[libc] [libcxx] [llvm] [clang] [flang] [clang-tools-extra] [compiler-rt] [LAA] Add a test case to show incorrect dependency classification (NFC). (PR #70473)

[Alexandros Lamprineas via cfe-commits]

https://github.com/labrinea updated https://github.com/llvm/llvm-project/pull/70473

>From 454d423f0018db65dbbc0739ddf9ecbb8d38fef6 Mon Sep 17 00:00:00 2001
From: Alexandros Lamprineas <alexandros.lamprineas at arm.com>
Date: Fri, 27 Oct 2023 16:45:11 +0100
Subject: [PATCH] [LAA] Add a test case to show incorrect dependency
 classification (NFC).

Currently the loop access analysis classifies this loop as unsafe to vectorize
because the memory dependencies are 'ForwardButPreventsForwarding'. However,
the access pattern is 'write-after-read' with no subsequent read accessing
the written memory locations. I can't see how store-to-load forwarding is
applicable here.

void vectorizable_Read_Write(int *A) {
  for (unsigned i = 1022; i >= 0; i--)
    A[i+1] = A[i] + 1;
}
---
 .../forward-negative-step.ll                  | 40 +++++++++++++++++++
 1 file changed, 40 insertions(+)
 create mode 100644 llvm/test/Analysis/LoopAccessAnalysis/forward-negative-step.ll

diff --git a/llvm/test/Analysis/LoopAccessAnalysis/forward-negative-step.ll b/llvm/test/Analysis/LoopAccessAnalysis/forward-negative-step.ll
new file mode 100644
index 000000000000000..89c1737fb730513
--- /dev/null
+++ b/llvm/test/Analysis/LoopAccessAnalysis/forward-negative-step.ll
@@ -0,0 +1,40 @@
+; RUN: opt -passes='print<access-info>' -disable-output  < %s 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-i128:128-n32:64-S128"
+
+; FIXME: This should be vectorizable
+
+; void vectorizable_Read_Write(int *A) {
+;  for (unsigned i = 1022; i >= 0; i--)
+;    A[i+1] = A[i] + 1;
+; }
+
+; CHECK: function 'vectorizable_Read_Write':
+; CHECK-NEXT:   for.body:
+; CHECK-NEXT:     Report: unsafe dependent memory operations in loop
+; CHECK-NEXT:     Forward loop carried data dependence that prevents store-to-load forwarding.
+; CHECK-NEXT:     Dependences:
+; CHECK-NEXT:       ForwardButPreventsForwarding:
+; CHECK-NEXT:           %0 = load i32, ptr %arrayidx, align 4 ->
+; CHECK-NEXT:           store i32 %add, ptr %gep, align 4
+
+define void @vectorizable_Read_Write(ptr nocapture %A) {
+entry:
+  %invariant.gep = getelementptr i32, ptr %A, i64 1
+  br label %for.body
+
+for.cond.cleanup:
+  ret void
+
+for.body:
+  %indvars.iv = phi i64 [ 1022, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds i32, ptr %A, i64 %indvars.iv
+  %0 = load i32, ptr %arrayidx, align 4
+  %add = add nsw i32 %0, 1
+  %gep = getelementptr i32, ptr %invariant.gep, i64 %indvars.iv
+  store i32 %add, ptr %gep, align 4
+  %indvars.iv.next = add nsw i64 %indvars.iv, -1
+  %cmp.not = icmp eq i64 %indvars.iv, 0
+  br i1 %cmp.not, label %for.cond.cleanup, label %for.body
+}
+