[llvm] [AMDGPU] Create new option for force flush load counter (PR #124974)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: Stanislav Mekhanoshin (rampitec)

<details>
<summary>Changes</summary>

In ceratin situations it is beneficial to wait for all outstanding
loads regardless of specific load's data we need. This may allow
to reduce a number of cache requests.

Fixes: SWDEV-511507

---
Full diff: https://github.com/llvm/llvm-project/pull/124974.diff


2 Files Affected:

- (modified) llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp (+8) 
- (added) llvm/test/CodeGen/AMDGPU/load-store-cnt.ll (+48) 


``````````diff
diff --git a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
index de2095fa60ffd4..3d6419778f4b1c 100644
--- a/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp
@@ -53,6 +53,11 @@ static cl::opt<bool>
                                "s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)"),
                       cl::init(false), cl::Hidden);
 
+static cl::opt<bool> ForceEmitZeroLoadFlag(
+    "amdgpu-waitcnt-load-forcezero",
+    cl::desc("Force all waitcnt load counters to wait until 0"),
+    cl::init(false), cl::Hidden);
+
 namespace {
 // Class of object that encapsulates latest instruction counter score
 // associated with the operand.  Used for determining whether
@@ -1850,6 +1855,9 @@ bool SIInsertWaitcnts::generateWaitcntInstBefore(MachineInstr &MI,
       Wait.BvhCnt = 0;
   }
 
+  if (ForceEmitZeroLoadFlag && Wait.LoadCnt != ~0u)
+    Wait.LoadCnt = 0;
+
   return generateWaitcnt(Wait, MI.getIterator(), *MI.getParent(), ScoreBrackets,
                          OldWaitcntInstr);
 }
diff --git a/llvm/test/CodeGen/AMDGPU/load-store-cnt.ll b/llvm/test/CodeGen/AMDGPU/load-store-cnt.ll
new file mode 100644
index 00000000000000..a7fccde4166713
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/load-store-cnt.ll
@@ -0,0 +1,48 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -march=amdgcn -mcpu=gfx1100 < %s | FileCheck --check-prefixes=DEFAULT %s
+; RUN: llc -march=amdgcn -mcpu=gfx1100 -amdgpu-waitcnt-load-forcezero < %s | FileCheck --check-prefixes=LDZERO %s
+
+define amdgpu_kernel void @copy(ptr addrspace(1) noalias nocapture readonly %src1, ptr addrspace(1) noalias nocapture readonly %src2, ptr addrspace(1) noalias nocapture writeonly %dst1, ptr addrspace(1) noalias nocapture writeonly %dst2) {
+; DEFAULT-LABEL: copy:
+; DEFAULT:       ; %bb.0:
+; DEFAULT-NEXT:    s_load_b256 s[0:7], s[4:5], 0x24
+; DEFAULT-NEXT:    v_and_b32_e32 v0, 0x3ff, v0
+; DEFAULT-NEXT:    s_delay_alu instid0(VALU_DEP_1)
+; DEFAULT-NEXT:    v_lshlrev_b32_e32 v0, 2, v0
+; DEFAULT-NEXT:    s_waitcnt lgkmcnt(0)
+; DEFAULT-NEXT:    s_clause 0x1
+; DEFAULT-NEXT:    global_load_b32 v1, v0, s[0:1]
+; DEFAULT-NEXT:    global_load_b32 v2, v0, s[2:3]
+; DEFAULT-NEXT:    s_waitcnt vmcnt(1)
+; DEFAULT-NEXT:    global_store_b32 v0, v1, s[4:5]
+; DEFAULT-NEXT:    s_waitcnt vmcnt(0)
+; DEFAULT-NEXT:    global_store_b32 v0, v2, s[6:7]
+; DEFAULT-NEXT:    s_endpgm
+;
+; LDZERO-LABEL: copy:
+; LDZERO:       ; %bb.0:
+; LDZERO-NEXT:    s_load_b256 s[0:7], s[4:5], 0x24
+; LDZERO-NEXT:    v_and_b32_e32 v0, 0x3ff, v0
+; LDZERO-NEXT:    s_delay_alu instid0(VALU_DEP_1)
+; LDZERO-NEXT:    v_lshlrev_b32_e32 v0, 2, v0
+; LDZERO-NEXT:    s_waitcnt lgkmcnt(0)
+; LDZERO-NEXT:    s_clause 0x1
+; LDZERO-NEXT:    global_load_b32 v1, v0, s[0:1]
+; LDZERO-NEXT:    global_load_b32 v2, v0, s[2:3]
+; LDZERO-NEXT:    s_waitcnt vmcnt(0)
+; LDZERO-NEXT:    s_clause 0x1
+; LDZERO-NEXT:    global_store_b32 v0, v1, s[4:5]
+; LDZERO-NEXT:    global_store_b32 v0, v2, s[6:7]
+; LDZERO-NEXT:    s_endpgm
+  %id = tail call i32 @llvm.amdgcn.workitem.id.x()
+  %idx = zext i32 %id to i64
+  %gep.ld1 = getelementptr inbounds nuw float, ptr addrspace(1) %src1, i64 %idx
+  %v1 = load float, ptr addrspace(1) %gep.ld1, align 4
+  %gep.ld2 = getelementptr inbounds nuw float, ptr addrspace(1) %src2, i64 %idx
+  %v2 = load float, ptr addrspace(1) %gep.ld2, align 4
+  %gep.st1 = getelementptr inbounds nuw float, ptr addrspace(1) %dst1, i64 %idx
+  store float %v1, ptr addrspace(1) %gep.st1, align 4
+  %gep.st2 = getelementptr inbounds nuw float, ptr addrspace(1) %dst2, i64 %idx
+  store float %v2, ptr addrspace(1) %gep.st2, align 4
+  ret void
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/124974