0,0 → 1,557 |
/* ehopt.c--optimize gcc exception frame information. |
Copyright 1998, 2000, 2001, 2003, 2005, 2007, 2008, 2009 |
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" |
#include "struc-symbol.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; |
}; |
|
static int get_cie_info (struct cie_info *); |
|
/* Extract information from the CIE. */ |
|
static int |
get_cie_info (struct cie_info *info) |
{ |
fragS *f; |
fixS *fix; |
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 (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0) |
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, |
}; |
|
/* 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 |
{ |
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 frame_data eh_frame_data; |
static struct frame_data debug_frame_data; |
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 (strncmp (segment_name (now_seg), ".eh_frame", 9) == 0 |
&& segment_name (now_seg)[9] != '_') |
d = &eh_frame_data; |
else if (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0) |
d = &debug_frame_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); |
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 (exp->X_add_symbol->bsym |
&& exp->X_op_symbol->bsym |
&& exp->X_add_symbol->sy_value.X_op == O_subtract |
&& exp->X_op_symbol->sy_value.X_op == O_constant |
&& ((exp->X_op == O_divide |
? exp->X_op_symbol->sy_value.X_add_number |
: (offsetT) 1 << exp->X_op_symbol->sy_value.X_add_number) |
== (offsetT) d->cie_info.code_alignment)) |
{ |
/* 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 (&exp->X_add_symbol->sy_value), |
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 < 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; |
|
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; |
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; |
|
default: |
md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4); |
break; |
} |
|
frag->fr_fix += frag->fr_subtype & 7; |
frag->fr_type = rs_fill; |
frag->fr_subtype = 0; |
frag->fr_offset = 0; |
} |