[llvm] [AMDGPU] Remove scope check in SIInsertWaitcnts::generateWaitcntInstBefore (PR #157821)

[via llvm-commits]

https://github.com/choikwa updated https://github.com/llvm/llvm-project/pull/157821

>From 55b34c2a5de061fb77350bf3d0071b0b93bf82ef Mon Sep 17 00:00:00 2001
From: Kevin Choi <kevin.choi at amd.com>
Date: Wed, 10 Sep 2025 01:00:15 -0500
Subject: [PATCH] [AMDGPU] Remove scope check in
 SIInsertWaitcnts::generateWaitcntInstBefore

This change was motivated by CK where many VMCNT(0)'s were generated due
to instructions lacking !alias.scope metadata. The two causes of this were
1) LowerLDSModule not tacking on scope metadata on a single LDS variable
2) IPSCCP pass before inliner replacing noalias ptr derivative with a
   global value, which made inliner unable to track it back to the noalias
   ptr argument.

However, it turns out that IPSCCP losing the scope information was largely
ineffectual as ScopedNoAliasAA was able to handle asymmetric condition,
where one MemLoc was missing scope, and still return NoAlias result.

AMDGPU however was checking for existence of scope in SIInsertWaitcnts and
conservatively treating it as aliasing all and inserted VMCNT(0) before
DS_READs, forcing it to wait for all previous LDS DMA instructions.

Since we know that ScopedNoAliasAA can handle asymmetry, we should also
allow AA query to determine if two MIs may alias.

Remove confusing comments, redundant check-prefix, move function attribute

turn into positive checks via script
---
 llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp  | 8 +-------
 llvm/test/CodeGen/AMDGPU/waitcnt-unscoped.ll | 1 -
 2 files changed, 1 insertion(+), 8 deletions(-)

diff --git a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
index b163a274396ff..ae75fb529dade 100644
--- a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
@@ -1941,13 +1941,7 @@ bool SIInsertWaitcnts::generateWaitcntInstBefore(MachineInstr &MI,
 
         // LOAD_CNT is only relevant to vgpr or LDS.
         unsigned RegNo = FIRST_LDS_VGPR;
-        // Only objects with alias scope info were added to LDSDMAScopes array.
-        // In the absense of the scope info we will not be able to disambiguate
-        // aliasing here. There is no need to try searching for a corresponding
-        // store slot. This is conservatively correct because in that case we
-        // will produce a wait using the first (general) LDS DMA wait slot which
-        // will wait on all of them anyway.
-        if (Ptr && Memop->getAAInfo() && Memop->getAAInfo().Scope) {
+        if (Ptr && Memop->getAAInfo()) {
           const auto &LDSDMAStores = ScoreBrackets.getLDSDMAStores();
           for (unsigned I = 0, E = LDSDMAStores.size(); I != E; ++I) {
             if (MI.mayAlias(AA, *LDSDMAStores[I], true))
diff --git a/llvm/test/CodeGen/AMDGPU/waitcnt-unscoped.ll b/llvm/test/CodeGen/AMDGPU/waitcnt-unscoped.ll
index 0bd8667d17e52..a00aca34252b1 100644
--- a/llvm/test/CodeGen/AMDGPU/waitcnt-unscoped.ll
+++ b/llvm/test/CodeGen/AMDGPU/waitcnt-unscoped.ll
@@ -26,7 +26,6 @@ define amdgpu_kernel void @test_waitcnt(ptr addrspace(1) %global_buffer, ptr add
 ; CHECK-NEXT:    ds_write_b32 v1, v3
 ; CHECK-NEXT:    ds_write_b32 v2, v3
 ; CHECK-NEXT:    ; sched_barrier mask(0x00000000)
-; CHECK-NEXT:    s_waitcnt vmcnt(0)
 ; CHECK-NEXT:    ds_read_b32 v1, v1
 ; CHECK-NEXT:    s_waitcnt lgkmcnt(0)
 ; CHECK-NEXT:    global_store_dword v0, v1, s[0:1] offset:16