https://github.com/kasuga-fj created
https://github.com/llvm/llvm-project/pull/133665
There is a problem with the current profitability check for vectorization in
LoopInterchange. There are both false positives and false negatives. The former
means that the heuristic may say that "an exchange is necessary to vectorize
the innermost loop" even though it's already possible. The latter means that
the heuristic may miss a case where an exchange is necessary to vectorize the
innermost loop. Note that this is not a dependency analysis problem. These
problems can occur even if the analysis is accurate (no overestimation).
This patch adds tests to clarify the cases that should be fixed. The root cause
of these cases is that the heuristic doesn't handle the direction of a
dependency correctly.
>From b53b7ce2b303ff9ea94d77b3ffe74d1697db9f3d Mon Sep 17 00:00:00 2001
From: Ryotaro Kasuga <kasuga.ryot...@fujitsu.com>
Date: Thu, 27 Mar 2025 07:04:27 +0000
Subject: [PATCH] [LoopInterchange] Add tests for the vectorization
profitability (NFC)
There is a problem with the current profitability check for
vectorization in LoopInterchange. There are both false positives and
false negatives. The former means that the heuristic may say that "an
exchange is necessary to vectorize the innermost loop" even though it's
already possible. The latter means that the heuristic may miss a case
where an exchange is necessary to vectorize the innermost loop. Note
that this is not a dependency analysis problem. These problems can
occur even if the analysis is accurate (no overestimation).
This patch adds tests to clarify the cases that should be fixed. The
root cause of these cases is that the heuristic doesn't handle the
direction of a dependency correctly.
---
.../profitability-vectorization-heuristic.ll | 108 ++++++++++++++++++
1 file changed, 108 insertions(+)
create mode 100644
llvm/test/Transforms/LoopInterchange/profitability-vectorization-heuristic.ll
diff --git
a/llvm/test/Transforms/LoopInterchange/profitability-vectorization-heuristic.ll
b/llvm/test/Transforms/LoopInterchange/profitability-vectorization-heuristic.ll
new file mode 100644
index 0000000000000..606117e70db86
--- /dev/null
+++
b/llvm/test/Transforms/LoopInterchange/profitability-vectorization-heuristic.ll
@@ -0,0 +1,108 @@
+; RUN: opt < %s -passes=loop-interchange -cache-line-size=64 \
+; RUN: -pass-remarks-output=%t -disable-output
-loop-interchange-profitabilities=vectorize
+; RUN: FileCheck -input-file %t %s
+
+@A = dso_local global [256 x [256 x float]] zeroinitializer
+@B = dso_local global [256 x [256 x float]] zeroinitializer
+@C = dso_local global [256 x [256 x float]] zeroinitializer
+
+; Check that the below loops are exchanged for vectorization.
+;
+; for (int i = 0; i < 256; i++) {
+; for (int j = 1; j < 256; j++) {
+; A[i][j] = A[i][j-1] + B[i][j];
+; C[i][j] += 1;
+; }
+; }
+;
+; FIXME: These loops are not exchanged at this time due to the problem of
+; profitablity heuristic for vectorization.
+
+; CHECK: --- !Missed
+; CHECK-NEXT: Pass: loop-interchange
+; CHECK-NEXT: Name: InterchangeNotProfitable
+; CHECK-NEXT: Function: interchange_necesasry_for_vectorization
+; CHECK-NEXT: Args:
+; CHECK-NEXT: - String: Interchanging loops is not considered to
improve cache locality nor vectorization.
+; CHECK-NEXT: ...
+define void @interchange_necesasry_for_vectorization() {
+entry:
+ br label %for.i.header
+
+for.i.header:
+ %i = phi i64 [ 1, %entry ], [ %i.next, %for.i.inc ]
+ br label %for.j.body
+
+for.j.body:
+ %j = phi i64 [ 1, %for.i.header ], [ %j.next, %for.j.body ]
+ %j.dec = add nsw i64 %j, -1
+ %a.load.index = getelementptr nuw inbounds [256 x [256 x float]], ptr @A,
i64 %i, i64 %j.dec
+ %b.index = getelementptr nuw inbounds [256 x [256 x float]], ptr @B, i64 %i,
i64 %j
+ %c.index = getelementptr nuw inbounds [256 x [256 x float]], ptr @C, i64 %i,
i64 %j
+ %a = load float, ptr %a.load.index, align 4
+ %b = load float, ptr %b.index, align 4
+ %c = load float, ptr %c.index, align 4
+ %add.0 = fadd float %a, %b
+ %a.store.index = getelementptr nuw inbounds [256 x [256 x float]], ptr @A,
i64 %i, i64 %j
+ store float %add.0, ptr %a.store.index, align 4
+ %add.1 = fadd float %c, 1.0
+ store float %add.1, ptr %c.index, align 4
+ %j.next = add nuw nsw i64 %j, 1
+ %cmp.j = icmp eq i64 %j.next, 256
+ br i1 %cmp.j, label %for.i.inc, label %for.j.body
+
+for.i.inc:
+ %i.next = add nuw nsw i64 %i, 1
+ %cmp.i = icmp eq i64 %i.next, 256
+ br i1 %cmp.i, label %exit, label %for.i.header
+
+exit:
+ ret void
+}
+
+; Check that the following innermost loop can be vectorized so that
+; interchangig is unnecessary.
+;
+; for (int i = 0; i < 256; i++)
+; for (int j = 1; j < 256; j++)
+; A[i][j-1] = A[i][j] + B[i][j];
+;
+; FIXME: These loops are exchanged at this time due to the problem of
+; profitablity heuristic for vectorization.
+
+; CHECK: --- !Passed
+; CHECK-NEXT: Pass: loop-interchange
+; CHECK-NEXT: Name: Interchanged
+; CHECK-NEXT: Function: interchange_unnecesasry_for_vectorization
+; CHECK-NEXT: Args:
+; CHECK-NEXT: - String: Loop interchanged with enclosing loop.
+define void @interchange_unnecesasry_for_vectorization() {
+entry:
+ br label %for.i.header
+
+for.i.header:
+ %i = phi i64 [ 1, %entry ], [ %i.next, %for.i.inc ]
+ br label %for.j.body
+
+for.j.body:
+ %j = phi i64 [ 1, %for.i.header ], [ %j.next, %for.j.body ]
+ %j.dec = add nsw i64 %j, -1
+ %a.load.index = getelementptr nuw inbounds [256 x [256 x float]], ptr @A,
i64 %i, i64 %j
+ %b.index = getelementptr nuw inbounds [256 x [256 x float]], ptr @B, i64 %i,
i64 %j
+ %a = load float, ptr %a.load.index, align 4
+ %b = load float, ptr %b.index, align 4
+ %add = fadd float %a, %b
+ %a.store.index = getelementptr nuw inbounds [256 x [256 x float]], ptr @A,
i64 %i, i64 %j.dec
+ store float %add, ptr %a.store.index, align 4
+ %j.next = add nuw nsw i64 %j, 1
+ %cmp.j = icmp eq i64 %j.next, 256
+ br i1 %cmp.j, label %for.i.inc, label %for.j.body
+
+for.i.inc:
+ %i.next = add nuw nsw i64 %i, 1
+ %cmp.i = icmp eq i64 %i.next, 256
+ br i1 %cmp.i, label %exit, label %for.i.header
+
+exit:
+ ret void
+}
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