[PATCH v1] RISC-V: bitmanip: improve constant-loading for (1ULL << 31) in DImode

[PATCH v1] RISC-V: bitmanip: improve constant-loading for (1ULL << 31) in DImode

[Philipp Tomsich]

The SINGLE_BIT_MASK_OPERAND() is overly restrictive, triggering for
bits above 31 only (to side-step any issues with the negative SImode
value 0x80000000).  This moves the special handling of this SImode
value (i.e. the check for -2147483648) to riscv.cc and relaxes the
SINGLE_BIT_MASK_OPERAND() test.

This changes the code-generation for loading (1ULL << 31) from:
	li	a0,1
	slli	a0,a0,31
to:
	bseti	a0,zero,31

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_build_integer_1): Rewrite value as
	-2147483648 for the single-bit case, when operating on 0x80000000
	in SImode.
	* gcc/config/riscv/riscv.h (SINGLE_BIT_MASK_OPERAND): Allow for
	any single-bit value, moving the special case for 0x80000000 to
	riscv_build_integer_1 (in riscv.c).

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>

---

 gcc/config/riscv/riscv.cc |  9 +++++++++
 gcc/config/riscv/riscv.h  | 11 ++++-------
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index f83dc796d88..fe8196f5c80 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -420,6 +420,15 @@ riscv_build_integer_1 (struct riscv_integer_op codes[RISCV_MAX_INTEGER_OPS],
       /* Simply BSETI.  */
       codes[0].code = UNKNOWN;
       codes[0].value = value;
+
+      /* RISC-V sign-extends all 32bit values that life in a 32bit
+	 register.  To avoid paradoxes, we thus need to use the
+	 sign-extended (negative) representation for the value, if we
+	 want to build 0x80000000 in SImode.  This will then expand
+	 to an ADDI/LI instruction.  */
+      if (mode == SImode && value == 0x80000000)
+	codes[0].value = -2147483648;
+
       return 1;
     }
 
diff --git a/gcc/config/riscv/riscv.h b/gcc/config/riscv/riscv.h
index 5083a1c24b0..6f7f4d3fbdc 100644
--- a/gcc/config/riscv/riscv.h
+++ b/gcc/config/riscv/riscv.h
@@ -528,13 +528,10 @@ enum reg_class
   (((VALUE) | ((1UL<<31) - IMM_REACH)) == ((1UL<<31) - IMM_REACH)	\
    || ((VALUE) | ((1UL<<31) - IMM_REACH)) + IMM_REACH == 0)
 
-/* If this is a single bit mask, then we can load it with bseti.  But this
-   is not useful for any of the low 31 bits because we can use addi or lui
-   to load them.  It is wrong for loading SImode 0x80000000 on rv64 because it
-   needs to be sign-extended.  So we restrict this to the upper 32-bits
-   only.  */
-#define SINGLE_BIT_MASK_OPERAND(VALUE) \
-  (pow2p_hwi (VALUE) && (ctz_hwi (VALUE) >= 32))
+/* If this is a single bit mask, then we can load it with bseti.  Special
+   handling of SImode 0x80000000 on RV64 is done in riscv_build_integer_1. */
+#define SINGLE_BIT_MASK_OPERAND(VALUE)					\
+  (pow2p_hwi (VALUE))
 
 /* Stack layout; function entry, exit and calling.  */
 
-- 
2.34.1

Re: [PATCH v1] RISC-V: bitmanip: improve constant-loading for (1ULL << 31) in DImode

[Andrew Pinski]

On Tue, May 24, 2022 at 3:57 PM Philipp Tomsich
<philipp.tomsich@vrull.eu> wrote:
>
> The SINGLE_BIT_MASK_OPERAND() is overly restrictive, triggering for
> bits above 31 only (to side-step any issues with the negative SImode
> value 0x80000000).  This moves the special handling of this SImode
> value (i.e. the check for -2147483648) to riscv.cc and relaxes the
> SINGLE_BIT_MASK_OPERAND() test.
>
> This changes the code-generation for loading (1ULL << 31) from:
>         li      a0,1
>         slli    a0,a0,31
> to:
>         bseti   a0,zero,31
>
> gcc/ChangeLog:
>
>         * config/riscv/riscv.cc (riscv_build_integer_1): Rewrite value as
>         -2147483648 for the single-bit case, when operating on 0x80000000
>         in SImode.
>         * gcc/config/riscv/riscv.h (SINGLE_BIT_MASK_OPERAND): Allow for
>         any single-bit value, moving the special case for 0x80000000 to
>         riscv_build_integer_1 (in riscv.c).
>
> Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
>
> ---
>
>  gcc/config/riscv/riscv.cc |  9 +++++++++
>  gcc/config/riscv/riscv.h  | 11 ++++-------
>  2 files changed, 13 insertions(+), 7 deletions(-)
>
> diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
> index f83dc796d88..fe8196f5c80 100644
> --- a/gcc/config/riscv/riscv.cc
> +++ b/gcc/config/riscv/riscv.cc
> @@ -420,6 +420,15 @@ riscv_build_integer_1 (struct riscv_integer_op codes[RISCV_MAX_INTEGER_OPS],
>        /* Simply BSETI.  */
>        codes[0].code = UNKNOWN;
>        codes[0].value = value;
> +
> +      /* RISC-V sign-extends all 32bit values that life in a 32bit
> +        register.  To avoid paradoxes, we thus need to use the
> +        sign-extended (negative) representation for the value, if we
> +        want to build 0x80000000 in SImode.  This will then expand
> +        to an ADDI/LI instruction.  */
> +      if (mode == SImode && value == 0x80000000)
> +       codes[0].value = -2147483648;

Instead it is better to use HOST_WIDE_INT_M1U<<31 here instead of the
number -2147483648 since I have no idea what that is.
Like for 0x80000000 to be HOST_WIDE_INT_1U<<31 ?

Thanks,
Andrew Pinski

> +
>        return 1;
>      }
>
> diff --git a/gcc/config/riscv/riscv.h b/gcc/config/riscv/riscv.h
> index 5083a1c24b0..6f7f4d3fbdc 100644
> --- a/gcc/config/riscv/riscv.h
> +++ b/gcc/config/riscv/riscv.h
> @@ -528,13 +528,10 @@ enum reg_class
>    (((VALUE) | ((1UL<<31) - IMM_REACH)) == ((1UL<<31) - IMM_REACH)      \
>     || ((VALUE) | ((1UL<<31) - IMM_REACH)) + IMM_REACH == 0)
>
> -/* If this is a single bit mask, then we can load it with bseti.  But this
> -   is not useful for any of the low 31 bits because we can use addi or lui
> -   to load them.  It is wrong for loading SImode 0x80000000 on rv64 because it
> -   needs to be sign-extended.  So we restrict this to the upper 32-bits
> -   only.  */
> -#define SINGLE_BIT_MASK_OPERAND(VALUE) \
> -  (pow2p_hwi (VALUE) && (ctz_hwi (VALUE) >= 32))
> +/* If this is a single bit mask, then we can load it with bseti.  Special
> +   handling of SImode 0x80000000 on RV64 is done in riscv_build_integer_1. */
> +#define SINGLE_BIT_MASK_OPERAND(VALUE)                                 \
> +  (pow2p_hwi (VALUE))
>
>  /* Stack layout; function entry, exit and calling.  */
>
> --
> 2.34.1
>