https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116429
--- Comment #2 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Michael Matz <m...@gcc.gnu.org>: https://gcc.gnu.org/g:e223ac9c225352e3aeea7180a3b56a96ecdbe2fd commit r15-3221-ge223ac9c225352e3aeea7180a3b56a96ecdbe2fd Author: Michael Matz <m...@suse.de> Date: Thu Aug 22 17:21:42 2024 +0200 LRA: Fix setup_sp_offset This is part of making m68k work with LRA. See PR116429. In short: setup_sp_offset is internally inconsistent. It wants to setup the sp_offset for newly generated instructions. sp_offset for an instruction is always the state of the sp-offset right before that instruction. For that it starts at the (assumed correct) sp_offset of the instruction right after the given (new) sequence, and then iterates that sequence forward simulating its effects on sp_offset. That can't ever be right: either it needs to start at the front and simulate forward, or start at the end and simulate backward. The former seems to be the more natural way. Funnily the local variable holding that instruction is also called 'before'. This changes it to the first variant: start before the sequence, do one simulation step to get the sp-offset state in front of the sequence and then continue simulating. More details: in the problematic testcase we start with this situation (sp_off before 550 is 0): 550: [--sp] = 0 sp_off = 0 {pushexthisi_const} 551: [--sp] = 37 sp_off = -4 {pushexthisi_const} 552: [--sp] = r37 sp_off = -8 {movsi_m68k2} 554: [--sp] = r116 - r37 sp_off = -12 {subsi3} 556: call sp_off = -16 insn 554 doesn't match its constraints and needs some reloads: Creating newreg=262, assigning class DATA_REGS to r262 554: r262:SI=r262:SI-r37:SI REG_ARGS_SIZE 0x10 Inserting insn reload before: 996: r262:SI=r116:SI Inserting insn reload after: 997: [--%sp:SI]=r262:SI Considering alt=0 of insn 997: (0) =g (1) damSKT 1 Non pseudo reload: reject++ overall=1,losers=0,rld_nregs=0 Choosing alt 0 in insn 997: (0) =g (1) damSKT {*movsi_m68k2} (sp_off=-16) Note how insn 997 (the after-reload) now has sp_off=-16 already. It all goes downhill from there. We end up with these insns: 552: [--sp] = r37 sp_off = -8 {movsi_m68k2} 996: r262 = r116 sp_off = -12 554: r262 = r262 - r37 sp_off = -12 997: [--sp] = r262 sp_off = -16 (!!! should be -12) 556: call sp_off = -16 The call insn sp_off remains at the correct -16, but internally it's already inconsistent here. If the sp_off before an insn is -16, and that insn pre_decs sp, then the after-insn sp_off should be -20. PR target/116429 * lra.cc (setup_sp_offset): Start with sp_offset from before the new sequence, not from after.
