[llvm] [AArch64] Define cost of i16->i32 udot/sdot instructions (PR #174102)

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm created https://github.com/llvm/llvm-project/pull/174102

i16 -> i32 dot-product operations are natively supported with SVE2p1 and SME2. This updates the cost-model so that those operations are recognized as cheap by the LoopVectorizer.

>From e077583b3c185103d26f7f9f73ce713f99b73f93 Mon Sep 17 00:00:00 2001
From: Sander de Smalen <sander.desmalen at arm.com>
Date: Wed, 31 Dec 2025 14:23:13 +0000
Subject: [PATCH] [AArch64] Define cost of i16->i32 udot/sdot instructions

i16 -> i32 dot-product operations are natively supported with
SVE2p1 and SME2. This updates the cost-model so that those operations
are recognized as cheap by the LoopVectorizer.
---
 .../AArch64/AArch64TargetTransformInfo.cpp    |  5 +++
 .../partial-reduce-add-sdot-i16-i32.ll        | 37 +++++++++++++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 llvm/test/Transforms/LoopVectorize/AArch64/partial-reduce-add-sdot-i16-i32.ll

diff --git a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
index fdf973d0cf1b7..ad2d50a28d284 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
@@ -5915,6 +5915,11 @@ InstructionCost AArch64TTIImpl::getPartialReductionCost(
     if (AccumLT.second.getScalarType() == MVT::i64 &&
         InputLT.second.getScalarType() == MVT::i16)
       return Cost;
+    // i16 -> i32 is natively supported with SVE2p1
+    if (AccumLT.second.getScalarType() == MVT::i32 &&
+        InputLT.second.getScalarType() == MVT::i16 &&
+        (ST->hasSVE2p1() || ST->hasSME2()))
+      return Cost;
     // i8 -> i64 is supported with an extra level of extends
     if (AccumLT.second.getScalarType() == MVT::i64 &&
         InputLT.second.getScalarType() == MVT::i8)
diff --git a/llvm/test/Transforms/LoopVectorize/AArch64/partial-reduce-add-sdot-i16-i32.ll b/llvm/test/Transforms/LoopVectorize/AArch64/partial-reduce-add-sdot-i16-i32.ll
new file mode 100644
index 0000000000000..d090f61a47821
--- /dev/null
+++ b/llvm/test/Transforms/LoopVectorize/AArch64/partial-reduce-add-sdot-i16-i32.ll
@@ -0,0 +1,37 @@
+; REQUIRES: asserts
+; RUN: opt -passes=loop-vectorize -force-vector-interleave=1 -force-vector-width=8 \
+; RUN:     -enable-epilogue-vectorization=false -debug-only=loop-vectorize         \
+; RUN:     -mattr=+sve2p1 -scalable-vectorization=off                              \
+; RUN:     -disable-output < %s 2>&1 | FileCheck %s --check-prefix=CHECK-FIXED
+; RUN: opt -passes=loop-vectorize -force-vector-interleave=1 -force-vector-width=8 \
+; RUN:     -enable-epilogue-vectorization=false -debug-only=loop-vectorize         \
+; RUN:     -mattr=+sve2p1 -scalable-vectorization=on                               \
+; RUN:     -disable-output < %s 2>&1 | FileCheck %s --check-prefix=CHECK-SCALABLE
+; RUN: opt -passes=loop-vectorize -force-vector-interleave=1 -force-vector-width=8 \
+; RUN:     -enable-epilogue-vectorization=false -debug-only=loop-vectorize         \
+; RUN:     -mattr=+sve2,+sme2 -scalable-vectorization=on                           \
+; RUN:     -disable-output < %s 2>&1 | FileCheck %s --check-prefix=CHECK-SCALABLE
+
+; CHECK-FIXED: Cost of 1 for VF 8: EXPRESSION vp<%8> = ir<%acc> + partial.reduce.add (ir<%load> sext to i32)
+; CHECK-SCALABLE: Cost of 1 for VF vscale x 8: EXPRESSION vp<%8> = ir<%acc> + partial.reduce.add (ir<%load> sext to i32)
+
+target triple = "aarch64"
+
+define i32 @sext_reduction_i16_to_i32(ptr %arr, i32 %n) vscale_range(1,16) {
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %loop ]
+  %acc = phi i32 [ 0, %entry ], [ %add, %loop ]
+  %gep = getelementptr inbounds i16, ptr %arr, i32 %iv
+  %load = load i16, ptr %gep
+  %sext = sext i16 %load to i32
+  %add = add i32 %acc, %sext
+  %iv.next = add i32 %iv, 1
+  %cmp = icmp ult i32 %iv.next, %n
+  br i1 %cmp, label %loop, label %exit
+
+exit:
+  ret i32 %add
+}