The CWSR trap handler needs to save and restore the VGPRs. When dynamic
VGPRs are in use, the fixed function hardware will only allocate enough
space for one VGPR block. The rest will have to be stored in scratch, at
offset 0.
This patch allocates the necessary space by:
- generating a prologue that checks at runtime if we're on a compute
queue (since CWSR only works on compute queues); for this we will have
to check the ME_ID bits of the ID_HW_ID2 register - if that is non-zero,
we can assume we're on a compute queue and initialize the SP and FP with
enough room for the dynamic VGPRs
- forcing all compute entry functions to use a FP so they can access
their locals/spills correctly (this isn't ideal but it's the quickest to
implement)
Note that at the moment we allocate enough space for the theoretical
maximum number of VGPRs that can be allocated dynamically (for blocks of
16 registers, this will be 128, of which we subtract the first 16, which
are already allocated by the fixed function hardware). Future patches
may decide to allocate less if they can prove the shader never allocates
that many blocks.
Also note that this should not affect any reported stack sizes (e.g. PAL
backend_stack_size etc).
This performs the minimal replacment of amdgpu-no-agpr to
amdgpu-agpr-alloc=0. Most of the test diffs are due to the new
attribute sorting later alphabetically.
We could do better by trying to perform range merging in the attributor,
and trying to pick non-0 values.
Remove the MIR scan to detect whether AGPRs are used or not,
and the special case for callable functions. This behavior was
confusing, and not overridable. The amdgpu-no-agpr attribute was
intended to avoid this imprecise heuristic for how many AGPRs to
allocate. It was also too confusing to make this interact with
the pending amdgpu-num-agpr replacement for amdgpu-no-agpr.
Also adds an xfail-ish test where the register allocator asserts
after allocation fails which I ran into.
Future work should reintroduce a more refined MIR scan to estimate
AGPR pressure for how to split AGPRs and VGPRs.
Occupancy (i.e., the number of waves per EU) depends, in addition to
register usage, on per-workgroup LDS usage as well as on the range of
possible workgroup sizes. Mirroring the latter, occupancy should
therefore be expressed as a range since different group sizes generally
yield different achievable occupancies.
`getOccupancyWithLocalMemSize` currently returns a scalar occupancy
based on the maximum workgroup size and LDS usage. With respect to the
workgroup size range, this scalar can be the minimum, the maximum, or
neither of the two of the range of achievable occupancies. This commit
fixes the function by making it compute and return the range of
achievable occupancies w.r.t. workgroup size and LDS usage; it also
renames it to `getOccupancyWithWorkGroupSizes` since it is the range of
workgroup sizes that produces the range of achievable occupancies.
Computing the achievable occupancy range is surprisingly involved.
Minimum/maximum workgroup sizes do not necessarily yield maximum/minimum
occupancies i.e., sometimes workgroup sizes inside the range yield the
occupancy bounds. The implementation finds these sizes in constant time;
heavy documentation explains the rationale behind the sometimes
relatively obscure calculations.
As a justifying example, consider a target with 10 waves / EU, 4 EUs/CU,
64-wide waves. Also consider a function with no LDS usage and a flat
workgroup size range of [513,1024].
- A group of 513 items requires 9 waves per group. Only 4 groups made up
of 9 waves each can fit fully on a CU at any given time, for a total of
36 waves on the CU, or 9 per EU. However, filling as much as possible
the remaining 40-36=4 wave slots without decreasing the number of groups
reveals that a larger group of 640 items yields 40 waves on the CU, or
10 per EU.
- Similarly, a group of 1024 items requires 16 waves per group. Only 2
groups made up of 16 waves each can fit fully on a CU ay any given time,
for a total of 32 waves on the CU, or 8 per EU. However, removing as
many waves as possible from the groups without being able to fit another
equal-sized group on the CU reveals that a smaller group of 896 items
yields 28 waves on the CU, or 7 per EU.
Therefore the achievable occupancy range for this function is not [8,9]
as the group size bounds directly yield, but [7,10].
Naturally this change causes a lot of test churn as instruction
scheduling is driven by achievable occupancy estimates. In most unit
tests the flat workgroup size range is the default [1,1024] which,
ignoring potential LDS limitations, would previously produce a scalar
occupancy of 8 (derived from 1024) on a lot of targets, whereas we now
consider the maximum occupancy to be 10 in such cases. Most tests are
updated automatically and checked manually for sanity. I also manually
changed some non-automatically generated assertions when necessary.
Fixes#118220.
Add a prologue to the kernel entry to handle cases where code designed
for kernarg preloading is executed on hardware equipped with
incompatible firmware. If hardware has compatible firmware the 256 bytes
at the start of the kernel entry will be skipped. This skipping is done
automatically by hardware that supports the feature.
A pass is added which is intended to be run at the very end of the
pipeline to avoid any optimizations that would assume the prologue is a
real predecessor block to the actual code start. In reality we have two
possible entry points for the function. 1. The optimized path that
supports kernarg preloading which begins at an offset of 256 bytes. 2.
The backwards compatible entry point which starts at offset 0.
Allocating wwm-registers and per-thread VGPR operands
together imposes many challenges in the way the
registers are reused during allocation. There are
times when regalloc reuses the registers of regular
VGPRs operations for wwm-operations in a small range
leading to unwantedly clobbering their inactive lanes
causing correctness issues that are hard to trace.
This patch splits the VGPR allocation pipeline further
to allocate wwm-registers first and the regular VGPR
operands in a separate pipeline. The splitting would
ensure that the physical registers used for wwm
allocations won't take part in the next allocation
pipeline to avoid any such clobbering.
Multiple conditions exist to decide whether callee save spills/restores
are required for amdgpu_cs_chain or amdgpu_cs_chain_preserve calling
conventions. This patch consolidates them all and moves to a single
place.
It is almost always simpler to use {} instead of std::nullopt to
initialize an empty ArrayRef. This patch changes all occurrences I could
find in LLVM itself. In future the ArrayRef(std::nullopt_t) constructor
could be deprecated or removed.
SIMachineFunctionInfo has a scan of the function body for inline asm
which may use AGPRs, or callees in SIMachineFunctionInfo. Move this
into the attributor, so it actually works interprocedurally.
Could probably avoid most of the test churn if this bothered to avoid
adding this on subtargets without AGPRs. We should also probably
try to delete the MIR scan in usesAGPRs but it seems to be trickier
to eliminate.
A new function attribute named amdgpu_num_work_groups is added. This
attribute, which consists of three integers, allows programmers to let
the compiler know the number of workgroups to be launched in each of the
three dimensions and do optimizations based on that information.
---------
Co-authored-by: Jun Wang <jun.wang7@amd.com>
At the moment, the emergency spill slot is a fixed object for entry
functions and chain functions, and a regular stack object otherwise.
This patch adopts the latter behaviour for entry/chain functions too. It
seems this was always the intention [1] and it will also save us a bit
of stack space in cases where the first stack object has a large
alignment.
[1]
34c8b835b1
CSR SGPR spilling currently uses the early available physical VGPRs. It
currently imposes a high register pressure while trying to allocate
large VGPR tuples within the default register budget.
This patch changes the spilling strategy by picking the VGPRs in the
reverse order, the highest available VGPR first and later after regalloc
shift them back to the lowest available range. With that, the initial
VGPRs would be available for allocation and possibility
of finding large number of contiguous registers will be more.
Physical VGPRs used for SGPR spills need to be tracked independent of
WWM reserved registers. The WWM reserved set contains extra registers
allocated during WWM pre-allocation pass.
This causes SGPR spills allocated after WWM pre-allocation to overlap
with WWM register usage, e.g. if frame pointer is spilt during
prologue/epilog insertion.
This patch replaces uses of StringRef::{starts,ends}with with
StringRef::{starts,ends}_with for consistency with
std::{string,string_view}::{starts,ends}_with in C++20.
I'm planning to deprecate and eventually remove
StringRef::{starts,ends}with.
Introduce a helper to check if a function is at the bottom of the stack,
i.e. if it's an entry function or a chain function.
This was suggested in #71913.
Switch to using immediate offsets instead of the SP register to access
objects on the current stack frame in chain functions. This means we no
longer need to reserve a SP register just for accesing stack objects and
it also allows us to set the SP (when one is actually needed) to the
stack size from the very beginning.
This only works if we use a FixedObject for the ScavengeFI, which is
what we do for entry functions anyway (and we generally want to keep
chain functions close to amdgpu_cs behaviour where we don't have a good
reason to diverge).
Teach prolog epilog insertion how to handle functions with the
amdgpu_cs_chain or amdgpu_cs_chain_preserve calling conventions.
For amdgpu_cs_chain functions, we only need to preserve the inactive
lanes of VGPRs above v8, and only in the presence of calls via
@llvm.amdgcn.cs.chain.
For amdgpu_cs_chain_preserve functions, we will also need to preserve
the active lanes for registers above the last argument VGPR. AFAICT
there's no direct way to find out what the last argument VGPR is, so
instead the patch uses the fact that chain calls from
amdgpu_cs_chain_preserve functions can't use more VGPRs than the
caller's VGPR arguments. In other words, it removes the operands of
SI_CS_CHAIN_TC instructions from the list of callee saved registers.
For both calling conventions, registers v0-v7 never need to be saved and
restored, so we should never add them as WWM spills.
Differential Revision: https://reviews.llvm.org/D156412
This patch adds the DAG isel changes for kernel argument preloading.
These changes are not usable with older firmware but subsequent patches
in the series will make the codegen backwards compatible. This patch
should only be submitted alongside that subsequent patch.
Preloading here begins from the start of the kernel arguments until the
amount of arguments indicated by the CL flag
amdgpu-kernarg-preload-count.
Aggregates and arguments passed by-ref are not supported.
Special care for the alignment of the kernarg segment is needed as well
as consideration of the alignment of addressable SGPR tuples when we
cannot directly use misaligned large tuples that the arguments are
loaded to.
Reviewed By: bcahoon
Differential Revision: https://reviews.llvm.org/D158579
Factor out and unify some common code that calculates and tracks the
number of user SGRPs.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D159439
Lower formal arguments and returns for functions with the
`amdgpu_cs_chain` and `amdgpu_cs_chain_preserve` calling conventions:
* Put `inreg` arguments into SGPRs, starting at s0, and other arguments
into VGPRs, starting at v8. No arguments should end up on the stack, if
we don't have enough registers we should error out.
* Lower the return (which is always void) as an S_ENDPGM.
* Set the ScratchRSrc register to s48:51, as described in the docs.
* Set the SP to s32, matching amdgpu_gfx. This might be revisited in a
future patch.
Differential Revision: https://reviews.llvm.org/D153517
This reverts commit a496c8be6e638ae58bb45f13113dbe3a4b7b23fd.
The workaround in c26dfc81e254c78dc23579cf3d1336f77249e1f6 should work
around the underlying problem with SUBREG_TO_REG.
And dependent commits.
Details in D150388.
This reverts commit 825b7f0ca5f2211ec3c93139f98d1e24048c225c.
This reverts commit 7a98f084c4d121244ef7286bc6503b6a181d446e.
This reverts commit b4a62b1fa546312d882fa12dfdcd015177d66826.
This reverts commit b7836d856206ec39509d42529f958c920368166b.
No conflicts in the code, few tests had conflicts in autogenerated CHECKs:
llvm/test/CodeGen/Thumb2/mve-float32regloops.ll
llvm/test/CodeGen/AMDGPU/fix-frame-reg-in-custom-csr-spills.ll
Reviewed By: alexfh
Differential Revision: https://reviews.llvm.org/D156381
Currently, the custom SGPR spill lowering pass spills
SGPRs into physical VGPR lanes and the remaining VGPRs
are used by regalloc for vector regclass allocation.
This imposes many restrictions that we ended up with
unsuccessful SGPR spilling when there won't be enough
VGPRs and we are forced to spill the leftover into
memory during PEI. The custom spill handling during PEI
has many edge cases and often breaks the compiler time
to time.
This patch implements spilling SGPRs into virtual VGPR
lanes. Since we now split the register allocation for
SGPRs and VGPRs, the virtual registers introduced for
the spill lanes would get allocated automatically in
the subsequent regalloc invocation for VGPRs.
Spill to virtual registers will always be successful,
even in the high-pressure situations, and hence it avoids
most of the edge cases during PEI. We are now left with
only the custom SGPR spills during PEI for special registers
like the frame pointer which is an unproblematic case.
Differential Revision: https://reviews.llvm.org/D124196
To reduce the register pressure during allocation,
when the allocator spills a virtual register that
corresponds to a whole wave mode operation, the
spill loads and restores should be activated for
all lanes by temporarily flipping all bits in exec
register to one just before the spills. It is not
implemented in the compiler as of today and this
patch enables the necessary support.
This is a pre-patch before the SGPR spill to virtual
VGPR lanes that would eventually causes the whole
wave register spills during allocation.
Reviewed By: arsenm, cdevadas
Differential Revision: https://reviews.llvm.org/D143759
Branch relaxation requires 2 additional SGPRs for AMDGPU to handle the
case when an indirect branch target is too far away. The register
scavanger may not find available registers, which causes a “did not find
scavenging index” assert to occur in assignRegToScavengingIndex.
In this patch, we estimate before register allocation whether an
indirect branch is likely to be needed, and reserve 2 SGPRs if the
branch distance is found to be above a threshold. The distance threshold
is an approximation as the exact code size and branch distance are
unknown prior to register allocation.
Patch by Corbin Robeck. Thanks!
Differential Review: https://reviews.llvm.org/D149775
Currently, the codegen support for llvm.amdgcn.workgroup.id*
intrinsics are enabled only for compute kernels. In addition,
this patch enables their selection for compute shaders on
subtargets that have architected SGPRs.
Differential Revision: https://reviews.llvm.org/D145045
This fixes what I consider to be an API flaw I've tripped over
multiple times. The point this is constructed isn't well defined, so
depending on where this is first called, you can conclude different
information based on the MachineFunction. For example, the AMDGPU
implementation inspected the MachineFrameInfo on construction for the
stack objects and if the frame has calls. This kind of worked in
SelectionDAG which visited all allocas up front, but broke in
GlobalISel which hasn't visited any of the IR when arguments are
lowered.
I've run into similar problems before with the MIR parser and trying
to make use of other MachineFunction fields, so I think it's best to
just categorically disallow dependency on the MachineFunction state in
the constructor and to always construct this at the same time as the
MachineFunction itself.
A missing feature I still could use is a way to access an custom
analysis pass on the IR here.
Currently, the custom SGPR spill lowering pass spills
SGPRs into physical VGPR lanes and the remaining VGPRs
are used by regalloc for vector regclass allocation.
This imposes many restrictions that we ended up with
unsuccessful SGPR spilling when there won't be enough
VGPRs and we are forced to spill the leftover into
memory during PEI. The custom spill handling during PEI
has many edge cases and often breaks the compiler time
to time.
This patch implements spilling SGPRs into virtual VGPR
lanes. Since we now split the register allocation for
SGPRs and VGPRs, the virtual registers introduced for
the spill lanes would get allocated automatically in
the subsequent regalloc invocation for VGPRs.
Spill to virtual registers will always be successful,
even in the high-pressure situations, and hence it avoids
most of the edge cases during PEI. We are now left with
only the custom SGPR spills during PEI for special registers
like the frame pointer which isn an unproblematic case.
This patch also implements the whole wave spills which
might occur if RA spills any live range of virtual registers
involved in the whole wave operations. Earlier, we had
been hand-picking registers for such machine operands.
But now with SGPR spills into virtual VGPR lanes, we are
exposing them to the allocator.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D124196
