SIGSEGV in vISA when the GRF RA iteration budget is exhausted under fail-safe RA (dg2, bmg)

[pvelesko]

ocloc dies with SIGSEGV inside libigc when the GRF register allocator exhausts its iteration budget while fail-safe RA is enabled: coloringRegAlloc() returns VISA_SUCCESS with G4_RegVars whose physical register is still null, and the first post-RA pass to touch one dereferences it.

Stock IGC, no local patches. Reproduces on IGC 2.36.3 (ocloc 26.22.38646.4, Arc A380) and 2.38.2 (ocloc 26.27.39122.11, Arc B570), -device dg2, dg2-g11 and bmg.

__attribute__((intel_reqd_sub_group_size(32)))
kernel void k(global const int *in, global int *out, global const uint *off) {
  int gid = get_global_id(0);
  bool p0 = in[gid + off[0]] > 0;  bool p1 = in[gid + off[1]] > 1;
  bool p2 = in[gid + off[2]] > 2;  bool p3 = in[gid + off[3]] > 3;
  bool p4 = in[gid + off[4]] > 4;  bool p5 = in[gid + off[5]] > 5;
  bool p6 = in[gid + off[6]] > 6;  bool p7 = in[gid + off[7]] > 7;
  int acc = 0;
  if (p0) acc += 1; else acc -= 1;   if (p1) acc += 2; else acc -= 2;
  if (p2) acc += 3; else acc -= 3;   if (p3) acc += 4; else acc -= 4;
  if (p4) acc += 5; else acc -= 5;   if (p5) acc += 6; else acc -= 6;
  if (p6) acc += 7; else acc -= 7;
  out[gid] = acc;
}

$ ocloc compile -file repro.cl -options "-cl-opt-disable -igc_opts 'VISAOptions=-maxRAIterations 1'" -device dg2
Segmentation fault
$ echo $?
139

Expected a build, or a failure naming the kernel that could not be allocated. Not a crash.

-maxRAIterations 2 and above pass, so the exhausted budget is the trigger. Not specific to -cl-opt-disable: the same kernel extended to 32 predicates crashes identically with optimization enabled.

I have a fix and can open a PR if useful.