| /* ehopt.c--optimize gcc exception frame information. |
| Copyright (C) 1998-2024 Free Software Foundation, Inc. |
| Written by Ian Lance Taylor <ian@cygnus.com>. |
| |
| This file is part of GAS, the GNU Assembler. |
| |
| GAS is free software; you can redistribute it and/or modify |
| it under the terms of the GNU General Public License as published by |
| the Free Software Foundation; either version 3, or (at your option) |
| any later version. |
| |
| GAS is distributed in the hope that it will be useful, |
| but WITHOUT ANY WARRANTY; without even the implied warranty of |
| MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| GNU General Public License for more details. |
| |
| You should have received a copy of the GNU General Public License |
| along with GAS; see the file COPYING. If not, write to the Free |
| Software Foundation, 51 Franklin Street - Fifth Floor, Boston, MA |
| 02110-1301, USA. */ |
| |
| #include "as.h" |
| #include "subsegs.h" |
| |
| /* We include this ELF file, even though we may not be assembling for |
| ELF, since the exception frame information is always in a format |
| derived from DWARF. */ |
| |
| #include "dwarf2.h" |
| |
| /* Try to optimize gcc 2.8 exception frame information. |
| |
| Exception frame information is emitted for every function in the |
| .eh_frame or .debug_frame sections. Simple information for a function |
| with no exceptions looks like this: |
| |
| __FRAME_BEGIN__: |
| .4byte .LLCIE1 / Length of Common Information Entry |
| .LSCIE1: |
| #if .eh_frame |
| .4byte 0x0 / CIE Identifier Tag |
| #elif .debug_frame |
| .4byte 0xffffffff / CIE Identifier Tag |
| #endif |
| .byte 0x1 / CIE Version |
| .byte 0x0 / CIE Augmentation (none) |
| .byte 0x1 / ULEB128 0x1 (CIE Code Alignment Factor) |
| .byte 0x7c / SLEB128 -4 (CIE Data Alignment Factor) |
| .byte 0x8 / CIE RA Column |
| .byte 0xc / DW_CFA_def_cfa |
| .byte 0x4 / ULEB128 0x4 |
| .byte 0x4 / ULEB128 0x4 |
| .byte 0x88 / DW_CFA_offset, column 0x8 |
| .byte 0x1 / ULEB128 0x1 |
| .align 4 |
| .LECIE1: |
| .set .LLCIE1,.LECIE1-.LSCIE1 / CIE Length Symbol |
| .4byte .LLFDE1 / FDE Length |
| .LSFDE1: |
| .4byte .LSFDE1-__FRAME_BEGIN__ / FDE CIE offset |
| .4byte .LFB1 / FDE initial location |
| .4byte .LFE1-.LFB1 / FDE address range |
| .byte 0x4 / DW_CFA_advance_loc4 |
| .4byte .LCFI0-.LFB1 |
| .byte 0xe / DW_CFA_def_cfa_offset |
| .byte 0x8 / ULEB128 0x8 |
| .byte 0x85 / DW_CFA_offset, column 0x5 |
| .byte 0x2 / ULEB128 0x2 |
| .byte 0x4 / DW_CFA_advance_loc4 |
| .4byte .LCFI1-.LCFI0 |
| .byte 0xd / DW_CFA_def_cfa_register |
| .byte 0x5 / ULEB128 0x5 |
| .byte 0x4 / DW_CFA_advance_loc4 |
| .4byte .LCFI2-.LCFI1 |
| .byte 0x2e / DW_CFA_GNU_args_size |
| .byte 0x4 / ULEB128 0x4 |
| .byte 0x4 / DW_CFA_advance_loc4 |
| .4byte .LCFI3-.LCFI2 |
| .byte 0x2e / DW_CFA_GNU_args_size |
| .byte 0x0 / ULEB128 0x0 |
| .align 4 |
| .LEFDE1: |
| .set .LLFDE1,.LEFDE1-.LSFDE1 / FDE Length Symbol |
| |
| The immediate issue we can address in the assembler is the |
| DW_CFA_advance_loc4 followed by a four byte value. The value is |
| the difference of two addresses in the function. Since gcc does |
| not know this value, it always uses four bytes. We will know the |
| value at the end of assembly, so we can do better. */ |
| |
| struct cie_info |
| { |
| unsigned code_alignment; |
| int z_augmentation; |
| }; |
| |
| /* Extract information from the CIE. */ |
| |
| static int |
| get_cie_info (struct cie_info *info) |
| { |
| fragS *f; |
| fixS *fix; |
| unsigned int offset; |
| char CIE_id; |
| char augmentation[10]; |
| int iaug; |
| int code_alignment = 0; |
| |
| /* We should find the CIE at the start of the section. */ |
| |
| f = seg_info (now_seg)->frchainP->frch_root; |
| fix = seg_info (now_seg)->frchainP->fix_root; |
| |
| /* Look through the frags of the section to find the code alignment. */ |
| |
| /* First make sure that the CIE Identifier Tag is 0/-1. */ |
| |
| if (startswith (segment_name (now_seg), ".debug_frame")) |
| CIE_id = (char)0xff; |
| else |
| CIE_id = 0; |
| |
| offset = 4; |
| while (f != NULL && offset >= f->fr_fix) |
| { |
| offset -= f->fr_fix; |
| f = f->fr_next; |
| } |
| if (f == NULL |
| || f->fr_fix - offset < 4 |
| || f->fr_literal[offset] != CIE_id |
| || f->fr_literal[offset + 1] != CIE_id |
| || f->fr_literal[offset + 2] != CIE_id |
| || f->fr_literal[offset + 3] != CIE_id) |
| return 0; |
| |
| /* Next make sure the CIE version number is 1. */ |
| |
| offset += 4; |
| while (f != NULL && offset >= f->fr_fix) |
| { |
| offset -= f->fr_fix; |
| f = f->fr_next; |
| } |
| if (f == NULL |
| || f->fr_fix - offset < 1 |
| || f->fr_literal[offset] != 1) |
| return 0; |
| |
| /* Skip the augmentation (a null terminated string). */ |
| |
| iaug = 0; |
| ++offset; |
| while (1) |
| { |
| while (f != NULL && offset >= f->fr_fix) |
| { |
| offset -= f->fr_fix; |
| f = f->fr_next; |
| } |
| if (f == NULL) |
| return 0; |
| |
| while (offset < f->fr_fix && f->fr_literal[offset] != '\0') |
| { |
| if ((size_t) iaug < (sizeof augmentation) - 1) |
| { |
| augmentation[iaug] = f->fr_literal[offset]; |
| ++iaug; |
| } |
| ++offset; |
| } |
| if (offset < f->fr_fix) |
| break; |
| } |
| ++offset; |
| while (f != NULL && offset >= f->fr_fix) |
| { |
| offset -= f->fr_fix; |
| f = f->fr_next; |
| } |
| if (f == NULL) |
| return 0; |
| |
| augmentation[iaug] = '\0'; |
| if (augmentation[0] == '\0') |
| { |
| /* No augmentation. */ |
| } |
| else if (strcmp (augmentation, "eh") == 0) |
| { |
| /* We have to skip a pointer. Unfortunately, we don't know how |
| large it is. We find out by looking for a matching fixup. */ |
| while (fix != NULL |
| && (fix->fx_frag != f || fix->fx_where != offset)) |
| fix = fix->fx_next; |
| if (fix == NULL) |
| offset += 4; |
| else |
| offset += fix->fx_size; |
| while (f != NULL && offset >= f->fr_fix) |
| { |
| offset -= f->fr_fix; |
| f = f->fr_next; |
| } |
| if (f == NULL) |
| return 0; |
| } |
| else if (augmentation[0] != 'z') |
| return 0; |
| |
| /* We're now at the code alignment factor, which is a ULEB128. If |
| it isn't a single byte, forget it. */ |
| |
| code_alignment = f->fr_literal[offset] & 0xff; |
| if ((code_alignment & 0x80) != 0) |
| code_alignment = 0; |
| |
| info->code_alignment = code_alignment; |
| info->z_augmentation = (augmentation[0] == 'z'); |
| |
| return 1; |
| } |
| |
| enum frame_state |
| { |
| state_idle, |
| state_saw_size, |
| state_saw_cie_offset, |
| state_saw_pc_begin, |
| state_seeing_aug_size, |
| state_skipping_aug, |
| state_wait_loc4, |
| state_saw_loc4, |
| state_error, |
| }; |
| |
| struct frame_data |
| { |
| enum frame_state state; |
| |
| int cie_info_ok; |
| struct cie_info cie_info; |
| |
| symbolS *size_end_sym; |
| fragS *loc4_frag; |
| int loc4_fix; |
| |
| int aug_size; |
| int aug_shift; |
| }; |
| |
| static struct eh_state |
| { |
| struct frame_data eh_data; |
| struct frame_data debug_data; |
| } frame; |
| |
| /* This function is called from emit_expr. It looks for cases which |
| we can optimize. |
| |
| Rather than try to parse all this information as we read it, we |
| look for a single byte DW_CFA_advance_loc4 followed by a 4 byte |
| difference. We turn that into a rs_cfa_advance frag, and handle |
| those frags at the end of the assembly. If the gcc output changes |
| somewhat, this optimization may stop working. |
| |
| This function returns non-zero if it handled the expression and |
| emit_expr should not do anything, or zero otherwise. It can also |
| change *EXP and *PNBYTES. */ |
| |
| int |
| check_eh_frame (expressionS *exp, unsigned int *pnbytes) |
| { |
| struct frame_data *d; |
| |
| /* Don't optimize. */ |
| if (flag_traditional_format) |
| return 0; |
| |
| #ifdef md_allow_eh_opt |
| if (! md_allow_eh_opt) |
| return 0; |
| #endif |
| |
| /* Select the proper section data. */ |
| if (startswith (segment_name (now_seg), ".eh_frame") |
| && segment_name (now_seg)[9] != '_') |
| d = &frame.eh_data; |
| else if (startswith (segment_name (now_seg), ".debug_frame")) |
| d = &frame.debug_data; |
| else |
| return 0; |
| |
| if (d->state >= state_saw_size && S_IS_DEFINED (d->size_end_sym)) |
| { |
| /* We have come to the end of the CIE or FDE. See below where |
| we set saw_size. We must check this first because we may now |
| be looking at the next size. */ |
| d->state = state_idle; |
| } |
| |
| switch (d->state) |
| { |
| case state_idle: |
| if (*pnbytes == 4) |
| { |
| /* This might be the size of the CIE or FDE. We want to know |
| the size so that we don't accidentally optimize across an FDE |
| boundary. We recognize the size in one of two forms: a |
| symbol which will later be defined as a difference, or a |
| subtraction of two symbols. Either way, we can tell when we |
| are at the end of the FDE because the symbol becomes defined |
| (in the case of a subtraction, the end symbol, from which the |
| start symbol is being subtracted). Other ways of describing |
| the size will not be optimized. */ |
| if ((exp->X_op == O_symbol || exp->X_op == O_subtract) |
| && ! S_IS_DEFINED (exp->X_add_symbol)) |
| { |
| d->state = state_saw_size; |
| d->size_end_sym = exp->X_add_symbol; |
| } |
| } |
| break; |
| |
| case state_saw_size: |
| case state_saw_cie_offset: |
| /* Assume whatever form it appears in, it appears atomically. */ |
| d->state = (enum frame_state) (d->state + 1); |
| break; |
| |
| case state_saw_pc_begin: |
| /* Decide whether we should see an augmentation. */ |
| if (! d->cie_info_ok |
| && ! (d->cie_info_ok = get_cie_info (&d->cie_info))) |
| d->state = state_error; |
| else if (d->cie_info.z_augmentation) |
| { |
| d->state = state_seeing_aug_size; |
| d->aug_size = 0; |
| d->aug_shift = 0; |
| } |
| else |
| d->state = state_wait_loc4; |
| break; |
| |
| case state_seeing_aug_size: |
| /* Bytes == -1 means this comes from an leb128 directive. */ |
| if ((int)*pnbytes == -1 && exp->X_op == O_constant) |
| { |
| d->aug_size = exp->X_add_number; |
| d->state = state_skipping_aug; |
| } |
| else if (*pnbytes == 1 && exp->X_op == O_constant) |
| { |
| unsigned char byte = exp->X_add_number; |
| d->aug_size |= (byte & 0x7f) << d->aug_shift; |
| d->aug_shift += 7; |
| if ((byte & 0x80) == 0) |
| d->state = state_skipping_aug; |
| } |
| else |
| d->state = state_error; |
| if (d->state == state_skipping_aug && d->aug_size == 0) |
| d->state = state_wait_loc4; |
| break; |
| |
| case state_skipping_aug: |
| if ((int)*pnbytes < 0) |
| d->state = state_error; |
| else |
| { |
| int left = (d->aug_size -= *pnbytes); |
| if (left == 0) |
| d->state = state_wait_loc4; |
| else if (left < 0) |
| d->state = state_error; |
| } |
| break; |
| |
| case state_wait_loc4: |
| if (*pnbytes == 1 |
| && exp->X_op == O_constant |
| && exp->X_add_number == DW_CFA_advance_loc4) |
| { |
| /* This might be a DW_CFA_advance_loc4. Record the frag and the |
| position within the frag, so that we can change it later. */ |
| frag_grow (1 + 4); |
| d->state = state_saw_loc4; |
| d->loc4_frag = frag_now; |
| d->loc4_fix = frag_now_fix (); |
| } |
| break; |
| |
| case state_saw_loc4: |
| d->state = state_wait_loc4; |
| if (*pnbytes != 4) |
| break; |
| if (exp->X_op == O_constant) |
| { |
| /* This is a case which we can optimize. The two symbols being |
| subtracted were in the same frag and the expression was |
| reduced to a constant. We can do the optimization entirely |
| in this function. */ |
| if (exp->X_add_number < 0x40) |
| { |
| d->loc4_frag->fr_literal[d->loc4_fix] |
| = DW_CFA_advance_loc | exp->X_add_number; |
| /* No more bytes needed. */ |
| return 1; |
| } |
| else if (exp->X_add_number < 0x100) |
| { |
| d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc1; |
| *pnbytes = 1; |
| } |
| else if (exp->X_add_number < 0x10000) |
| { |
| d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc2; |
| *pnbytes = 2; |
| } |
| } |
| else if (exp->X_op == O_subtract && d->cie_info.code_alignment == 1) |
| { |
| /* This is a case we can optimize. The expression was not |
| reduced, so we can not finish the optimization until the end |
| of the assembly. We set up a variant frag which we handle |
| later. */ |
| frag_var (rs_cfa, 4, 0, 1 << 3, make_expr_symbol (exp), |
| d->loc4_fix, (char *) d->loc4_frag); |
| return 1; |
| } |
| else if ((exp->X_op == O_divide |
| || exp->X_op == O_right_shift) |
| && d->cie_info.code_alignment > 1) |
| { |
| if (symbol_symbolS (exp->X_add_symbol) |
| && symbol_constant_p (exp->X_op_symbol) |
| && S_GET_SEGMENT (exp->X_op_symbol) == absolute_section |
| && ((exp->X_op == O_divide |
| ? *symbol_X_add_number (exp->X_op_symbol) |
| : (offsetT) 1 << *symbol_X_add_number (exp->X_op_symbol)) |
| == (offsetT) d->cie_info.code_alignment)) |
| { |
| expressionS *symval; |
| |
| symval = symbol_get_value_expression (exp->X_add_symbol); |
| if (symval->X_op == O_subtract) |
| { |
| /* This is a case we can optimize as well. The |
| expression was not reduced, so we can not finish |
| the optimization until the end of the assembly. |
| We set up a variant frag which we handle later. */ |
| frag_var (rs_cfa, 4, 0, d->cie_info.code_alignment << 3, |
| make_expr_symbol (symval), |
| d->loc4_fix, (char *) d->loc4_frag); |
| return 1; |
| } |
| } |
| } |
| break; |
| |
| case state_error: |
| /* Just skipping everything. */ |
| break; |
| } |
| |
| return 0; |
| } |
| |
| /* The function estimates the size of a rs_cfa variant frag based on |
| the current values of the symbols. It is called before the |
| relaxation loop. We set fr_subtype{0:2} to the expected length. */ |
| |
| int |
| eh_frame_estimate_size_before_relax (fragS *frag) |
| { |
| offsetT diff; |
| int ca = frag->fr_subtype >> 3; |
| int ret; |
| |
| diff = resolve_symbol_value (frag->fr_symbol); |
| |
| gas_assert (ca > 0); |
| diff /= ca; |
| if (diff == 0) |
| ret = -1; |
| else if (diff < 0x40) |
| ret = 0; |
| else if (diff < 0x100) |
| ret = 1; |
| else if (diff < 0x10000) |
| ret = 2; |
| else |
| ret = 4; |
| |
| frag->fr_subtype = (frag->fr_subtype & ~7) | (ret & 7); |
| |
| return ret; |
| } |
| |
| /* This function relaxes a rs_cfa variant frag based on the current |
| values of the symbols. fr_subtype{0:2} is the current length of |
| the frag. This returns the change in frag length. */ |
| |
| int |
| eh_frame_relax_frag (fragS *frag) |
| { |
| int oldsize, newsize; |
| |
| oldsize = frag->fr_subtype & 7; |
| if (oldsize == 7) |
| oldsize = -1; |
| newsize = eh_frame_estimate_size_before_relax (frag); |
| return newsize - oldsize; |
| } |
| |
| /* This function converts a rs_cfa variant frag into a normal fill |
| frag. This is called after all relaxation has been done. |
| fr_subtype{0:2} will be the desired length of the frag. */ |
| |
| void |
| eh_frame_convert_frag (fragS *frag) |
| { |
| offsetT diff; |
| fragS *loc4_frag; |
| int loc4_fix, ca; |
| |
| loc4_frag = (fragS *) frag->fr_opcode; |
| loc4_fix = (int) frag->fr_offset; |
| |
| diff = resolve_symbol_value (frag->fr_symbol); |
| |
| ca = frag->fr_subtype >> 3; |
| gas_assert (ca > 0); |
| diff /= ca; |
| switch (frag->fr_subtype & 7) |
| { |
| case 0: |
| gas_assert (diff < 0x40); |
| loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | diff; |
| break; |
| |
| case 1: |
| gas_assert (diff < 0x100); |
| loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc1; |
| frag->fr_literal[frag->fr_fix] = diff; |
| break; |
| |
| case 2: |
| gas_assert (diff < 0x10000); |
| loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc2; |
| md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 2); |
| break; |
| |
| case 4: |
| md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4); |
| break; |
| |
| case 7: |
| gas_assert (diff == 0); |
| frag->fr_fix -= 8; |
| break; |
| |
| default: |
| abort (); |
| } |
| |
| frag->fr_fix += frag->fr_subtype & 7; |
| frag->fr_type = rs_fill; |
| frag->fr_subtype = 0; |
| frag->fr_offset = 0; |
| } |
| |
| void |
| eh_begin (void) |
| { |
| memset (&frame, 0, sizeof (frame)); |
| } |