Prefer to keep uniform (non-divergent) multiplies on the scalar ALU when
possible. This significantly improves some game cases by eliminating
v_readfirstlane instructions when the result feeds into a scalar
operation, like the address calculation for a scalar load or store.
Since isDivergent is only an approximation of whether a value is in
SGPRs, it can potentially regress some situations where a uniform value
ends up in a VGPR. These should be rare in real code, although the test
changes do contain a number of examples.
Most of the test changes are just using s_mul instead of v_mul/mad which
is generally better for both register pressure and latency (at least on
GFX10 where sgpr pressure doesn't affect occupancy and vector ALU
instructions have significantly longer latency than scalar ALU). Some
R600 tests now use MULLO_INT instead of MUL_UINT24.
GlobalISel appears to handle more scenarios in the desirable way,
although it can also be thrown off and fails to select the 24-bit
multiplies in some cases.
Alternative solution considered and rejected was to allow selecting
MUL_[UI]24 to S_MUL_I32. I've rejected this because the definition of
those SD operations works is don't-care on the most significant 8 bits,
and this fact is used in some combines via SimplifyDemandedBits.
Based on a patch by Nicolai Hähnle.
Differential Revision: https://reviews.llvm.org/D97063
Exec mask manipulation inserted by SIWholeQuadMode barriers to
instruction scheduling. Move the entire pass after the machine
instruction scheduler and make changes so pass is correct for
non-SSA operation. These changes should leave the pass still
usable pre-scheduler, although tests have be updated to reflect
post-scheduler results.
Reviewed By: nhaehnle
Differential Revision: https://reviews.llvm.org/D88081
Add a new llvm.amdgcn.ballot intrinsic modeled on the ballot function
in GLSL and other shader languages. It returns a bitfield containing the
result of its boolean argument in all active lanes, and zero in all
inactive lanes.
This is intended to replace the existing llvm.amdgcn.icmp and
llvm.amdgcn.fcmp intrinsics after a suitable transition period.
Use the new intrinsic in the atomic optimizer pass.
Differential Revision: https://reviews.llvm.org/D65088
Summary:
Add support for gfx10, where all DPP operations are confined to work
within a single row of 16 lanes, and wave32.
Reviewers: arsenm, sheredom, critson, rampitec
Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, t-tye, hiraditya, jfb, dstuttard, tpr, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D65644
llvm-svn: 369745
This change incorporates an effort by Connor Abbot to change how we deal
with WWM operations potentially trashing valid values in inactive lanes.
Previously, the SIFixWWMLiveness pass would work out which registers
were being trashed within WWM regions, and ensure that the register
allocator did not have any values it was depending on resident in those
registers if the WWM section would trash them. This worked perfectly
well, but would cause sometimes severe register pressure when the WWM
section resided before divergent control flow (or at least that is where
I mostly observed it).
This fix instead runs through the WWM sections and pre allocates some
registers for WWM. It then reserves these registers so that the register
allocator cannot use them. This results in a significant register
saving on some WWM shaders I'm working with (130 -> 104 VGPRs, with just
this change!).
Differential Revision: https://reviews.llvm.org/D59295
llvm-svn: 357400
This commit fixes the DPP sequence in the atomic optimizer (which was
previously missing the row_shr:3 step), and works around a read_register
exec bug by using a ballot instead.
Differential Revision: https://reviews.llvm.org/D57737
llvm-svn: 353703
This commit adds a new IR level pass to the AMDGPU backend to perform
atomic optimizations. It works by:
- Running through a function and finding atomicrmw add/sub or uses of
the atomic buffer intrinsics for add/sub.
- If all arguments except the value to be added/subtracted are uniform,
record the value to be optimized.
- Run through the atomic operations we can optimize and, depending on
whether the value is uniform/divergent use wavefront wide operations
(DPP in the divergent case) to calculate the total amount to be
atomically added/subtracted.
- Then let only a single lane of each wavefront perform the atomic
operation, reducing the total number of atomic operations in flight.
- Lastly we recombine the result from the single lane to each lane of
the wavefront, and calculate our individual lanes offset into the
final result.
Differential Revision: https://reviews.llvm.org/D51969
llvm-svn: 343973
Jay Foad