On Tue, 24 May 2022 at 11:50, Richard Biener via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> When facing multiple PHI defs and one feeding the other we can
> postpone processing uses of one and thus can proceed.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.
>
> 2022-05-20 Richard Biener <rguent...@suse.de>
>
> PR tree-optimization/100221
> * tree-ssa-dse.cc (contains_phi_arg): New function.
> (dse_classify_store): Postpone PHI defs that feed another PHI in defs.
>
> * gcc.dg/tree-ssa/ssa-dse-44.c: New testcase.
> * gcc.dg/tree-ssa/ssa-dse-45.c: Likewise.
> ---
> gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-44.c | 19 +++++++++
> gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-45.c | 24 +++++++++++
> gcc/tree-ssa-dse.cc | 46 +++++++++++++++++++---
> 3 files changed, 84 insertions(+), 5 deletions(-)
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-44.c
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-45.c
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-44.c
> b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-44.c
> new file mode 100644
> index 00000000000..aaec41d7bdf
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-44.c
> @@ -0,0 +1,19 @@
> +/* { dg-do link } */
> +/* { dg-options "-O -fdump-tree-dse1-details" } */
> +
> +extern void foo(void);
> +int a, b;
> +static int c;
> +int main()
> +{
> + if (c)
> + foo ();
> + int *g = &c;
> + int **h = &g;
> + int ***h1 = &h;
> + if (a)
> + while (b)
> + b = 0;
> +}
> +
> +/* { dg-final { scan-tree-dump "Deleted dead store: g = &c;" "dse1" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-45.c
> b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-45.c
> new file mode 100644
> index 00000000000..fd92d7b599a
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-45.c
> @@ -0,0 +1,24 @@
> +/* { dg-do link } */
> +/* { dg-options "-O" } */
> +
> +extern void foo(void);
> +int a, b;
> +static int c;
> +static void f() {
> + while (a)
> + for (; b; b--)
> + ;
> +}
> +void i() {
> + if (c)
> + foo();
> + int *g = &c;
> + {
> + int **h[1] = {&g};
> + f();
> + }
> +}
> +int main() {
> + i();
> + return 0;
> +}
> diff --git a/gcc/tree-ssa-dse.cc b/gcc/tree-ssa-dse.cc
> index 881a2d0f98d..ea50de789b1 100644
> --- a/gcc/tree-ssa-dse.cc
> +++ b/gcc/tree-ssa-dse.cc
> @@ -898,6 +898,17 @@ dse_optimize_redundant_stores (gimple *stmt)
> }
> }
>
> +/* Return whether PHI contains ARG as an argument. */
> +
> +static bool
> +contains_phi_arg (gphi *phi, tree arg)
> +{
> + for (unsigned i = 0; i < gimple_phi_num_args (phi); ++i)
> + if (gimple_phi_arg_def (phi, i) == arg)
> + return true;
> + return false;
> +}
Hi Richard,
Just wondering if contains_phi_arg could be a more general purpose
utility function, and moved to gimple.cc
or tree-ssa.cc than be specific to tree-ssa-dse.cc ?
Thanks,
Prathamesh
> +
> /* A helper of dse_optimize_stmt.
> Given a GIMPLE_ASSIGN in STMT that writes to REF, classify it
> according to downstream uses and defs. Sets *BY_CLOBBER_P to true
> @@ -949,8 +960,8 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
> return DSE_STORE_LIVE;
>
> auto_vec<gimple *, 10> defs;
> - gimple *first_phi_def = NULL;
> - gimple *last_phi_def = NULL;
> + gphi *first_phi_def = NULL;
> + gphi *last_phi_def = NULL;
> FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
> {
> /* Limit stmt walking. */
> @@ -973,8 +984,8 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
> {
> defs.safe_push (use_stmt);
> if (!first_phi_def)
> - first_phi_def = use_stmt;
> - last_phi_def = use_stmt;
> + first_phi_def = as_a <gphi *> (use_stmt);
> + last_phi_def = as_a <gphi *> (use_stmt);
> }
> }
> /* If the statement is a use the store is not dead. */
> @@ -1046,6 +1057,7 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
> use_operand_p use_p;
> tree vdef = (gimple_code (def) == GIMPLE_PHI
> ? gimple_phi_result (def) : gimple_vdef (def));
> + gphi *phi_def;
> /* If the path to check starts with a kill we do not need to
> process it further.
> ??? With byte tracking we need only kill the bytes currently
> @@ -1079,7 +1091,31 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
> && bitmap_bit_p (visited,
> SSA_NAME_VERSION
> (PHI_RESULT (use_stmt))))))
> - defs.unordered_remove (i);
> + {
> + defs.unordered_remove (i);
> + if (def == first_phi_def)
> + first_phi_def = NULL;
> + else if (def == last_phi_def)
> + last_phi_def = NULL;
> + }
> + /* If def is a PHI and one of its arguments is another PHI node
> still
> + in consideration we can defer processing it. */
> + else if ((phi_def = dyn_cast <gphi *> (def))
> + && ((last_phi_def
> + && phi_def != last_phi_def
> + && contains_phi_arg (phi_def,
> + gimple_phi_result
> (last_phi_def)))
> + || (first_phi_def
> + && phi_def != first_phi_def
> + && contains_phi_arg
> + (phi_def, gimple_phi_result
> (first_phi_def)))))
> + {
> + defs.unordered_remove (i);
> + if (phi_def == first_phi_def)
> + first_phi_def = NULL;
> + else if (phi_def == last_phi_def)
> + last_phi_def = NULL;
> + }
> else
> ++i;
> }
> --
> 2.35.3
