For RV64, the shamt field in slli.uw is 6 bits wide. While the encoding
space currently reserves a wider shamt-field (for use is a future RV128
ISA), setting the additional bit to 1 will not map to slli.uw for RV64
and needs to be treated as an illegal instruction.

Note that this encoding being reserved for a future RV128 does not imply
that no other instructions for RV64-only could be added in this encoding
space in the future.

As the implementation is separate from the gen_shifti helpers, we keep
it that way and add the check for the shamt-width here.

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
---

(no changes since v3)

Changes in v3:
- Instead of defining a new decoding format, we treat slli.uw as if it
  had a 7bit-wide field for shamt (the 7th bit is reserved for RV128)
  and check for validity of the encoding in C code.

 target/riscv/insn_trans/trans_rvb.c.inc | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/target/riscv/insn_trans/trans_rvb.c.inc b/target/riscv/insn_trans/trans_rvb.c.inc
index 3cdd70a2b9..dcc7b6893d 100644
--- a/target/riscv/insn_trans/trans_rvb.c.inc
+++ b/target/riscv/insn_trans/trans_rvb.c.inc
@@ -430,6 +430,15 @@ static bool trans_slli_uw(DisasContext *ctx, arg_slli_uw *a)
     REQUIRE_64BIT(ctx);
     REQUIRE_ZBA(ctx);
 
+    /*
+     * The shamt field is only 6 bits for RV64 (with the 7th bit
+     * remaining reserved for RV128).  If the reserved bit is set
+     * on RV64, the encoding is illegal.
+     */
+    if (a->shamt >= TARGET_LONG_BITS) {
+        return false;
+    }
+
     TCGv source1 = tcg_temp_new();
     gen_get_gpr(source1, a->rs1);
 
-- 
2.25.1

On Thu, Aug 26, 2021 at 3:09 AM Philipp Tomsich
<philipp.tomsich@vrull.eu> wrote:
>
> For RV64, the shamt field in slli.uw is 6 bits wide. While the encoding
> space currently reserves a wider shamt-field (for use is a future RV128
> ISA), setting the additional bit to 1 will not map to slli.uw for RV64
> and needs to be treated as an illegal instruction.
>
> Note that this encoding being reserved for a future RV128 does not imply
> that no other instructions for RV64-only could be added in this encoding
> space in the future.
>
> As the implementation is separate from the gen_shifti helpers, we keep
> it that way and add the check for the shamt-width here.
>
> Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

> ---
>
> (no changes since v3)
>
> Changes in v3:
> - Instead of defining a new decoding format, we treat slli.uw as if it
>   had a 7bit-wide field for shamt (the 7th bit is reserved for RV128)
>   and check for validity of the encoding in C code.
>
>  target/riscv/insn_trans/trans_rvb.c.inc | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/target/riscv/insn_trans/trans_rvb.c.inc b/target/riscv/insn_trans/trans_rvb.c.inc
> index 3cdd70a2b9..dcc7b6893d 100644
> --- a/target/riscv/insn_trans/trans_rvb.c.inc
> +++ b/target/riscv/insn_trans/trans_rvb.c.inc
> @@ -430,6 +430,15 @@ static bool trans_slli_uw(DisasContext *ctx, arg_slli_uw *a)
>      REQUIRE_64BIT(ctx);
>      REQUIRE_ZBA(ctx);
>
> +    /*
> +     * The shamt field is only 6 bits for RV64 (with the 7th bit
> +     * remaining reserved for RV128).  If the reserved bit is set
> +     * on RV64, the encoding is illegal.
> +     */
> +    if (a->shamt >= TARGET_LONG_BITS) {
> +        return false;
> +    }
> +
>      TCGv source1 = tcg_temp_new();
>      gen_get_gpr(source1, a->rs1);
>
> --
> 2.25.1
>
>