nir/from_ssa: Don't try to read an invalid instruction

Right now, the nir_instr_prev function function blindly looks up the
previous element in the exec list and casts it to an instruction even if
it's the tail sentinel.  The next commit will change this to return null if
it's the first instruction.  Making this change first avoids getting a
segfault between commits.  The only reason we never noticed is that, thanks
to the way things are laid out in nir_block, the casted instruction's type
was never parallal_copy.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

diff --git a/src/glsl/nir/nir_from_ssa.c b/src/glsl/nir/nir_from_ssa.c
index 3625237dd3dc41d67ac43e5190ed911358ddf2c4..7c5009577673537440eebdd5dae79fd0786b37e8 100644 (file)
--- a/src/glsl/nir/nir_from_ssa.c
+++ b/src/glsl/nir/nir_from_ssa.c
@@ -272,7 +272,7 @@ get_parallel_copy_at_end_of_block(nir_block *block)
    if (last_instr->type == nir_instr_type_jump)
       last_instr = nir_instr_prev(last_instr);
 
-   if (last_instr->type == nir_instr_type_parallel_copy)
+   if (last_instr && last_instr->type == nir_instr_type_parallel_copy)
       return nir_instr_as_parallel_copy(last_instr);
    else
       return NULL;