Re: [PATCH] aarch64: Fix aarch64_ldp_reg_operand predicate not to allow all subreg [PR113221]

[Alex Coplan <alex.coplan@arm.com>]

Hi Andrew,

On 16/01/2024 19:29, Andrew Pinski wrote:
> So the problem here is that aarch64_ldp_reg_operand will all subreg even subreg of lo_sum.
> When LRA tries to fix that up, all things break. So the fix is to change the check to only
> allow reg and subreg of regs.

Thanks a lot for tracking this down, I really appreciate having some help with
the bug-fixing.  Sorry for not getting to it sooner myself, I'm working on
PR113089 which ended up taking longer than expected to fix.

> 
> Note the tendancy here is to use register_operand but that checks the mode of the register
> but we need to allow a mismatch modes for this predicate for now.

Yeah, due to the design of the patterns using special predicates we need
to allow a mode mismatch with the contextual mode.

The patch broadly LGTM (although I can't approve), but I've left a
couple of minor comments below.

> 
> Built and tested for aarch64-linux-gnu with no regressions
> (Also tested with the LD/ST pair pass back on).
> 
> 	PR target/113221
> 
> gcc/ChangeLog:
> 
> 	* config/aarch64/predicates.md (aarch64_ldp_reg_operand): For subreg,
> 	only allow REG operands isntead of allowing all.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.c-torture/compile/pr113221-1.c: New test.
> 
> Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>
> ---
>  gcc/config/aarch64/predicates.md                 |  8 +++++++-
>  gcc/testsuite/gcc.c-torture/compile/pr113221-1.c | 12 ++++++++++++
>  2 files changed, 19 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/gcc.c-torture/compile/pr113221-1.c
> 
> diff --git a/gcc/config/aarch64/predicates.md b/gcc/config/aarch64/predicates.md
> index 8a204e48bb5..256268517d8 100644
> --- a/gcc/config/aarch64/predicates.md
> +++ b/gcc/config/aarch64/predicates.md
> @@ -313,7 +313,13 @@ (define_predicate "pmode_plus_operator"
>  
>  (define_special_predicate "aarch64_ldp_reg_operand"
>    (and
> -    (match_code "reg,subreg")
> +    (ior
> +      (match_code "reg")
> +      (and
> +       (match_code "subreg")
> +       (match_test "GET_CODE (SUBREG_REG (op)) == REG")

This could be just REG_P (SUBREG_REG (op)) in the match_test.

> +      )
> +    )

I think it would be more in keeping with the style in the rest of the file to
have the closing parens on the same line as the SUBREG_REG match_test.

>      (match_test "aarch64_ldpstp_operand_mode_p (GET_MODE (op))")
>      (ior
>        (match_test "mode == VOIDmode")
> diff --git a/gcc/testsuite/gcc.c-torture/compile/pr113221-1.c b/gcc/testsuite/gcc.c-torture/compile/pr113221-1.c
> new file mode 100644
> index 00000000000..152a510786e
> --- /dev/null
> +++ b/gcc/testsuite/gcc.c-torture/compile/pr113221-1.c
> @@ -0,0 +1,12 @@
> +/* { dg-options "-fno-move-loop-invariants -funroll-all-loops" } */

Does this need to be dg-additional-options?  Naively I would expect the
dg-options clause to override the torture options (and potentially any
options provided in RUNTESTFLAGS, e.g. to re-enable the ldp/stp pass).

Thanks again for the patch, and apologies for the oversight on my part: I'd
missed that register_operand also checks the code inside the subreg.

Alex

> +/* PR target/113221 */
> +/* This used to ICE after the `load/store pair fusion pass` was added
> +   due to the predicate aarch64_ldp_reg_operand allowing too much. */
> +
> +
> +void bar();
> +void foo(int* b) {
> +  for (;;)
> +    *b++ = (long)bar;
> +}
> +
> -- 
> 2.39.3
>