On Thu, Mar 23, 2017 at 3:58 PM, Segher Boessenkool
<seg...@kernel.crashing.org> wrote:
> The algorithm fwprop uses never reconsiders a possible propagation,
> although it could succeed if the def (in the def-use to propagate)
> has changed. This causes fwprop to do infinite propagations, like
> in the scenario in the PR, where we end up with effectively
> B = A
> A = B
> D = A
> where only propagations into the last statement are still tried, and
> that loops (it becomes D = B, then back to D = A, etc.)
>
> Fixing this properly isn't easy; this patch instead limits the number
> of propagations performed to the number of uses we originally had,
> which is the maximum number of propagations that can be done if there
> are no such infinite loops.
>
> Bootstrapped and regression checked on powerpc64-linux {-m64,-m32};
> is this okay for trunk?
https://gcc.gnu.org/ml/gcc-patches/2017-02/msg01485.html
?
>
> Segher
>
>
> 2017-03-23 Segher Boessenkool <seg...@kernel.crashing.org>
>
> PR rtl-optimization/79405
> * fwprop.c (propagations_left): New variable.
> (forward_propagate_into): Decrement it.
> (fwprop_init): Initialize it.
> (fw_prop): If the variable has reached zero, stop propagating.
> (fwprop_addr): Ditto.
>
> gcc/testsuite/
> PR rtl-optimization/79405
> gcc.dg/pr79405.c: New testcase.
>
> ---
> gcc/fwprop.c | 17 ++++++++++++++++
> gcc/testsuite/gcc.dg/pr79405.c | 45
> ++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 62 insertions(+)
> create mode 100644 gcc/testsuite/gcc.dg/pr79405.c
>
> diff --git a/gcc/fwprop.c b/gcc/fwprop.c
> index 285fb1a..0ab25ef 100644
> --- a/gcc/fwprop.c
> +++ b/gcc/fwprop.c
> @@ -120,6 +120,13 @@ static vec<df_ref> use_def_ref;
> static vec<df_ref> reg_defs;
> static vec<df_ref> reg_defs_stack;
>
> +/* The maximum number of propagations that are still allowed. If we do
> + more propagations than originally we had uses, we must have ended up
> + in a propagation loop, as in PR79405. Until the algorithm fwprop
> + uses can obviously not get into such loops we need a workaround like
> + this. */
> +static int propagations_left;
> +
> /* The MD bitmaps are trimmed to include only live registers to cut
> memory usage on testcases like insn-recog.c. Track live registers
> in the basic block and do not perform forward propagation if the
> @@ -1407,6 +1414,8 @@ forward_propagate_into (df_ref use)
> if (forward_propagate_and_simplify (use, def_insn, def_set)
> || forward_propagate_subreg (use, def_insn, def_set))
> {
> + propagations_left--;
> +
> if (cfun->can_throw_non_call_exceptions
> && find_reg_note (use_insn, REG_EH_REGION, NULL_RTX)
> && purge_dead_edges (DF_REF_BB (use)))
> @@ -1434,6 +1443,8 @@ fwprop_init (void)
> active_defs = XNEWVEC (df_ref, max_reg_num ());
> if (flag_checking)
> active_defs_check = sparseset_alloc (max_reg_num ());
> +
> + propagations_left = DF_USES_TABLE_SIZE ();
> }
>
> static void
> @@ -1480,6 +1491,9 @@ fwprop (void)
>
> for (i = 0; i < DF_USES_TABLE_SIZE (); i++)
> {
> + if (!propagations_left)
> + break;
> +
> df_ref use = DF_USES_GET (i);
> if (use)
> if (DF_REF_TYPE (use) == DF_REF_REG_USE
> @@ -1540,6 +1554,9 @@ fwprop_addr (void)
> end, and we'll go through them as well. */
> for (i = 0; i < DF_USES_TABLE_SIZE (); i++)
> {
> + if (!propagations_left)
> + break;
> +
> df_ref use = DF_USES_GET (i);
> if (use)
> if (DF_REF_TYPE (use) != DF_REF_REG_USE
> diff --git a/gcc/testsuite/gcc.dg/pr79405.c b/gcc/testsuite/gcc.dg/pr79405.c
> new file mode 100644
> index 0000000..c17baff
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr79405.c
> @@ -0,0 +1,45 @@
> +/* PR rtl-optimization/79405 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2" } */
> +
> +char cz;
> +long long int xx, u2;
> +
> +void
> +qv (int js, int wl)
> +{
> + if (js != 0)
> + {
> + short int sc;
> + int *at = (int *)≻
> + long long int gx = 0;
> +
> + for (;;)
> + {
> + *at = 0;
> + js /= sc;
> +
> + for (wl = 0; wl < 2; ++wl)
> + {
> + xx = gx;
> + u2 %= xx > 0;
> + cz /= u2;
> +
> + fa:
> + if (cz != u2)
> + {
> + gx |= js;
> + cz = gx / js;
> + }
> + }
> + }
> +
> + yq:
> + wl /= 0x80000000;
> + u2 = wl;
> + u2 |= (wl != 0) | (wl != 0 && gx != 0);
> + js = u2;
> + goto fa;
> + }
> + goto yq;
> +}
> --
> 1.9.3
>
