https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121843
--- Comment #2 from Jeevitha <jeevitha at gcc dot gnu.org> ---
The ICE occurs during the shrink-wrapping phase of the pro_and_epilogue pass.
After shrink-wrapping completes, the pass attempts to insert
NOTE_INSN_PROLOGUE_END and NOTE_INSN_EPILOGUE_BEG notes. The epilogue
note insertion between BB5 and EXIT succeeds normally. However, when
inserting the prologue note on the BB6 → BB2 edge, the note is inserted
before insn 54 (pc=L5), which is a jump instruction.
Since there is no non-note instruction preceding this jump,
prev_nonnote_insn(before) returns NULL, leaving last NULL. The
subsequent call to returnjump_p(last) then dereferences a NULL pointer,
triggering the ICE.
RTL (pro_and_epilogue)
1: NOTE_INSN_DELETED
2: NOTE_INSN_FUNCTION_BEG
53: NOTE_INSN_BASIC_BLOCK 6
60: NOTE_INSN_PROLOGUE_END <-- NOTE inserted here (no prior non-note
insn)
54: pc=L5 <-- jump instruction immediately follows
57: L57:
56: NOTE_INSN_BASIC_BLOCK 7
51: %r23:DI=[%r1:DI-0x48]
REG_CFA_RESTORE %r23:DI
5: L5:
6: NOTE_INSN_BASIC_BLOCK 2
7: debug begin stmt marker
8: {asm_operands;clobber ca:SI;}
REG_LABEL_TARGET code_label
Relevant code in commit_one_edge_insertion (cfgrtl.cc):
if (before)
{
emit_insn_before_noloc (insns, before, bb); /* NOTE_INSN_PROLOGUE_END
inserted here */
last = prev_nonnote_insn (before); /* Returns NULL → last is
NULL */
}
else
last = emit_insn_after_noloc (insns, after, bb);
if (returnjump_p (last)) /* NULL dereference → ICE */
With -fno-omit-frame-pointer, stack-pointer update instructions are emitted
before the jump, so prev_nonnote_insn returns a valid insn and the ICE is
avoided:
RTL (pro_and_epilogue) with -fno-omit-frame-pointer:
1: NOTE_INSN_DELETED
2: NOTE_INSN_FUNCTION_BEG
58: NOTE_INSN_BASIC_BLOCK 2
65: [%r1:DI-0x8]=%r31:DI
66: {[%r1:DI-0x80]=%r1:DI;%r1:DI=%r1:DI-0x80;}
REG_FRAME_RELATED_EXPR %r1:DI=%r1:DI-0x80
67: %r31:DI=%r1:DI
68: NOTE_INSN_PROLOGUE_END <-- NOTE safely inserted after real insns
59: pc=L5
62: L62:
61: NOTE_INSN_BASIC_BLOCK 3
56: %r23:DI=[%r31:DI+0x38]
REG_CFA_RESTORE %r23:DI
