[llvm] [VPlan] Reject Argmin/Argmax with intermediate ops. (PR #218944)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/218944

handleMultiUseReductions currently only supports a single min/max reduction step, not multiple intermediate ops.

Bail out instead of crashing in that case.

>From 81429007098f278105bde4571e307cdbba39382b Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Wed, 26 Aug 2026 14:31:34 +0100
Subject: [PATCH] [VPlan] Reject Argmin/Argmax with intermediate ops.

handleMultiUseReductions currently only supports a single min/max
reduction step, not multiple intermediate ops.

Bail out instead of crashing in that case.
---
 .../Vectorize/VPlanConstruction.cpp           | 13 ++++--
 .../LoopVectorize/select-smax-last-index.ll   | 44 +++++++++++++++++++
 2 files changed, 53 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp b/llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp
index ad963fe9ff158..459e8f86cb337 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp
@@ -2061,9 +2061,16 @@ bool VPlanTransforms::handleMultiUseReductions(VPlan &Plan,
     // ComputeReductionResult.
     assert(MinOrMaxOp->getNumUsers() == 2 &&
            "MinOrMaxOp must have exactly 2 users");
-    VPValue *MinOrMaxOpValue = MinOrMaxOp->getOperand(0);
-    if (MinOrMaxOpValue == MinOrMaxPhiR)
+    // MinOrMaxOp must combine MinOrMaxPhiR directly with the new element;
+    // reject multi-step min/max chains (e.g. max(l, max(k, phi))), which
+    // this transform does not handle.
+    VPValue *MinOrMaxOpValue;
+    if (MinOrMaxOp->getOperand(0) == MinOrMaxPhiR)
       MinOrMaxOpValue = MinOrMaxOp->getOperand(1);
+    else if (MinOrMaxOp->getOperand(1) == MinOrMaxPhiR)
+      MinOrMaxOpValue = MinOrMaxOp->getOperand(0);
+    else
+      return false;
 
     VPValue *CmpOpA;
     VPValue *CmpOpB;
@@ -2085,8 +2092,6 @@ bool VPlanTransforms::handleMultiUseReductions(VPlan &Plan,
 
     VPInstruction *MinOrMaxResult =
         findUserOf<VPInstruction::ComputeReductionResult>(MinOrMaxOp);
-    assert(is_contained(MinOrMaxPhiR->users(), MinOrMaxOp) &&
-           "one user must be MinOrMaxOp");
     assert(MinOrMaxResult && "MinOrMaxResult must be a user of MinOrMaxOp");
 
     // Cmp must be used by the select of a FindLastIV chain.
diff --git a/llvm/test/Transforms/LoopVectorize/select-smax-last-index.ll b/llvm/test/Transforms/LoopVectorize/select-smax-last-index.ll
index 4477e8c3e0909..3235ab99df7ab 100644
--- a/llvm/test/Transforms/LoopVectorize/select-smax-last-index.ll
+++ b/llvm/test/Transforms/LoopVectorize/select-smax-last-index.ll
@@ -733,3 +733,47 @@ exit:
   ret i64 %res
 }
 
+define i64 @test_smax_idx_intermediate_smax_op(ptr %src, i64 %n, i64 %k) {
+; CHECK-LABEL: define i64 @test_smax_idx_intermediate_smax_op(
+; CHECK-SAME: ptr [[SRC:%.*]], i64 [[N:%.*]], i64 [[K:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    br label %[[LOOP:.*]]
+; CHECK:       [[LOOP]]:
+; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[IV_NEXT:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[MAX_IDX:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[MAX_IDX_NEXT:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[MAX_VAL:%.*]] = phi i64 [ -2147483648, %[[ENTRY]] ], [ [[MAX_VAL_NEXT:%.*]], %[[LOOP]] ]
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i64, ptr [[SRC]], i64 [[IV]]
+; CHECK-NEXT:    [[L:%.*]] = load i64, ptr [[GEP]], align 4
+; CHECK-NEXT:    [[TMP:%.*]] = call i64 @llvm.smax.i64(i64 [[K]], i64 [[MAX_VAL]])
+; CHECK-NEXT:    [[MAX_VAL_NEXT]] = call i64 @llvm.smax.i64(i64 [[L]], i64 [[TMP]])
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sle i64 [[MAX_VAL]], [[L]]
+; CHECK-NEXT:    [[MAX_IDX_NEXT]] = select i1 [[CMP]], i64 [[IV]], i64 [[MAX_IDX]]
+; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
+; CHECK-NEXT:    [[EXITCOND_NOT:%.*]] = icmp eq i64 [[IV_NEXT]], [[N]]
+; CHECK-NEXT:    br i1 [[EXITCOND_NOT]], label %[[EXIT:.*]], label %[[LOOP]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    [[RES:%.*]] = phi i64 [ [[MAX_IDX_NEXT]], %[[LOOP]] ]
+; CHECK-NEXT:    ret i64 [[RES]]
+;
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
+  %max.idx = phi i64 [ 0, %entry ], [ %max.idx.next, %loop ]
+  %max.val = phi i64 [ -2147483648, %entry ], [ %max.val.next, %loop ]
+  %gep = getelementptr i64, ptr %src, i64 %iv
+  %l = load i64, ptr %gep
+  %tmp = call i64 @llvm.smax.i64(i64 %k, i64 %max.val)
+  %max.val.next = call i64 @llvm.smax.i64(i64 %l, i64 %tmp)
+  %cmp = icmp sle i64 %max.val, %l
+  %max.idx.next = select i1 %cmp, i64 %iv, i64 %max.idx
+  %iv.next = add nuw nsw i64 %iv, 1
+  %exitcond.not = icmp eq i64 %iv.next, %n
+  br i1 %exitcond.not, label %exit, label %loop
+
+exit:
+  %res = phi i64 [ %max.idx.next, %loop ]
+  ret i64 %res
+}
+