AMDGPU: Handle invariant loads when considering if a load can be scalar

[arsenm]

Doesn't touch the globalisel version because the handling
there looks a bit broken.

[arsenm]

• AMDGPU: Use ConstantPool as source value for DAG lowered kernarg loads #168917 : 2 dependent PRs (#168980 , #170309 )
• AMDGPU: Handle invariant when lowering global loads #168914
• AMDGPU: Add baseline test for split/widen invariant loads #168913
• AMDGPU: Handle invariant loads when considering if a load can be scalar #168787 👈 (View in Graphite)
• Reapply "DAG: Allow select ptr combine for non-0 address spaces" (#168292) #168786
• AMDGPU: Fix treating divergent loads as uniform #168785
• main

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[llvmbot]

@llvm/pr-subscribers-backend-amdgpu

Author: Matt Arsenault (arsenm)

Changes

Doesn't touch the globalisel version because the handling
there looks a bit broken.

---

Full diff: https://github.com/llvm/llvm-project/pull/168787.diff

2 Files Affected:

• (modified) llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp (+2-1)
• (modified) llvm/test/CodeGen/AMDGPU/invariant-load-no-alias-store.ll (+14-1)

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp b/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp
index 6a0a9e3d3e5ac..6c36f8ad9b6a9 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp
@@ -4437,7 +4437,8 @@ bool AMDGPUDAGToDAGISel::isUniformLoad(const SDNode *N) const {
          Ld->getAlign() >=
              Align(std::min(MMO->getSize().getValue().getKnownMinValue(),
                             uint64_t(4))) &&
-         ((Ld->getAddressSpace() == AMDGPUAS::CONSTANT_ADDRESS ||
+         (MMO->isInvariant() ||
+          (Ld->getAddressSpace() == AMDGPUAS::CONSTANT_ADDRESS ||
            Ld->getAddressSpace() == AMDGPUAS::CONSTANT_ADDRESS_32BIT) ||
           (Subtarget->getScalarizeGlobalBehavior() &&
            Ld->getAddressSpace() == AMDGPUAS::GLOBAL_ADDRESS &&
diff --git a/llvm/test/CodeGen/AMDGPU/invariant-load-no-alias-store.ll b/llvm/test/CodeGen/AMDGPU/invariant-load-no-alias-store.ll
index 6815050d0a441..23970d454526c 100644
--- a/llvm/test/CodeGen/AMDGPU/invariant-load-no-alias-store.ll
+++ b/llvm/test/CodeGen/AMDGPU/invariant-load-no-alias-store.ll
@@ -10,7 +10,7 @@
 ; GCN-DAG: buffer_load_dwordx2 [[PTR:v\[[0-9]+:[0-9]+\]]],
 ; GCN-DAG: v_mov_b32_e32 [[K:v[0-9]+]], 0x1c8007b
 ; GCN: buffer_store_dword [[K]], [[PTR]]
-define amdgpu_kernel void @test_merge_store_constant_i16_invariant_global_pointer_load(ptr addrspace(1) dereferenceable(4096) nonnull %in) #0 {
+define void @test_merge_store_constant_i16_invariant_global_pointer_load(ptr addrspace(1) dereferenceable(4096) nonnull %in) #0 {
   %ptr = load ptr addrspace(1), ptr addrspace(1) %in, !invariant.load !0
   %ptr.1 = getelementptr i16, ptr addrspace(1) %ptr, i64 1
   store i16 123, ptr addrspace(1) %ptr, align 4
@@ -30,6 +30,19 @@ define amdgpu_kernel void @test_merge_store_constant_i16_invariant_constant_poin
   ret void
 }
 
+; Invariant global load should be equivalently handled to constant.
+; GCN-LABEL: {{^}}test_merge_store_global_i16_invariant_uniform_global_pointer_load:
+; GCN: s_load_dwordx2 s[[[SPTR_LO:[0-9]+]]:[[SPTR_HI:[0-9]+]]]
+; GCN: v_mov_b32_e32 [[K:v[0-9]+]], 0x1c8007b
+; GCN: buffer_store_dword [[K]], off, s[[[SPTR_LO]]:
+define amdgpu_kernel void @test_merge_store_global_i16_invariant_uniform_global_pointer_load(ptr addrspace(1) dereferenceable(4096) nonnull %in) #0 {
+  %ptr = load ptr addrspace(1), ptr addrspace(1) %in, !invariant.load !0
+  %ptr.1 = getelementptr i16, ptr addrspace(1) %ptr, i64 1
+  store i16 123, ptr addrspace(1) %ptr, align 4
+  store i16 456, ptr addrspace(1) %ptr.1
+  ret void
+}
+
 !0 = !{}
 
 attributes #0 = { nounwind }

[github-actions]

🐧 Linux x64 Test Results

• 186425 tests passed
• 4867 tests skipped

[jayfoad]

This is saying that invariant loads can be selected to SMEM instructions regardless of any "no clobber" check, right? I think that's safe...