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Avoid undefined behaviour in m68hc11 md_begin

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Given p = A where p is a pointer to some type and A is an array of
that type, then the expression p - 1 + 1 evokes undefined behaviour
according to the C standard.

gcc-13 -fsanitize=address,undefined complains about this, but not
where the undefined behaviour actually occurs at tc-m68hc11.c:646.
Instead you get an error: "tc-m68hc11.c:708:20: runtime error: store
to address 0x62600000016c with insufficient space for an object of
type 'int'".  Which is a lie.  There most definitely is space there.
Oh well, diagnostics are sometimes hard to get right.  The UB is easy
to avoid.

	PR 30279
	* config/tc-m68hc11.c (md_begin): Avoid undefined pointer
	decrement.  Remove unnecessary cast.
This commit is contained in:
Alan Modra
2023-03-28 20:25:26 +10:30
parent d7f0f10189
commit c61b7b7b8e
1 changed files with 15 additions and 17 deletions
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32
gas/config/tc-m68hc11.c
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@@ -643,7 +643,7 @@ md_begin (void)
(int (*) (const void*, const void*)) cmp_opcode);
opc = XNEWVEC (struct m68hc11_opcode_def, num_opcodes);
m68hc11_opcode_defs = opc--;
m68hc11_opcode_defs = opc;
/* Insert unique names into hash table. The M6811 instruction set
has several identical opcode names that have different opcodes based
@@ -655,19 +655,18 @@ md_begin (void)
if (strcmp (prev_name, opcodes->name))
{
prev_name = (char *) opcodes->name;
prev_name = opcodes->name;
opc++;
opc->format = 0;
opc->min_operands = 100;
opc->max_operands = 0;
opc->nb_modes = 0;
opc->opcode = opcodes;
opc->used = 0;
str_hash_insert (m68hc11_hash, opcodes->name, opc, 0);
(opc - 1)->format = 0;
(opc - 1)->min_operands = 100;
(opc - 1)->max_operands = 0;
(opc - 1)->nb_modes = 0;
(opc - 1)->opcode = opcodes;
(opc - 1)->used = 0;
str_hash_insert (m68hc11_hash, opcodes->name, opc - 1, 0);
}
opc->nb_modes++;
opc->format |= opcodes->format;
(opc - 1)->nb_modes++;
(opc - 1)->format |= opcodes->format;
/* See how many operands this opcode needs. */
expect = 0;
@@ -700,14 +699,13 @@ md_begin (void)
expect++;
}
if (expect < opc->min_operands)
opc->min_operands = expect;
if (expect < (opc - 1)->min_operands)
(opc - 1)->min_operands = expect;
if (IS_CALL_SYMBOL (opcodes->format))
expect++;
if (expect > opc->max_operands)
opc->max_operands = expect;
if (expect > (opc - 1)->max_operands)
(opc - 1)->max_operands = expect;
}
opc++;
m68hc11_nb_opcode_defs = opc - m68hc11_opcode_defs;
if (flag_print_opcodes)