GlobalIsel::Combiner::tryDCE does not update liveness.

[hiraditya]

Repro:
$ sed 's/iXLen/i32/g' llvm-project/llvm/test/CodeGen/RISCV/GlobalISel/vararg.ll > liveness-varargs.ll
$ ./bin/llc -mtriple=riscv64 -global-isel -verify-machineinstrs liveness-varargs.ll -debug

Before Combiner

# *** IR Dump After Verify generated machine code (machineverifier) ***:
# Machine code for function va1: IsSSA, TracksLiveness
Frame Objects:
  fi#-2: size=8, align=16, fixed, at location [SP-64]
  fi#-1: size=56, align=8, fixed, at location [SP-56]
  fi#0: size=8, align=8, at location [SP]
save/restore points:
save points are empty
restore points are empty
Function Live Ins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17

bb.1 (%ir-block.0):
  liveins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17
  %0:_(p0) = COPY $x10
  %1:_(p0) = G_FRAME_INDEX %fixed-stack.1
  %2:_(s64) = G_CONSTANT i64 8
  %3:_(s64) = COPY $x11
  G_STORE %3:_(s64), %1:_(p0) :: (store (s64) into %fixed-stack.1)
  %4:_(p0) = G_PTR_ADD %1:_, %2:_(s64)
  %5:_(s64) = COPY $x12
  G_STORE %5:_(s64), %4:_(p0) :: (store (s64) into %fixed-stack.1 + 8)
  %6:_(p0) = G_PTR_ADD %4:_, %2:_(s64)
  %7:_(s64) = COPY $x13
  G_STORE %7:_(s64), %6:_(p0) :: (store (s64) into %fixed-stack.1 + 16)
  %8:_(p0) = G_PTR_ADD %6:_, %2:_(s64)
  %9:_(s64) = COPY $x14
  G_STORE %9:_(s64), %8:_(p0) :: (store (s64) into %fixed-stack.1 + 24)
  %10:_(p0) = G_PTR_ADD %8:_, %2:_(s64)
  %11:_(s64) = COPY $x15
  G_STORE %11:_(s64), %10:_(p0) :: (store (s64) into %fixed-stack.1 + 32)
  %12:_(p0) = G_PTR_ADD %10:_, %2:_(s64)
  %13:_(s64) = COPY $x16
  G_STORE %13:_(s64), %12:_(p0) :: (store (s64) into %fixed-stack.1 + 40)
  %14:_(p0) = G_PTR_ADD %12:_, %2:_(s64)
  %15:_(s64) = COPY $x17
  G_STORE %15:_(s64), %14:_(p0) :: (store (s64) into %fixed-stack.1 + 48)
  %16:_(p0) = G_PTR_ADD %14:_, %2:_(s64)
  %17:_(p0) = G_FRAME_INDEX %stack.0.va
  G_VASTART %17:_(p0) :: (store (s64) into %ir.va)
  %18:_(p0) = G_LOAD %17:_(p0) :: (dereferenceable load (p0) from %ir.va, align 4)
  %19:_(s64) = G_CONSTANT i64 4
  %20:_(p0) = nuw nusw inbounds G_PTR_ADD %18:_, %19:_(s64)
  G_STORE %20:_(p0), %17:_(p0) :: (store (p0) into %ir.va, align 4)
  %21:_(s32) = G_LOAD %18:_(p0) :: (load (s32) from %ir.argp.cur)
  %22:_(s64) = G_ANYEXT %21:_(s32)
  $x10 = COPY %22:_(s64)
  PseudoRET implicit $x10

# End machine code for function va1.

Somewhere in between

Combiner iteration #1
Dead: %16:_(p0) = G_PTR_ADD %14:_, %2:_(s64)
Dead: %0:_(p0) = COPY $x10

After the copy instruction is removed, X10 is no longer a live in.

After combiner

# *** IR Dump After RISCVPreLegalizerCombiner (riscv-prelegalizer-combiner) ***:
# Machine code for function va1: IsSSA, TracksLiveness
Frame Objects:
  fi#-2: size=8, align=16, fixed, at location [SP-64]
  fi#-1: size=56, align=8, fixed, at location [SP-56]
  fi#0: size=8, align=8, at location [SP]
save/restore points:
save points are empty
restore points are empty
Function Live Ins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17

bb.1 (%ir-block.0):
  liveins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17
  %1:_(p0) = G_FRAME_INDEX %fixed-stack.1
  %2:_(s64) = G_CONSTANT i64 8
  %3:_(s64) = COPY $x11
  G_STORE %3:_(s64), %1:_(p0) :: (store (s64) into %fixed-stack.1)
  %4:_(p0) = G_PTR_ADD %1:_, %2:_(s64)
  %5:_(s64) = COPY $x12
  G_STORE %5:_(s64), %4:_(p0) :: (store (s64) into %fixed-stack.1 + 8)
  %23:_(s64) = G_CONSTANT i64 16
  %6:_(p0) = G_PTR_ADD %1:_, %23:_(s64)
  %7:_(s64) = COPY $x13
  G_STORE %7:_(s64), %6:_(p0) :: (store (s64) into %fixed-stack.1 + 16)
  %24:_(s64) = G_CONSTANT i64 24
  %8:_(p0) = G_PTR_ADD %1:_, %24:_(s64)
  %9:_(s64) = COPY $x14
  G_STORE %9:_(s64), %8:_(p0) :: (store (s64) into %fixed-stack.1 + 24)
  %25:_(s64) = G_CONSTANT i64 32
  %10:_(p0) = G_PTR_ADD %1:_, %25:_(s64)
  %11:_(s64) = COPY $x15
  G_STORE %11:_(s64), %10:_(p0) :: (store (s64) into %fixed-stack.1 + 32)
  %26:_(s64) = G_CONSTANT i64 40
  %12:_(p0) = G_PTR_ADD %1:_, %26:_(s64)
  %13:_(s64) = COPY $x16
  G_STORE %13:_(s64), %12:_(p0) :: (store (s64) into %fixed-stack.1 + 40)
  %27:_(s64) = G_CONSTANT i64 48
  %14:_(p0) = G_PTR_ADD %1:_, %27:_(s64)
  %15:_(s64) = COPY $x17
  G_STORE %15:_(s64), %14:_(p0) :: (store (s64) into %fixed-stack.1 + 48)
  %17:_(p0) = G_FRAME_INDEX %stack.0.va
  G_VASTART %17:_(p0) :: (store (s64) into %ir.va)
  %18:_(p0) = G_LOAD %17:_(p0) :: (dereferenceable load (p0) from %ir.va, align 4)
  %19:_(s64) = G_CONSTANT i64 4
  %20:_(p0) = nuw nusw inbounds G_PTR_ADD %18:_, %19:_(s64)
  G_STORE %20:_(p0), %17:_(p0) :: (store (p0) into %ir.va, align 4)
  %22:_(s64) = G_LOAD %18:_(p0) :: (load (s32) from %ir.argp.cur)
  $x10 = COPY %22:_(s64)
  PseudoRET implicit $x10

# End machine code for function va1.

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

Author: AdityaK (hiraditya)

Repro: $ sed 's/iXLen/i32/g' llvm-project/llvm/test/CodeGen/RISCV/GlobalISel/vararg.ll > liveness-varargs.ll $ ./bin/llc -mtriple=riscv64 -global-isel -verify-machineinstrs liveness-varargs.ll -debug

Before Combiner

# *** IR Dump After Verify generated machine code (machineverifier) ***:
# Machine code for function va1: IsSSA, TracksLiveness
Frame Objects:
  fi#-2: size=8, align=16, fixed, at location [SP-64]
  fi#-1: size=56, align=8, fixed, at location [SP-56]
  fi#<!-- -->0: size=8, align=8, at location [SP]
save/restore points:
save points are empty
restore points are empty
Function Live Ins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17

bb.1 (%ir-block.0):
  liveins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17
  %0:_(p0) = COPY $x10
  %1:_(p0) = G_FRAME_INDEX %fixed-stack.1
  %2:_(s64) = G_CONSTANT i64 8
  %3:_(s64) = COPY $x11
  G_STORE %3:_(s64), %1:_(p0) :: (store (s64) into %fixed-stack.1)
  %4:_(p0) = G_PTR_ADD %1:_, %2:_(s64)
  %5:_(s64) = COPY $x12
  G_STORE %5:_(s64), %4:_(p0) :: (store (s64) into %fixed-stack.1 + 8)
  %6:_(p0) = G_PTR_ADD %4:_, %2:_(s64)
  %7:_(s64) = COPY $x13
  G_STORE %7:_(s64), %6:_(p0) :: (store (s64) into %fixed-stack.1 + 16)
  %8:_(p0) = G_PTR_ADD %6:_, %2:_(s64)
  %9:_(s64) = COPY $x14
  G_STORE %9:_(s64), %8:_(p0) :: (store (s64) into %fixed-stack.1 + 24)
  %10:_(p0) = G_PTR_ADD %8:_, %2:_(s64)
  %11:_(s64) = COPY $x15
  G_STORE %11:_(s64), %10:_(p0) :: (store (s64) into %fixed-stack.1 + 32)
  %12:_(p0) = G_PTR_ADD %10:_, %2:_(s64)
  %13:_(s64) = COPY $x16
  G_STORE %13:_(s64), %12:_(p0) :: (store (s64) into %fixed-stack.1 + 40)
  %14:_(p0) = G_PTR_ADD %12:_, %2:_(s64)
  %15:_(s64) = COPY $x17
  G_STORE %15:_(s64), %14:_(p0) :: (store (s64) into %fixed-stack.1 + 48)
  %16:_(p0) = G_PTR_ADD %14:_, %2:_(s64)
  %17:_(p0) = G_FRAME_INDEX %stack.0.va
  G_VASTART %17:_(p0) :: (store (s64) into %ir.va)
  %18:_(p0) = G_LOAD %17:_(p0) :: (dereferenceable load (p0) from %ir.va, align 4)
  %19:_(s64) = G_CONSTANT i64 4
  %20:_(p0) = nuw nusw inbounds G_PTR_ADD %18:_, %19:_(s64)
  G_STORE %20:_(p0), %17:_(p0) :: (store (p0) into %ir.va, align 4)
  %21:_(s32) = G_LOAD %18:_(p0) :: (load (s32) from %ir.argp.cur)
  %22:_(s64) = G_ANYEXT %21:_(s32)
  $x10 = COPY %22:_(s64)
  PseudoRET implicit $x10

# End machine code for function va1.

Somewhere in between

Combiner iteration #<!-- -->1
Dead: %16:_(p0) = G_PTR_ADD %14:_, %2:_(s64)
Dead: %0:_(p0) = COPY $x10

After the copy instruction is removed, X10 is no longer a live in.

After combiner

# *** IR Dump After RISCVPreLegalizerCombiner (riscv-prelegalizer-combiner) ***:
# Machine code for function va1: IsSSA, TracksLiveness
Frame Objects:
  fi#-2: size=8, align=16, fixed, at location [SP-64]
  fi#-1: size=56, align=8, fixed, at location [SP-56]
  fi#<!-- -->0: size=8, align=8, at location [SP]
save/restore points:
save points are empty
restore points are empty
Function Live Ins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17

bb.1 (%ir-block.0):
  liveins: $x10, $x11, $x12, $x13, $x14, $x15, $x16, $x17
  %1:_(p0) = G_FRAME_INDEX %fixed-stack.1
  %2:_(s64) = G_CONSTANT i64 8
  %3:_(s64) = COPY $x11
  G_STORE %3:_(s64), %1:_(p0) :: (store (s64) into %fixed-stack.1)
  %4:_(p0) = G_PTR_ADD %1:_, %2:_(s64)
  %5:_(s64) = COPY $x12
  G_STORE %5:_(s64), %4:_(p0) :: (store (s64) into %fixed-stack.1 + 8)
  %23:_(s64) = G_CONSTANT i64 16
  %6:_(p0) = G_PTR_ADD %1:_, %23:_(s64)
  %7:_(s64) = COPY $x13
  G_STORE %7:_(s64), %6:_(p0) :: (store (s64) into %fixed-stack.1 + 16)
  %24:_(s64) = G_CONSTANT i64 24
  %8:_(p0) = G_PTR_ADD %1:_, %24:_(s64)
  %9:_(s64) = COPY $x14
  G_STORE %9:_(s64), %8:_(p0) :: (store (s64) into %fixed-stack.1 + 24)
  %25:_(s64) = G_CONSTANT i64 32
  %10:_(p0) = G_PTR_ADD %1:_, %25:_(s64)
  %11:_(s64) = COPY $x15
  G_STORE %11:_(s64), %10:_(p0) :: (store (s64) into %fixed-stack.1 + 32)
  %26:_(s64) = G_CONSTANT i64 40
  %12:_(p0) = G_PTR_ADD %1:_, %26:_(s64)
  %13:_(s64) = COPY $x16
  G_STORE %13:_(s64), %12:_(p0) :: (store (s64) into %fixed-stack.1 + 40)
  %27:_(s64) = G_CONSTANT i64 48
  %14:_(p0) = G_PTR_ADD %1:_, %27:_(s64)
  %15:_(s64) = COPY $x17
  G_STORE %15:_(s64), %14:_(p0) :: (store (s64) into %fixed-stack.1 + 48)
  %17:_(p0) = G_FRAME_INDEX %stack.0.va
  G_VASTART %17:_(p0) :: (store (s64) into %ir.va)
  %18:_(p0) = G_LOAD %17:_(p0) :: (dereferenceable load (p0) from %ir.va, align 4)
  %19:_(s64) = G_CONSTANT i64 4
  %20:_(p0) = nuw nusw inbounds G_PTR_ADD %18:_, %19:_(s64)
  G_STORE %20:_(p0), %17:_(p0) :: (store (p0) into %ir.va, align 4)
  %22:_(s64) = G_LOAD %18:_(p0) :: (load (s32) from %ir.argp.cur)
  $x10 = COPY %22:_(s64)
  PseudoRET implicit $x10

# End machine code for function va1.

[hiraditya]

Moreover, live variable analysis that runs later doesn't update the liveness.

[arsenm]

I don't see the issue? The live-ins didn't change. %0 is dead to begin with, and is deleted, so that's also fine?

[hiraditya]

I don't see the issue? The live-ins didn't change. %0 is dead to begin with, and is deleted, so that's also fine?

%0 is dead but live-ins accounted for the use of %x10 there. After %0 is deleted, %x10 should be removed from live-in because %x10 is defined later before any usage.

[arsenm]

I don't see the issue? The live-ins didn't change. %0 is dead to begin with, and is deleted, so that's also fine?

%0 is dead but live-ins accounted for the use of %x10 there. After %0 is deleted, %x10 should be removed from live-in because %x10 is defined later before any usage.

This is not a problem, and not something anything in GlobalISel should be trying to solve. If we want to clean up liveins that aren't actually live in, that's a problem for the register allocation pipeline