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DAGCombiner: Assume invariant load cannot alias a store

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The motivation is to allow GatherAllAliases / FindBetterChain
to not give up on dependent loads of a pointer from constant memory.

This is important for AMDGPU, because most loads are pointers
derived from a load of a kernel argument from constant memory.

llvm-svn: 241948
This commit is contained in:
Matt Arsenault 2015-07-10 22:17:40 +00:00
parent a9909befd9
commit e1819eb4e3
2 changed files with 44 additions and 0 deletions
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9
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
View File

@ -13844,6 +13844,15 @@ bool DAGCombiner::isAlias(LSBaseSDNode *Op0, LSBaseSDNode *Op1) const {
// If they are both volatile then they cannot be reordered.
if (Op0->isVolatile() && Op1->isVolatile()) return true;
// If one operation reads from invariant memory, and the other may store, they
// cannot alias. These should really be checking the equivalent of mayWrite,
// but it only matters for memory nodes other than load /store.
if (Op0->isInvariant() && Op1->writeMem())
return false;
if (Op1->isInvariant() && Op0->writeMem())
return false;
// Gather base node and offset information.
SDValue Base1, Base2;
int64_t Offset1, Offset2;

35
test/CodeGen/AMDGPU/invariant-load-no-alias-store.ll Normal file
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@ -0,0 +1,35 @@
; RUN: llc -march=amdgcn -verify-machineinstrs < %s | FileCheck -check-prefix=GCN %s
; GatherAllAliases gives up on trying to analyze cases where the
; pointer may have been loaded from an aliased store, so make sure
; that this works and allows moving the stores to a better chain to
; allow them to be merged merged when it's clear the pointer is loaded
; from constant/invariant memory.
; GCN-LABEL: {{^}}test_merge_store_constant_i16_invariant_global_pointer_load:
; GCN: buffer_load_dwordx2 [[PTR:v\[[0-9]+:[0-9]+\]]],
; GCN: v_mov_b32_e32 [[K:v[0-9]+]], 0x1c8007b
; GCN: buffer_store_dword [[K]], [[PTR]]
define void @test_merge_store_constant_i16_invariant_global_pointer_load(i16 addrspace(1)* addrspace(1)* dereferenceable(4096) nonnull %in) #0 {
%ptr = load i16 addrspace(1)*, i16 addrspace(1)* addrspace(1)* %in, !invariant.load !0
%ptr.1 = getelementptr i16, i16 addrspace(1)* %ptr, i64 1
store i16 123, i16 addrspace(1)* %ptr, align 4
store i16 456, i16 addrspace(1)* %ptr.1
ret void
}
; GCN-LABEL: {{^}}test_merge_store_constant_i16_invariant_constant_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]], s{{\[}}[[SPTR_LO]]:
define void @test_merge_store_constant_i16_invariant_constant_pointer_load(i16 addrspace(1)* addrspace(2)* dereferenceable(4096) nonnull %in) #0 {
%ptr = load i16 addrspace(1)*, i16 addrspace(1)* addrspace(2)* %in, !invariant.load !0
%ptr.1 = getelementptr i16, i16 addrspace(1)* %ptr, i64 1
store i16 123, i16 addrspace(1)* %ptr, align 4
store i16 456, i16 addrspace(1)* %ptr.1
ret void
}
!0 = !{}
attributes #0 = { nounwind }