AArch64: Fix __sync_val_compare_and_swap [PR111404]

__sync_val_compare_and_swap may be used on 128-bit types and either calls the
outline atomic code or uses an inline loop.  On AArch64 LDXP is only atomic if
the value is stored successfully using STXP, but the current implementations
do not perform the store if the comparison fails.  In this case the value
returned is not read atomically.

gcc/ChangeLog:
	PR target/111404
	* config/aarch64/aarch64.cc (aarch64_split_compare_and_swap):
	For 128-bit store the loaded value and loop if needed.

libgcc/ChangeLog:
	PR target/111404
	* config/aarch64/lse.S (__aarch64_cas16_acq_rel): Execute STLXP using
	either new value or loaded value.
This commit is contained in:
Wilco Dijkstra 2023-11-30 16:14:35 +00:00
parent 18d8a50a04
commit df8958e6bc
2 changed files with 35 additions and 5 deletions

View File

@ -21272,11 +21272,11 @@ aarch64_split_compare_and_swap (rtx operands[])
mem = operands[1];
oldval = operands[2];
newval = operands[3];
is_weak = (operands[4] != const0_rtx);
model_rtx = operands[5];
scratch = operands[7];
mode = GET_MODE (mem);
model = memmodel_from_int (INTVAL (model_rtx));
is_weak = operands[4] != const0_rtx && mode != TImode;
/* When OLDVAL is zero and we want the strong version we can emit a tighter
loop:
@ -21337,6 +21337,33 @@ aarch64_split_compare_and_swap (rtx operands[])
else
aarch64_gen_compare_reg (NE, scratch, const0_rtx);
/* 128-bit LDAXP is not atomic unless STLXP succeeds. So for a mismatch,
store the returned value and loop if the STLXP fails. */
if (mode == TImode)
{
rtx_code_label *label3 = gen_label_rtx ();
emit_jump_insn (gen_rtx_SET (pc_rtx, gen_rtx_LABEL_REF (Pmode, label3)));
emit_barrier ();
emit_label (label2);
aarch64_emit_store_exclusive (mode, scratch, mem, rval, model_rtx);
if (aarch64_track_speculation)
{
/* Emit an explicit compare instruction, so that we can correctly
track the condition codes. */
rtx cc_reg = aarch64_gen_compare_reg (NE, scratch, const0_rtx);
x = gen_rtx_NE (GET_MODE (cc_reg), cc_reg, const0_rtx);
}
else
x = gen_rtx_NE (VOIDmode, scratch, const0_rtx);
x = gen_rtx_IF_THEN_ELSE (VOIDmode, x,
gen_rtx_LABEL_REF (Pmode, label1), pc_rtx);
aarch64_emit_unlikely_jump (gen_rtx_SET (pc_rtx, x));
label2 = label3;
}
emit_label (label2);
/* If we used a CBNZ in the exchange loop emit an explicit compare with RVAL

View File

@ -163,6 +163,8 @@ see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
#define tmp0 16
#define tmp1 17
#define tmp2 15
#define tmp3 14
#define tmp4 13
/* Start and end a function. */
.macro STARTFN name
@ -233,10 +235,11 @@ STARTFN NAME(cas)
0: LDXP x0, x1, [x4]
cmp x0, x(tmp0)
ccmp x1, x(tmp1), #0, eq
bne 1f
STXP w(tmp2), x2, x3, [x4]
cbnz w(tmp2), 0b
1: BARRIER
csel x(tmp2), x2, x0, eq
csel x(tmp3), x3, x1, eq
STXP w(tmp4), x(tmp2), x(tmp3), [x4]
cbnz w(tmp4), 0b
BARRIER
ret
#endif