On Thu, Jun 4, 2026 at 3:45 PM Richard Sandiford
<[email protected]> wrote:
>
> This PR had the equivalent of:
>
> L1:
> x = PHI(1, z)
> y = -x
> ...
> L2:
> z = PHI(x, y)
>
> where z could be simplified to:
>
> z = PHI(x, x)
>
> i.e. to x. Changing z triggered a need to replace x with a new SSA name,
> but z was left being replaced with x, rather than x's replacement.
>
> Since the replacement values are (of course) restricted to using
> dominating definitions, a single RPO pass is enough to calculate the
> transitive closure of replacement values.
>
> Bootstrapped & regression-tested on aarch64-linux-gnu, powerpc64le-linux-gnu
> and x86_64-linux-gnu. OK to install?
OK.
Richard.
> Richard
>
>
> gcc/
> PR tree-optimization/125599
> * gimple-ssa-backprop.cc (backprop::execute): Calculate the
> transitive closure of replacement values.
>
> gcc/testsuite/
> PR tree-optimization/125599
> * g++.dg/torture/pr125599.C: New test.
> ---
> gcc/gimple-ssa-backprop.cc | 15 +++++++++++++++
> gcc/testsuite/g++.dg/torture/pr125599.C | 19 +++++++++++++++++++
> 2 files changed, 34 insertions(+)
> create mode 100644 gcc/testsuite/g++.dg/torture/pr125599.C
>
> diff --git a/gcc/gimple-ssa-backprop.cc b/gcc/gimple-ssa-backprop.cc
> index 5d7295c9d8e..f32f18f1014 100644
> --- a/gcc/gimple-ssa-backprop.cc
> +++ b/gcc/gimple-ssa-backprop.cc
> @@ -1107,6 +1107,21 @@ backprop::execute ()
> propagate_change (v);
> }
>
> + /* Now that we have a complete set of values that need to change,
> + calculate the transitive closure of replacement values. */
> + bool changed = false;
> + for (unsigned int i = m_vars.length (); i-- > 0;)
> + if (var_info *v = m_vars[i])
> + if (v->var != v->new_value)
> + if (var_info *newv = lookup_operand (v->new_value))
> + {
> + gcc_assert (newv->index > i);
> + if (!changed && dump_file && (dump_flags & TDF_DETAILS))
> + fprintf (dump_file, "\n");
> + set_new_value (v, newv->new_value);
> + changed = true;
> + }
> +
> if (dump_file && (dump_flags & TDF_DETAILS))
> fprintf (dump_file, "\n");
>
> diff --git a/gcc/testsuite/g++.dg/torture/pr125599.C
> b/gcc/testsuite/g++.dg/torture/pr125599.C
> new file mode 100644
> index 00000000000..63ca61a3384
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/torture/pr125599.C
> @@ -0,0 +1,19 @@
> +/* { dg-additional-options "-w" } */
> +
> +double tetallnormal_A[1][4];
> +struct tetgenmesh {
> + bool lu_decmp(double[][4], int *, double *, int);
> + void tetallnormal();
> +};
> +int lu_decmp_n;
> +bool tetgenmesh::lu_decmp(double[][4], int *, double *d, int) {
> + double biggest, pivotindex, k;
> + *d = 1.0;
> + for (; lu_decmp_n; k++)
> + if (biggest == pivotindex != k)
> + *d = -*d;
> +}
> +void tetgenmesh::tetallnormal() {
> + double D;
> + int indx[](lu_decmp(tetallnormal_A, indx, &D, 0));
> +}
> --
> 2.54.0
>
