[all-commits] [llvm/llvm-project] f9a8c6: [AMDGPU] Save VGPR of whole wave when spilling

[Sebastian Neubauer via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: f9a8c6a0e50540f68e6740a849a7caf5e4d46ca6
      https://github.com/llvm/llvm-project/commit/f9a8c6a0e50540f68e6740a849a7caf5e4d46ca6
  Author: Sebastian Neubauer <sebastian.neubauer at amd.com>
  Date:   2021-04-12 (Mon, 12 Apr 2021)

  Changed paths:
    M llvm/lib/Target/AMDGPU/SIFrameLowering.cpp
    M llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp
    M llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.h
    M llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp
    M llvm/lib/Target/AMDGPU/SIRegisterInfo.h
    M llvm/test/CodeGen/AMDGPU/control-flow-fastregalloc.ll
    M llvm/test/CodeGen/AMDGPU/frame-setup-without-sgpr-to-vgpr-spills.ll
    M llvm/test/CodeGen/AMDGPU/partial-sgpr-to-vgpr-spills.ll
    M llvm/test/CodeGen/AMDGPU/sgpr-spill.mir
    M llvm/test/CodeGen/AMDGPU/si-spill-sgpr-stack.ll
    M llvm/test/CodeGen/AMDGPU/spill-m0.ll
    M llvm/test/CodeGen/AMDGPU/spill-scavenge-offset.ll
    A llvm/test/CodeGen/AMDGPU/spill-sgpr-stack-no-sgpr.ll
    M llvm/test/CodeGen/AMDGPU/spill-special-sgpr.mir

  Log Message:
  -----------
  [AMDGPU] Save VGPR of whole wave when spilling

Spilling SGPRs to scratch uses a temporary VGPR. LLVM currently cannot
determine if a VGPR is used in other lanes or not, so we need to save
all lanes of the VGPR. We even need to save the VGPR if it is marked as
dead.

The generated code depends on two things:
- Can we scavenge an SGPR to save EXEC?
- And can we scavenge a VGPR?

If we can scavenge an SGPR, we
- save EXEC into the SGPR
- set the needed lane mask
- save the temporary VGPR
- write the spilled SGPR into VGPR lanes
- save the VGPR again to the target stack slot
- restore the VGPR
- restore EXEC

If we were not able to scavenge an SGPR, we do the same operations, but
everytime the temporary VGPR is written to memory, we
- write VGPR to memory
- flip exec (s_not exec, exec)
- write VGPR again (previously inactive lanes)

Surprisingly often, we are able to scavenge an SGPR, even though we are
at the brink of running out of SGPRs.
Scavenging a VGPR does not have a great effect (saves three instructions
if no SGPR was scavenged), but we need to know if the VGPR we use is
live before or not, otherwise the machine verifier complains.

Differential Revision: https://reviews.llvm.org/D96336