The premise here is to allow non-kernel functions to locate external LDS variables without using LDS or extra magic SGPRs to do so.
1/ First it crawls the callgraph to work out which external LDS variables are reachable from a given kernel
2/ Then it creates a new `extern char[0]` variable for each kernel, which will alias all the other extern LDS variables because that's the documented behaviour of these variables
3/ The address of that variable is written to a lookup table. The global variable is tagged with metadata to track what address it was allocated at by codegen
4/ The assembler builds the lookup table using the metadata
5/ Any non-kernel functions use the same magic intrinsic used by table lookups of non-dynamic LDS variables to find the address to use
Heavy overlap with the code paths taken for other lowering, in particular the same intrinsic is used to pass the dynamic scope information through the same sgpr as for table lookups of static LDS.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D144233
Post ISel, LDS variables are absolute values. Representing them as
such is simpler than the frame recalculation currently used to build assembler
tables from their addresses.
This is a precursor to lowering dynamic/external LDS accesses from non-kernel
functions.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D144221
AMDGPU implements some handy code for expanding all constexpr
users of LDS globals. Extract the core logic into ReplaceConstant,
so that it can be reused elsewhere.
Renames the current lowering scheme to "module" and introduces two new
ones, "kernel" and "table", plus a "hybrid" that chooses between those three
on a per-variable basis.
Unit tests are set up to pass with the default lowering of "module" or "hybrid"
with this patch defaulting to "module", which will be a less dramatic codegen
change relative to the current. This reflects the sparsity of test coverage for
the table lowering method. Hybrid is better than module in every respect and
will be default in a subsequent patch.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D139433
Renames the current lowering scheme to "module" and introduces two new
ones, "kernel" and "table", plus a "hybrid" that chooses between those three
on a per-variable basis.
Unit tests are set up to pass with the default lowering of "module" or "hybrid"
with this patch defaulting to "module", which will be a less dramatic codegen
change relative to the current. This reflects the sparsity of test coverage for
the table lowering method. Hybrid is better than module in every respect and
will be default in a subsequent patch.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D139433
isKernelCC != isKernel(F->getCallingConv())
There's a test case (lower-kernel-lds.ll) that explicitly skips amdgpu_ps
so this change picks the isKernel predicate that continues to skip that
calling convention.
isKernel returns true for AMDGPU_KERNEL and SPIR_KERNEL. isKernelCC also
returns true for other calling conventions.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D136599
The association between kernel and struct is done by symbol name.
This doesn't work robustly for anonymous kernels as shown by the modified
test case.
An alternative association between function and struct can be constructed
if necessary, probably though metadata, but on the basis that we currently
miscompile anonymous kernels and that they are difficult to construct from
application code and difficult to call from the runtime, this patch makes
it a fatal error for now.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D134741
Bug noted in D112717 can be sidestepped with this change.
Expanding all ConstantExpr involved with LDS up front makes the variable specialisation simpler. Excludes ConstantExpr that don't access LDS to avoid disturbing codegen elsewhere.
Reviewed By: rampitec
Differential Revision: https://reviews.llvm.org/D133422
Currently LDS variables are removed by the lower module pass
if they have a use which is caught by the replace with struct control flow.
This makes tests brittle to changes to that control flow which induces
noise when trying to improve lowering. Some tests already check that
variables are removed, while others checked that they are not removed.
LDS variables are not (currently) externally accessible, and if that
changes the machinery which makes them externally accessible will look
like a use. This change therefore breaks no applications.
Reviewed By: rampitec
Differential Revision: https://reviews.llvm.org/D133028
It errors out in the Bazel CI:
AMDGPULowerModuleLDSPass.cpp:384:12: error: chosen constructor is
explicit in copy-initialization
return {SGV, std::move(Map)};
Reviewed By: rupprecht
Differential Revision: https://reviews.llvm.org/D130623
Introduces a string attribute, amdgpu-requires-module-lds, to allow
eliding the module.lds block from kernels. Will allocate the block as before
if the attribute is missing or has its default value of true.
Patch uses the new attribute to detect the simplest possible instance of this,
where a kernel makes no calls and thus cannot call any functions that use LDS.
Tests updated to match, coverage was already good. Interesting cases is in
lower-module-lds-offsets where annotating the kernel allows the backend to pick
a different (in this case better) variable ordering than previously. A later
patch will avoid moving kernel variables into module.lds when the kernel can
have this attribute, allowing optimal ordering and locally unused variable
elimination.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D122091
Currently the superalign option only increases the alignment of
variables that are moved into the module.lds block. Change that to all LDS
variables. Also only increase the alignment once, instead of once per function.
Reviewed By: rampitec
Differential Revision: https://reviews.llvm.org/D115488
When adding alias.scope and noalias metadata to a memcpy function,
the alias.scope and noalias metadata from the operands are merged.
The rule for merging alias.scope is to take the intersection of
the domains and the union of the scopes within those domains.
The rule for merging noalias is to take the intersection.
The bug is that AMDGPULowerModuleLDS was using concatenation for
both alias.scope and noalias. For example, when f1 and f2 are added
to the LDS structure and there is a memcpy(f2, f1, sizeof(f1)).
Then, concatenation creates noalias metadata for the memcpy that
includes both {f1, f2}. That means that the memcpy is assumed
not to alias a prior load of f2, which enables the optimizer to
remove a load of f2 that occurs after mempcy.
The function MDNode::getmostGenericAliasScope defines the semantics
for alias.scope. There is a function, combineMetadata in Local.cpp,
that uses intersect for noalias.
Differential Revision: https://reviews.llvm.org/D110049
Alias analysis is unable to disambiguate accesses to the structure
fields without it unlike distinct variables. As a result we cannot
combine ds_read and ds_write operations in a case of any store in
between which always considered clobbering.
Differential Revision: https://reviews.llvm.org/D108315
The main motivation behind pointer replacement of LDS use within non-kernel
functions is - to *avoid* subsequent LDS lowering pass from directly packing
LDS (assume large LDS) into a struct type which would otherwise cause allocating
huge memory for struct instance within every kernel.
Reviewed By: rampitec
Differential Revision: https://reviews.llvm.org/D103225
