Support for XNACK and SRAMECC is not static on some GPUs. We must be able
to differentiate between different scenarios for these dynamic subtarget
features.
The possible settings are:
- Unsupported: The GPU has no support for XNACK/SRAMECC.
- Any: Preference is unspecified. Use conservative settings that can run anywhere.
- Off: Request support for XNACK/SRAMECC Off
- On: Request support for XNACK/SRAMECC On
GCNSubtarget will track the four options based on the following criteria. If
the subtarget does not support XNACK/SRAMECC we say the setting is
"Unsupported". If no subtarget features for XNACK/SRAMECC are requested we
must support "Any" mode. If the subtarget features XNACK/SRAMECC exist in the
feature string when initializing the subtarget, the settings are "On/Off".
The defaults are updated to be conservatively correct, meaning if no setting
for XNACK or SRAMECC is explicitly requested, defaults will be used which
generate code that can be run anywhere. This corresponds to the "Any" setting.
Differential Revision: https://reviews.llvm.org/D85882
Clustering loads has caching benefits, but as far as I know there is no
advantage to clustering stores on any AMDGPU subtargets.
The disadvantage is that it tends to increase register pressure and
restricts scheduling freedom.
Differential Revision: https://reviews.llvm.org/D85530
Summary:
BaseMemOpClusterMutation::apply forms store chains by looking for
control (i.e. non-data) dependencies from one mem op to another.
In the test case, clusterNeighboringMemOps successfully clusters the
loads, and then adds artificial edges to the loads' successors as
described in the comment:
// Copy successor edges from SUa to SUb. Interleaving computation
// dependent on SUa can prevent load combining due to register reuse.
The effect of this is that *data* dependencies from one load to a store
are copied as *artificial* dependencies from a different load to the
same store.
Then when BaseMemOpClusterMutation::apply looks at the stores, it finds
that some of them have a control dependency on a previous load, which
breaks the chains and means that the stores are not all considered part
of the same chain and won't all be clustered.
The fix is to only consider non-artificial control dependencies when
forming chains.
Subscribers: MatzeB, jvesely, nhaehnle, hiraditya, javed.absar, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D71717
Austin Kerbow