Unmerges have the same fundamental problem as G_TRUNC, and G_TRUNC
could be implemented in terms of G_UNMERGE_VALUES. Reducing the number
of elements in unmerge results ends up producing the original unmerge
type profile, so the artifact combiner needs to eliminate the
intermediate illegal registers. This avoids infinite looping in the
legalizer in a future change.
Assuming an unmerge has each result unmerged the same way, this ends
up producing a new unmerge of the source for every definition. I'm not
sure if the artifact combiner should either insert temporary merges
here and erase the original merge, or if the combiner should look at
uses from defs rather than defs from uses for unmerges.
In a few cases this regresses from using 16-bit shifts for 8-bit
values to using 32-bit shifts, but I think these can be legalized
later (the other legalization rules don't try very hard to use 16-bit
shifts either).
Most notably, we were incorrectly reporting <3 x s16> as a legal type
for these. Make sure these aren't legal to help make progress on
fixing the artifact combiner and vector legalizer
rules. Unfortunately, this means spreading the -global-isel-abort=0
hack, although this doesn't change the legalizer result in any
situation.
Implicit uses of non-register value types places impossible to satisfy
constraints on the legalizer / artifact combiner. These prevent
writing sensible legalize rules for the artifacts without triggering
infinite loops in the legalizer.
The verifier really needs to enforce this, but I'm not sure what the
exact conditions would look like yet.
Use pad with undef and unmerge with unused results. This is annoyingly
similar to several other places in LegalizerHelper, but they're all
slightly different.
The legalizer produces a lot of these, and they make reading legalized
MIR annoying. For some reason, this does seem to sometimes introduce
copies of implicit def, which is dumb.
If we have s_pack_* instructions, legalize this to
G_BUILD_VECTOR_TRUNC from s32 elements. This is closer to how how the
s_pack_* instructions really behave.
If we don't have s_pack_ instructions, expand this by creating a merge
to s32 and bitcasting. This expands to the expected bit operations. I
think this eventually should go in a new bitcast legalize action type
in LegalizerHelper.
We already directly emit the shift operations in RegBankSelect for the
vector case. This could possibly be cleaned up, but I also may want to
defer doing this expansion to selection anyway. I'll see about that
when I try to actually match VOP3P instructions.
This breaks the selection of the build_vector since tablegen doesn't
know how to match G_BUILD_VECTOR_TRUNC yet, so just xfail it for now.
Summary:
Currently, Legalizer aborts if it’s unable to legalize artifacts. However, it’s
possible to combine them after processing the rest of the instruction because
the legalization is likely to generate more artifacts that allow ArtifactCombiner
to combine away them.
Instead, move illegal artifacts to another list called RetryList and wait until all of the
instruction in InstList are legalized. After that, check if there is any new artifacts and
try to combine them again if that’s the case. If not, abort. The idea is similar to D59339,
but the approach is a bit different.
This patch fixes the issue described above, but the legalizer still may be unable to handle
some cases depending on when to legalize artifacts. So, in the long run, we probably need
a different legalization strategy that handles this dependency in a better way.
Reviewers: dsanders, aditya_nandakumar, qcolombet, arsenm, aemerson, paquette
Reviewed By: dsanders
Subscribers: jvesely, wdng, nhaehnle, rovka, javed.absar, hiraditya, Petar.Avramovic, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D65894
llvm-svn: 369805
This is necessary for handling <3 x s16> on AMDGPU, assuming this
should be handled as 2 separate legalization actions. The alternative
would be for fewerElementsVector to handle 3->2.
llvm-svn: 369547
AMDGPU sometimes has legal s16 and <2 x s16> operations, but all
registers are really 32-bit. An unmerge destination really should ben
widened to a 32-bit register. If widening a scalarizing vector with a
target size that matches the vector size, bitcast to integer and
extract the relevant bits with shifts.
I'm not sure if this is the right place for this. This could arguably
be part of widenScalar for the result. I also have a growing feeling
that we're missing a bitcast legalize action.
llvm-svn: 367604
Apparently the check for legal instructions during instruction
select does not happen without an asserts build, so these would
successfully select in release, and fail in debug.
Make s16 and/or/xor legal. These can just be selected directly
to the 32-bit operation, as is already done in SelectionDAG, so just
make them legal.
llvm-svn: 366210
Petar Avramovic