, ptr %gep.src, align 1
%rdx.next = fadd float %rdx, %l
%rdx.next = fadd reassoc float %rdx, %l
%ec = icmp ne i64 %iv.next, 1000
br i1 %ec, label %loop, label %exit

exit:
ret float %rdx.next
}

define i32 @test_smin(ptr %src, i64 %n) {
; CHECK-LABEL: define i32 @test_smin(
; CHECK-SAME: ptr [[SRC:%.*]], i64 [[N:%.*]]) {
Expand Down
133 changes: 133 additions & 0 deletions llvm/test/Transforms/LoopUnroll/runtime-unroll-reductions.ll
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Expand Up @@ -220,6 +220,137 @@ exit:
ret i32 %res
}

define float @test_fadd_reduction(ptr %a, i64 %n) {
; CHECK-LABEL: define float @test_fadd_reduction(
; CHECK-SAME: ptr [[A:%.*]], i64 [[N:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: [[TMP0:%.*]] = add i64 [[N]], -1
; CHECK-NEXT: [[XTRAITER:%.*]] = and i64 [[N]], 1
; CHECK-NEXT: [[TMP1:%.*]] = icmp ult i64 [[TMP0]], 1
; CHECK-NEXT: br i1 [[TMP1]], label %[[LOOP_EPIL_PREHEADER:.*]], label %[[ENTRY_NEW:.*]]
; CHECK: [[ENTRY_NEW]]:
; CHECK-NEXT: [[UNROLL_ITER:%.*]] = sub i64 [[N]], [[XTRAITER]]
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ 0, %[[ENTRY_NEW]] ], [ [[IV_NEXT_1:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[RDX_1:%.*]] = phi float [ -0.000000e+00, %[[ENTRY_NEW]] ], [ [[RDX_NEXT_1:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[RDX:%.*]] = phi float [ 0.000000e+00, %[[ENTRY_NEW]] ], [ [[RDX_NEXT:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[NITER:%.*]] = phi i64 [ 0, %[[ENTRY_NEW]] ], [ [[NITER_NEXT_1:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[GEP_A:%.*]] = getelementptr inbounds nuw float, ptr [[A]], i64 [[IV]]
; CHECK-NEXT: [[TMP2:%.*]] = load float, ptr [[GEP_A]], align 16
; CHECK-NEXT: [[RDX_NEXT]] = fadd reassoc float [[RDX]], [[TMP2]]
; CHECK-NEXT: [[IV_NEXT:%.*]] = add nuw nsw i64 [[IV]], 1
; CHECK-NEXT: [[GEP_A_1:%.*]] = getelementptr inbounds nuw float, ptr [[A]], i64 [[IV_NEXT]]
; CHECK-NEXT: [[TMP3:%.*]] = load float, ptr [[GEP_A_1]], align 16
; CHECK-NEXT: [[RDX_NEXT_1]] = fadd reassoc float [[RDX_1]], [[TMP3]]
; CHECK-NEXT: [[IV_NEXT_1]] = add nuw nsw i64 [[IV]], 2
; CHECK-NEXT: [[NITER_NEXT_1]] = add i64 [[NITER]], 2
; CHECK-NEXT: [[NITER_NCMP_1:%.*]] = icmp eq i64 [[NITER_NEXT_1]], [[UNROLL_ITER]]
; CHECK-NEXT: br i1 [[NITER_NCMP_1]], label %[[EXIT_UNR_LCSSA:.*]], label %[[LOOP]], !llvm.loop [[LOOP5:![0-9]+]]
; CHECK: [[EXIT_UNR_LCSSA]]:
; CHECK-NEXT: [[RES_PH:%.*]] = phi float [ [[RDX_NEXT_1]], %[[LOOP]] ]
; CHECK-NEXT: [[IV_UNR:%.*]] = phi i64 [ [[IV_NEXT_1]], %[[LOOP]] ]
; CHECK-NEXT: [[RDX_UNR:%.*]] = phi float [ [[RDX_NEXT_1]], %[[LOOP]] ]
; CHECK-NEXT: [[BIN_RDX:%.*]] = fadd reassoc float [[RDX_NEXT_1]], [[RDX_NEXT]]
; CHECK-NEXT: [[LCMP_MOD:%.*]] = icmp ne i64 [[XTRAITER]], 0
; CHECK-NEXT: br i1 [[LCMP_MOD]], label %[[LOOP_EPIL_PREHEADER]], label %[[EXIT:.*]]
; CHECK: [[LOOP_EPIL_PREHEADER]]:
; CHECK-NEXT: [[IV_EPIL_INIT:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[IV_UNR]], %[[EXIT_UNR_LCSSA]] ]
; CHECK-NEXT: [[RDX_EPIL_INIT:%.*]] = phi float [ 0.000000e+00, %[[ENTRY]] ], [ [[BIN_RDX]], %[[EXIT_UNR_LCSSA]] ]
; CHECK-NEXT: [[LCMP_MOD2:%.*]] = icmp ne i64 [[XTRAITER]], 0
; CHECK-NEXT: call void @llvm.assume(i1 [[LCMP_MOD2]])
; CHECK-NEXT: br label %[[LOOP_EPIL:.*]]
; CHECK: [[LOOP_EPIL]]:
; CHECK-NEXT: [[GEP_A_EPIL:%.*]] = getelementptr inbounds nuw float, ptr [[A]], i64 [[IV_EPIL_INIT]]
; CHECK-NEXT: [[TMP4:%.*]] = load float, ptr [[GEP_A_EPIL]], align 16
; CHECK-NEXT: [[RDX_NEXT_EPIL:%.*]] = fadd reassoc float [[RDX_EPIL_INIT]], [[TMP4]]
; CHECK-NEXT: br label %[[EXIT]]
; CHECK: [[EXIT]]:
; CHECK-NEXT: [[RES:%.*]] = phi float [ [[BIN_RDX]], %[[EXIT_UNR_LCSSA]] ], [ [[RDX_NEXT_EPIL]], %[[LOOP_EPIL]] ]
; CHECK-NEXT: ret float [[RES]]
;
entry:
br label %loop

loop:
%iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
%rdx = phi float [ 0.0, %entry ], [ %rdx.next, %loop ]
%gep.a = getelementptr inbounds nuw float, ptr %a, i64 %iv
%1 = load float, ptr %gep.a, align 16
%rdx.next = fadd reassoc float %rdx, %1
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@fhahn fhahn Nov 24, 2025

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Could you also add a test with a different fast-math flag, to make sure we don't introduce additional accumulators in that case?

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@juliannagele juliannagele Nov 24, 2025

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done, thanks!

%iv.next = add nuw nsw i64 %iv, 1
%ec = icmp eq i64 %iv.next, %n
br i1 %ec, label %exit, label %loop, !llvm.loop !0

exit:
%res = phi float [ %rdx.next, %loop ]
ret float %res
}

define float @test_fadd_no_reassoc(ptr %a, i64 %n) {
; CHECK-LABEL: define float @test_fadd_no_reassoc(
; CHECK-SAME: ptr [[A:%.*]], i64 [[N:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: [[TMP0:%.*]] = add i64 [[N]], -1
; CHECK-NEXT: [[XTRAITER:%.*]] = and i64 [[N]], 1
; CHECK-NEXT: [[TMP1:%.*]] = icmp ult i64 [[TMP0]], 1
; CHECK-NEXT: br i1 [[TMP1]], label %[[LOOP_EPIL_PREHEADER:.*]], label %[[ENTRY_NEW:.*]]
; CHECK: [[ENTRY_NEW]]:
; CHECK-NEXT: [[UNROLL_ITER:%.*]] = sub i64 [[N]], [[XTRAITER]]
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ 0, %[[ENTRY_NEW]] ], [ [[IV_NEXT_1:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[RDX:%.*]] = phi float [ 0.000000e+00, %[[ENTRY_NEW]] ], [ [[RDX_NEXT_1:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[NITER:%.*]] = phi i64 [ 0, %[[ENTRY_NEW]] ], [ [[NITER_NEXT_1:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[GEP_A:%.*]] = getelementptr inbounds nuw float, ptr [[A]], i64 [[IV]]
; CHECK-NEXT: [[TMP2:%.*]] = load float, ptr [[GEP_A]], align 16
; CHECK-NEXT: [[RDX_NEXT:%.*]] = fadd float [[RDX]], [[TMP2]]
; CHECK-NEXT: [[IV_NEXT:%.*]] = add nuw nsw i64 [[IV]], 1
; CHECK-NEXT: [[GEP_A_1:%.*]] = getelementptr inbounds nuw float, ptr [[A]], i64 [[IV_NEXT]]
; CHECK-NEXT: [[TMP3:%.*]] = load float, ptr [[GEP_A_1]], align 16
; CHECK-NEXT: [[RDX_NEXT_1]] = fadd float [[RDX_NEXT]], [[TMP3]]
; CHECK-NEXT: [[IV_NEXT_1]] = add nuw nsw i64 [[IV]], 2
; CHECK-NEXT: [[NITER_NEXT_1]] = add i64 [[NITER]], 2
; CHECK-NEXT: [[NITER_NCMP_1:%.*]] = icmp eq i64 [[NITER_NEXT_1]], [[UNROLL_ITER]]
; CHECK-NEXT: br i1 [[NITER_NCMP_1]], label %[[EXIT_UNR_LCSSA:.*]], label %[[LOOP]], !llvm.loop [[LOOP6:![0-9]+]]
; CHECK: [[EXIT_UNR_LCSSA]]:
; CHECK-NEXT: [[RES_PH:%.*]] = phi float [ [[RDX_NEXT_1]], %[[LOOP]] ]
; CHECK-NEXT: [[IV_UNR:%.*]] = phi i64 [ [[IV_NEXT_1]], %[[LOOP]] ]
; CHECK-NEXT: [[RDX_UNR:%.*]] = phi float [ [[RDX_NEXT_1]], %[[LOOP]] ]
; CHECK-NEXT: [[LCMP_MOD:%.*]] = icmp ne i64 [[XTRAITER]], 0
; CHECK-NEXT: br i1 [[LCMP_MOD]], label %[[LOOP_EPIL_PREHEADER]], label %[[EXIT:.*]]
; CHECK: [[LOOP_EPIL_PREHEADER]]:
; CHECK-NEXT: [[IV_EPIL_INIT:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[IV_UNR]], %[[EXIT_UNR_LCSSA]] ]
; CHECK-NEXT: [[RDX_EPIL_INIT:%.*]] = phi float [ 0.000000e+00, %[[ENTRY]] ], [ [[RDX_UNR]], %[[EXIT_UNR_LCSSA]] ]
; CHECK-NEXT: [[LCMP_MOD2:%.*]] = icmp ne i64 [[XTRAITER]], 0
; CHECK-NEXT: call void @llvm.assume(i1 [[LCMP_MOD2]])
; CHECK-NEXT: br label %[[LOOP_EPIL:.*]]
; CHECK: [[LOOP_EPIL]]:
; CHECK-NEXT: [[GEP_A_EPIL:%.*]] = getelementptr inbounds nuw float, ptr [[A]], i64 [[IV_EPIL_INIT]]
; CHECK-NEXT: [[TMP4:%.*]] = load float, ptr [[GEP_A_EPIL]], align 16
; CHECK-NEXT: [[RDX_NEXT_EPIL:%.*]] = fadd float [[RDX_EPIL_INIT]], [[TMP4]]
; CHECK-NEXT: br label %[[EXIT]]
; CHECK: [[EXIT]]:
; CHECK-NEXT: [[RES:%.*]] = phi float [ [[RES_PH]], %[[EXIT_UNR_LCSSA]] ], [ [[RDX_NEXT_EPIL]], %[[LOOP_EPIL]] ]
; CHECK-NEXT: ret float [[RES]]
;
entry:
br label %loop

loop:
%iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
%rdx = phi float [ 0.0, %entry ], [ %rdx.next, %loop ]
%gep.a = getelementptr inbounds nuw float, ptr %a, i64 %iv
%1 = load float, ptr %gep.a, align 16
%rdx.next = fadd float %rdx, %1
%iv.next = add nuw nsw i64 %iv, 1
%ec = icmp eq i64 %iv.next, %n
br i1 %ec, label %exit, label %loop, !llvm.loop !0

exit:
%res = phi float [ %rdx.next, %loop ]
ret float %res
}


!0 = distinct !{!0, !1}
Expand All @@ -234,4 +365,6 @@ exit:
; CHECK: [[LOOP2]] = distinct !{[[LOOP2]], [[META1]]}
; CHECK: [[LOOP3]] = distinct !{[[LOOP3]], [[META1]]}
; CHECK: [[LOOP4]] = distinct !{[[LOOP4]], [[META1]]}
; CHECK: [[LOOP5]] = distinct !{[[LOOP5]], [[META1]]}
; CHECK: [[LOOP6]] = distinct !{[[LOOP6]], [[META1]]}
;.