On Thu, Jan 23, 2025 at 2:05 PM Richard Sandiford
<[email protected]> wrote:
>
> rtl-ssa uses degenerate phis to maintain an RPO list of
> accesses in which every use is of the RPO-previous definition.
> Thus, if it finds that a phi is always equal to a particular
> value V, it sometimes needs to keep the phi and make V the
> single input, rather than replace all uses of the phi with V.
>
> The code to do that rerouted the phi's first input to the single
> value V. But as this PR shows, it failed to unlink the uses of
> the other inputs.
>
> The specific problem in the PR was that we had:
>
> x = PHI<x(a), V(b)>
>
> The code replaced the first input with V and removed the second
> input from the phi, but it didn't unlink the use of V associated
> with that second input.
>
> Bootstrapped & regression-tested on aarch64-linux-gnu. Also
> spot-checked on a riscv64-linux-gnu cross-compiler. OK to install?
OK.
Richard.
> Richard
>
>
> gcc/
> PR rtl-optimization/118562
> * rtl-ssa/blocks.cc (function_info::replace_phi): When converting
> to a degenerate phi, make sure to remove all uses of the previous
> inputs.
>
> gcc/testsuite/
> PR rtl-optimization/118562
> * gcc.dg/torture/pr118562.c: New test.
> ---
> gcc/rtl-ssa/blocks.cc | 19 +++++++++++++++----
> gcc/testsuite/gcc.dg/torture/pr118562.c | 18 ++++++++++++++++++
> 2 files changed, 33 insertions(+), 4 deletions(-)
> create mode 100644 gcc/testsuite/gcc.dg/torture/pr118562.c
>
> diff --git a/gcc/rtl-ssa/blocks.cc b/gcc/rtl-ssa/blocks.cc
> index e175f8ce460..953fd9e516e 100644
> --- a/gcc/rtl-ssa/blocks.cc
> +++ b/gcc/rtl-ssa/blocks.cc
> @@ -424,14 +424,25 @@ function_info::replace_phi (phi_info *phi, set_info
> *new_value)
> {
> // We need to keep the phi around for its local uses.
> // Turn it into a degenerate phi, if it isn't already.
> - use_info *use = phi->input_use (0);
> - if (use->def () != new_value)
> - update_use (use);
> + use_info *single_use = nullptr;
> + for (auto *use : phi->inputs ())
> + if (!single_use)
> + single_use = use;
> + else if (use->def () == new_value)
> + {
> + remove_use (single_use);
> + single_use = use;
> + }
> + else
> + remove_use (use);
> +
> + if (single_use->def () != new_value)
> + update_use (single_use);
>
> if (phi->is_degenerate ())
> return;
>
> - phi->make_degenerate (use);
> + phi->make_degenerate (single_use);
>
> // Redirect all phi users to NEW_VALUE.
> while (use_info *phi_use = phi->last_phi_use ())
> diff --git a/gcc/testsuite/gcc.dg/torture/pr118562.c
> b/gcc/testsuite/gcc.dg/torture/pr118562.c
> new file mode 100644
> index 00000000000..82161e76015
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/torture/pr118562.c
> @@ -0,0 +1,18 @@
> +/* { dg-additional-options "-march=rv64gv -mabi=lp64" { target { rv64 } } }
> */
> +
> +float b[2], c[2];
> +void d();
> +int h1();
> +void e(float * __restrict h) {
> + int f;
> + for (int f = 0; f < 4; f++) {
> + if (h1())
> + d();
> + }
> + for (int g = 0; g < 4; g++) {
> + c[0] = h[0] - b[0];
> + c[1] = h[1] - b[1];
> + d();
> + h += 1;
> + }
> +}
> --
> 2.25.1
>
