RE: [RFC] expr: don't clear SUBREG_PROMOTED_VAR_P flag for a promoted subreg [target/111466]

["Roger Sayle" <roger@nextmovesoftware.com>]

I agree that this looks dubious.  Normally, if the middle-end/optimizers
wish to reuse a SUBREG in a context where the flags are not valid, it
should create a new one with the desired flags, rather than "mutate"
an existing (and possibly shared) RTX.

I wonder if creating a new SUBREG here also fixes your problem?
I'm not sure that clearing SUBREG_PROMOTED_VAR_P is needed
at all, but given its motivation has been lost to history, it would
good to have a plan B, if Jeff's alpha testing uncovers a subtle issue.

Roger
--

> -----Original Message-----
> From: Vineet Gupta <vineetg@rivosinc.com>
> Sent: 28 September 2023 22:44
> To: gcc-patches@gcc.gnu.org; Robin Dapp <rdapp.gcc@gmail.com>
> Cc: kito.cheng@gmail.com; Jeff Law <jeffreyalaw@gmail.com>; Palmer Dabbelt
> <palmer@rivosinc.com>; gnu-toolchain@rivosinc.com; Roger Sayle
> <roger@nextmovesoftware.com>; Jakub Jelinek <jakub@redhat.com>; Jivan
> Hakobyan <jivanhakobyan9@gmail.com>; Vineet Gupta <vineetg@rivosinc.com>
> Subject: [RFC] expr: don't clear SUBREG_PROMOTED_VAR_P flag for a promoted
> subreg [target/111466]
> 
> RISC-V suffers from extraneous sign extensions, despite/given the ABI
guarantee
> that 32-bit quantities are sign-extended into 64-bit registers, meaning
incoming SI
> function args need not be explicitly sign extended (so do SI return values
as most
> ALU insns implicitly sign-extend too.)
> 
> Existing REE doesn't seem to handle this well and there are various ideas
floating
> around to smarten REE about it.
> 
> RISC-V also seems to correctly implement middle-end hook PROMOTE_MODE
> etc.
> 
> Another approach would be to prevent EXPAND from generating the
sign_extend
> in the first place which this patch tries to do.
> 
> The hunk being removed was introduced way back in 1994 as
>    5069803972 ("expand_expr, case CONVERT_EXPR .. clear the promotion
flag")
> 
> This survived full testsuite run for RISC-V rv64gc with surprisingly no
> fallouts: test results before/after are exactly same.
> 
> |                               | # of unexpected case / # of unique
unexpected case
> |                               |          gcc |          g++ |
gfortran |
> | rv64imafdc_zba_zbb_zbs_zicond/|  264 /    87 |    5 /     2 |   72 /
12 |
> |    lp64d/medlow
> 
> Granted for something so old to have survived, there must be a valid
reason.
> Unfortunately the original change didn't have additional commentary or a
test
> case. That is not to say it can't/won't possibly break things on other
arches/ABIs,
> hence the RFC for someone to scream that this is just bonkers, don't do
this :-)
> 
> I've explicitly CC'ed Jakub and Roger who have last touched subreg
promoted
> notes in expr.cc for insight and/or screaming ;-)
> 
> Thanks to Robin for narrowing this down in an amazing debugging session @
GNU
> Cauldron.
> 
> ```
> foo2:
> 	sext.w	a6,a1             <-- this goes away
> 	beq	a1,zero,.L4
> 	li	a5,0
> 	li	a0,0
> .L3:
> 	addw	a4,a2,a5
> 	addw	a5,a3,a5
> 	addw	a0,a4,a0
> 	bltu	a5,a6,.L3
> 	ret
> .L4:
> 	li	a0,0
> 	ret
> ```
> 
> Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>
> Co-developed-by: Robin Dapp <rdapp.gcc@gmail.com>
> ---
>  gcc/expr.cc                               |  7 -------
>  gcc/testsuite/gcc.target/riscv/pr111466.c | 15 +++++++++++++++
>  2 files changed, 15 insertions(+), 7 deletions(-)  create mode 100644
> gcc/testsuite/gcc.target/riscv/pr111466.c
> 
> diff --git a/gcc/expr.cc b/gcc/expr.cc
> index 308ddc09e631..d259c6e53385 100644
> --- a/gcc/expr.cc
> +++ b/gcc/expr.cc
> @@ -9332,13 +9332,6 @@ expand_expr_real_2 (sepops ops, rtx target,
> machine_mode tmode,
>  	  op0 = expand_expr (treeop0, target, VOIDmode,
>  			     modifier);
> 
> -	  /* If the signedness of the conversion differs and OP0 is
> -	     a promoted SUBREG, clear that indication since we now
> -	     have to do the proper extension.  */
> -	  if (TYPE_UNSIGNED (TREE_TYPE (treeop0)) != unsignedp
> -	      && GET_CODE (op0) == SUBREG)
> -	    SUBREG_PROMOTED_VAR_P (op0) = 0;
> -
>  	  return REDUCE_BIT_FIELD (op0);
>  	}
> 
> diff --git a/gcc/testsuite/gcc.target/riscv/pr111466.c
> b/gcc/testsuite/gcc.target/riscv/pr111466.c
> new file mode 100644
> index 000000000000..007792466a51
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/riscv/pr111466.c
> @@ -0,0 +1,15 @@
> +/* Simplified varaint of gcc.target/riscv/zba-adduw.c.  */
> +
> +/* { dg-do compile } */
> +/* { dg-options "-march=rv64gc_zba_zbs -mabi=lp64" } */
> +/* { dg-skip-if "" { *-*-* } { "-O0" } } */
> +
> +int foo2(int unused, int n, unsigned y, unsigned delta){
> +  int s = 0;
> +  unsigned int x = 0;
> +  for (;x<n;x +=delta)
> +    s += x+y;
> +  return s;
> +}
> +
> +/* { dg-final { scan-assembler "\msext\M" } } */
> --
> 2.34.1