On Wed, Aug 12, 2026 at 11:59 PM Andrea Pinski
<[email protected]> wrote:
>
> r17-3204-ge02ce3b8c4574c added a no_delete to simple_dce_from_worklist.
> With the no_delete option, for calls where the lhs would be removed,
> simple_dce_from_worklist would insert an assignment after the call.
> But with exceptions the call is required to be last stmt in the basic block.
> Note inserting before the call would not work either because of requirements
> of returns twice functions need to be the first stmt of the basic block (
> see gimple_verify_flow_info).
> So the fix is instead just not removing the lhs for no_delete case.
>
> Pushed as obvious after a bootstrap/test on x86_64-linux-gnu.
>
> PR tree-optimization/126815
>
> gcc/ChangeLog:
>
> * tree-ssa-dce.cc (simple_dce_from_worklist): Just don't
> remove the lhs for no_delete case rather than adding a
> new stmt.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/torture/pr126815-1.C: New test.
>
> Signed-off-by: Andrea Pinski <[email protected]>
> ---
> gcc/testsuite/g++.dg/torture/pr126815-1.C | 20 ++++++++++++++++++++
> gcc/tree-ssa-dce.cc | 13 +++----------
> 2 files changed, 23 insertions(+), 10 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/torture/pr126815-1.C
>
> diff --git a/gcc/testsuite/g++.dg/torture/pr126815-1.C
> b/gcc/testsuite/g++.dg/torture/pr126815-1.C
> new file mode 100644
> index 00000000000..227fdb4505f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/torture/pr126815-1.C
> @@ -0,0 +1,20 @@
> +// PR tree-optimization/126815
> +// { dg-do compile }
> +
> +
> +struct Guard { ~Guard (); }; // EH cleanup: makes f() end its basic
> block
> +int f (int);
> +void sink (int);
> +
> +void h (int a, int b, int c)
> +{
> + Guard g;
> + int t;
> + if (c)
> + t = f (a); // throwing call feeds the PHI
> + else
> + t = b; // non-constant: prevents jump threading
> + if (t == 42) // t's only use
> + __builtin_unreachable ();
> + sink (a);
> +}
> diff --git a/gcc/tree-ssa-dce.cc b/gcc/tree-ssa-dce.cc
> index bf818ad2c91..9fdd102442a 100644
> --- a/gcc/tree-ssa-dce.cc
> +++ b/gcc/tree-ssa-dce.cc
> @@ -2224,19 +2224,12 @@ simple_dce_from_worklist (bitmap worklist, bitmap
> need_eh_cleanup,
> if (gimple_has_side_effects (t))
> {
> gcall *call = dyn_cast <gcall *> (t);
> - if (call)
> + // For no delete don't remove the lhs.
> + if (call && no_delete)
I just noticed after pushing this patch I messed up the condition. It
should have been call && !no_delete.
Whoops.
Will update and push a patch to fix that. Yes I didn't run into a
bootstrap or testcase failure with it so maybe there was no testcase
testing it though.
> {
> gimple_call_set_lhs (call, NULL_TREE);
> update_stmt (call);
> - if (no_delete)
> - {
> - tree zero = build_zero_cst (TREE_TYPE (def));
> - gassign *new_stmt = gimple_build_assign (def, zero);
> - gimple_stmt_iterator gsi = gsi_for_stmt (t);
> - gsi_insert_after (&gsi, new_stmt, GSI_SAME_STMT);
> - }
> - else
> - release_ssa_name (def);
> + release_ssa_name (def);
> }
> continue;
> }
> --
> 2.43.0
>
