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Fix miscompilation of function containing __builtin_unreachable

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This is a wrong-code generation on the SPARC for a function containing
a call to __builtin_unreachable caused by the delay slot scheduling pass,
and more specifically the find_end_label function which has these lines:

  /* Otherwise, see if there is a label at the end of the function. If there
     is, it must be that RETURN insns aren't needed, so that is our return
     label and we don't have to do anything else.  */

The comment was correct 20 years ago but no longer is nowadays in the
presence of RTL epilogues and calls to __builtin_unreachable, so the
patch just removes the associated two lines of code:

  else if (LABEL_P (insn))
    *plabel = as_a <rtx_code_label *> (insn);

and otherwise contains just adjustments to the commentary.

gcc/
	PR rtl-optimization/117327
	* reorg.cc (find_end_label): Do not return a dangling label at the
	end of the function and adjust commentary.

gcc/testsuite/
	* gcc.c-torture/execute/20241029-1.c: New test.
This commit is contained in:
Eric Botcazou 2024-10-29 21:40:34 +01:00
parent 9dd9a88b75
commit 7211155732
2 changed files with 38 additions and 23 deletions
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38
gcc/reorg.cc
View File

@ -336,13 +336,14 @@ insn_sets_resource_p (rtx insn, struct resources *res,
return resource_conflicts_p (&insn_sets, res);
}
/* Find a label at the end of the function or before a RETURN. If there
is none, try to make one. If that fails, returns 0.
/* Find a label before a RETURN. If there is none, try to make one; if this
fails, return 0. KIND is either ret_rtx or simple_return_rtx, indicating
which type of RETURN we're looking for.
The property of such a label is that it is placed just before the
epilogue or a bare RETURN insn, so that another bare RETURN can be
turned into a jump to the label unconditionally. In particular, the
label cannot be placed before a RETURN insn with a filled delay slot.
The property of the label is that it is placed just before a bare RETURN
insn, so that another bare RETURN can be turned into a jump to the label
unconditionally. In particular, the label cannot be placed before a
RETURN insn with a filled delay slot.
??? There may be a problem with the current implementation. Suppose
we start with a bare RETURN insn and call find_end_label. It may set
@ -353,9 +354,7 @@ insn_sets_resource_p (rtx insn, struct resources *res,
Note that this is probably mitigated by the following observation:
once function_return_label is made, it is very likely the target of
a jump, so filling the delay slot of the RETURN will be much more
difficult.
KIND is either simple_return_rtx or ret_rtx, indicating which type of
return we're looking for. */
difficult. */
static rtx_code_label *
find_end_label (rtx kind)
@ -375,10 +374,7 @@ find_end_label (rtx kind)
if (*plabel)
return *plabel;
/* Otherwise, see if there is a label at the end of the function. If there
is, it must be that RETURN insns aren't needed, so that is our return
label and we don't have to do anything else. */
/* Otherwise, scan the insns backward from the end of the function. */
insn = get_last_insn ();
while (NOTE_P (insn)
|| (NONJUMP_INSN_P (insn)
@ -386,9 +382,8 @@ find_end_label (rtx kind)
|| GET_CODE (PATTERN (insn)) == CLOBBER)))
insn = PREV_INSN (insn);
/* When a target threads its epilogue we might already have a
suitable return insn. If so put a label before it for the
function_return_label. */
/* First, see if there is a RETURN at the end of the function. If so,
put the label before it. */
if (BARRIER_P (insn)
&& JUMP_P (PREV_INSN (insn))
&& PATTERN (PREV_INSN (insn)) == kind)
@ -397,8 +392,8 @@ find_end_label (rtx kind)
rtx_code_label *label = gen_label_rtx ();
LABEL_NUSES (label) = 0;
/* Put the label before any USE insns that may precede the RETURN
insn. */
/* Put the label before any USE insns that may precede the
RETURN insn. */
while (GET_CODE (temp) == USE)
temp = PREV_INSN (temp);
@ -406,15 +401,12 @@ find_end_label (rtx kind)
*plabel = label;
}
else if (LABEL_P (insn))
*plabel = as_a <rtx_code_label *> (insn);
/* If the basic block reordering pass has moved the return insn to some
other place, try to locate it again and put the label there. */
else
{
rtx_code_label *label = gen_label_rtx ();
LABEL_NUSES (label) = 0;
/* If the basic block reorder pass moves the return insn to
some other place try to locate it again and put our
function_return_label there. */
while (insn && ! (JUMP_P (insn) && (PATTERN (insn) == kind)))
insn = PREV_INSN (insn);
if (insn)

23
gcc/testsuite/gcc.c-torture/execute/20241029-1.c Normal file
View File

@ -0,0 +1,23 @@
/* PR rtl-optimization/117327 */
/* Testcase by Brad Moody <brad.moody@oracle.com> */
__attribute__((noinline))
void foo(int *self, int *x)
{
__builtin_puts ("foo\n");
if (x) {
while (1) {
++*self;
if (*self == 6) break;
if (*self == 7) __builtin_unreachable();
}
}
}
int main (void)
{
int y = 0;
foo (&y, 0);
return 0;
}