On Wed, Aug 20, 2025 at 1:03 PM Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> While testing a later patch, I found that create_degenerate_phi
> had an inverted test for bitmap_set_bit. It was assuming that
> the return value was the previous bit value, rather than a
> "something changed" value. :(
>
> Also, the call to add_live_out_use shouldn't be conditional
> on the DF_LR_OUT operation, since the register could be live-out
> because of uses later in the same EBB (which do not require a
> live-out use to be added to the rtl-ssa instruction). Instead,
> add_live_out should itself check whether a live-out use already exists.
>
> Tested on aarch64-linux-gnu, powerpc64le-linux-gnu and
> x86_64-linux-gnu. OK to install?
OK. Is the inverted test worth backporting?
> Richard
>
>
> gcc/
> * rtl-ssa/blocks.cc (function_info::create_degenerate_phi): Fix
> inverted test of bitmap_set_bit. Call add_live_out_use even
> if the register was previously live-out from the predecessor block.
> Instead...
> (function_info::add_live_out_use): ...check here whether a live-out
> use already exists.
> ---
> gcc/rtl-ssa/blocks.cc | 11 +++++------
> 1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/gcc/rtl-ssa/blocks.cc b/gcc/rtl-ssa/blocks.cc
> index 953fd9e516e..a57b9e15f13 100644
> --- a/gcc/rtl-ssa/blocks.cc
> +++ b/gcc/rtl-ssa/blocks.cc
> @@ -315,15 +315,14 @@ function_info::add_live_out_use (bb_info *bb, set_info
> *def)
>
> // If the end of the block already has an artificial use, that use
> // acts to make DEF live at the appropriate point.
> - use_info *use = def->last_nondebug_insn_use ();
> - if (use && use->insn () == bb->end_insn ())
> + if (find_use (def, bb->end_insn ()).matching_use ())
> return;
>
> // Currently there is no need to maintain a backward link from the end
> // instruction to the list of live-out uses. Such a list would be
> // expensive to update if it was represented using the usual insn_info
> // access arrays.
> - use = allocate<use_info> (bb->end_insn (), def->resource (), def);
> + auto *use = allocate<use_info> (bb->end_insn (), def->resource (), def);
> use->set_is_live_out_use (true);
> add_use (use);
> }
> @@ -540,12 +539,12 @@ function_info::create_degenerate_phi (ebb_info *ebb,
> set_info *def)
> basic_block pred_cfg_bb = single_pred (ebb->first_bb ()->cfg_bb ());
> bb_info *pred_bb = this->bb (pred_cfg_bb);
>
> - if (!bitmap_set_bit (DF_LR_IN (ebb->first_bb ()->cfg_bb ()), regno))
> + if (bitmap_set_bit (DF_LR_IN (ebb->first_bb ()->cfg_bb ()), regno))
> {
> // The register was not previously live on entry to EBB and
> // might not have been live on exit from PRED_BB either.
> - if (bitmap_set_bit (DF_LR_OUT (pred_cfg_bb), regno))
> - add_live_out_use (pred_bb, def);
> + bitmap_set_bit (DF_LR_OUT (pred_cfg_bb), regno);
> + add_live_out_use (pred_bb, def);
> }
> else
> {
> --
> 2.43.0
>
