/* ehopt.c--optimize gcc exception frame information.
Copyright (C) 1998-2024 Free Software Foundation, Inc.
Written by Ian Lance Taylor <
[email protected]>.
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));
}
