https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103619
--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:e75a0a03588977c8c758091f9b50d456a5f67227 commit r12-5994-ge75a0a03588977c8c758091f9b50d456a5f67227 Author: Jakub Jelinek <ja...@redhat.com> Date: Wed Dec 15 10:41:02 2021 +0100 dwarf2cfi: Improve cfa_reg comparisons [PR103619] On Tue, Dec 14, 2021 at 10:32:21AM -0700, Jeff Law wrote: > I think the attached testcase should trigger on c6x with -mbig-endian -O2 -g Thanks. Finally I see what's going on. c6x doesn't really need the CFA with span > 1 (and I bet neither does armbe), the only reason why dwf_cfa_reg is called is that the code in 13 cases just tries to compare the CFA against dwf_cfa_reg (some_reg). And that dwf_cfa_reg on some reg that usually isn't a CFA reg results in targetm.dwarf_register_span hook call, which on targets like c6x or armeb and others for some registers creates a PARALLEL with various REGs in it, then the loop with the assertion and finally operator== which just notes that the reg is different and fails. This seems compile time memory and time inefficient. The following so far untested patch instead adds an extra operator== and != for comparison of cfa_reg with rtx, which has the most common case where it is a different register number done early without actually invoking dwf_cfa_reg. This means the assertion in dwf_cfa_reg can stay as is (at least until some big endian target needs to have hard frame pointer or stack pointer with span > 1 as well). I've removed a different assertion there because it is redundant - dwf_regno already has exactly that assertion in it too. And I've included those 2 tweaks to avoid creating a REG in GC memory when we can use {stack,hard_frame}_pointer_rtx which is already initialized to the same REG we need by init_emit_regs. On Tue, Dec 14, 2021 at 03:05:37PM -0700, Jeff Law wrote: > So if someone is unfamiliar with the underlying issues here and needs to > twiddle dwarf2cfi, how are they supposed to know if they should compare > directly or use dwf_cfa_reg? Comparison without dwf_cfa_reg should be used whenever possible, because for registers which are never CFA related that won't call targetm.dwarf_register_span uselessly. The only comparisons with dwf_cfa_reg I've kept are the: regno = dwf_cfa_reg (XEXP (XEXP (dest, 0), 0)); if (cur_cfa->reg == regno) offset -= cur_cfa->offset; else if (cur_trace->cfa_store.reg == regno) offset -= cur_trace->cfa_store.offset; else { gcc_assert (cur_trace->cfa_temp.reg == regno); offset -= cur_trace->cfa_temp.offset; } and struct cfa_reg regno = dwf_cfa_reg (XEXP (dest, 0)); if (cur_cfa->reg == regno) offset = -cur_cfa->offset; else if (cur_trace->cfa_store.reg == regno) offset = -cur_trace->cfa_store.offset; else { gcc_assert (cur_trace->cfa_temp.reg == regno); offset = -cur_trace->cfa_temp.offset; } and there are 2 reasons for it: 1) there is an assertion, which guarantees it must compare equal to one of those 3 cfa related struct cfa_reg structs, so it must be some CFA related register (so, right now, targetm.dwarf_register_span shouldn't return non-NULL in those on anything but gcn) 2) it is compared 3 times in a row, so for the GCN case doing if (cur_cfa->reg == XEXP (XEXP (dest, 0), 0)) offset -= cur_cfa->offset; else if (cur_trace->cfa_store.reg == XEXP (XEXP (dest, 0), 0)) offset -= cur_trace->cfa_store.offset; else { gcc_assert (cur_trace->cfa_temp.reg == XEXP (XEXP (dest, 0), 0)); offset -= cur_trace->cfa_temp.offset; } could actually create more GC allocated garbage than the way it is written now. But doing it that way would work fine. I think for most of the comparisons even comparing with dwf_cfa_reg would work but be less compile time/memory efficient (e.g. those assertions that it is equal to some CFA related cfa_reg or in any spots where only the CFA related regs may appear in the frame related patterns). I'm aware just of a single spot where comparison with dwf_cfa_reg doesn't work (when the assert is in dwf_cfa_reg), that is the spot that was ICEing on your testcase, where we save arbitrary call saved register: if (REG_P (src) && REGNO (src) != STACK_POINTER_REGNUM && REGNO (src) != HARD_FRAME_POINTER_REGNUM && cur_cfa->reg == src) 2021-12-15 Jakub Jelinek <ja...@redhat.com> PR debug/103619 * dwarf2cfi.c (dwf_cfa_reg): Remove gcc_assert. (operator==, operator!=): New overloaded operators. (dwarf2out_frame_debug_adjust_cfa, dwarf2out_frame_debug_cfa_offset, dwarf2out_frame_debug_expr): Compare vars with cfa_reg type directly with REG rtxes rather than with dwf_cfa_reg results on those REGs. (create_cie_data): Use stack_pointer_rtx instead of gen_rtx_REG (Pmode, STACK_POINTER_REGNUM). (execute_dwarf2_frame): Use hard_frame_pointer_rtx instead of gen_rtx_REG (Pmode, HARD_FRAME_POINTER_REGNUM).