On Tue 14. Sep 2021 at 11:15, LIU Zhiwei <zhiwei_...@c-sky.com> wrote:
>
> On 2021/9/11 下午10:00, Philipp Tomsich wrote:
> > Assume clzw being executed on a register that is not sign-extended, such
> > as for the following sequence that uses (1ULL << 63) | 392 as the operand
> > to clzw:
> > bseti a2, zero, 63
> > addi a2, a2, 392
> > clzw a3, a2
> > The correct result of clzw would be 23, but the current implementation
> > returns -32 (as it performs a 64bit clz, which results in 0 leading zero
> > bits, and then subtracts 32).
>
> As the MSB word of a3 has been cleaned, the result of current
> implementation will be 23. So there is no
> error here.
Zhiwei,
bits [63:32] on rs (arg1) are not zero-extended, as ctx->w is not being set
(the EXT_ZERO doesn’t have any effect, unless ctx->w is true). Please see
the earlier discussion on this topic in v9 and v10.
Thanks,
Philipp.
Thanks,
> Zhiwei
>
> > Fix this by changing the implementation to:
> > 1. shift the original register up by 32
> > 2. performs a target-length (64bit) clz
> > 3. return 32 if no bits are set
> >
> > Marking this instruction as 'w-form' (i.e., setting ctx->w) would not
> > correctly model the behaviour, as the instruction should not perform
> > a zero-extensions on the input (after all, it is not a .uw instruction)
> > and the result is always in the range 0..32 (so neither a sign-extension
> > nor a zero-extension on the result will ever be needed). Consequently,
> > we do not set ctx->w and mark the instruction as EXT_NONE.
> >
> > Signed-off-by: Philipp Tomsich <philipp.toms...@vrull.eu>
> > ---
> >
> > Changes in v11:
> > - Swaps out the EXT_ZERO to EXT_NONE, as no extension is to be performed.
> >
> > Changes in v10:
> > - New patch, fixing correctnes for clzw called on a register with
> undefined
> > (as in: not properly sign-extended) upper bits.
> >
> > target/riscv/insn_trans/trans_rvb.c.inc | 8 +++++---
> > 1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/target/riscv/insn_trans/trans_rvb.c.inc
> b/target/riscv/insn_trans/trans_rvb.c.inc
> > index 6c85c89f6d..73d1e45026 100644
> > --- a/target/riscv/insn_trans/trans_rvb.c.inc
> > +++ b/target/riscv/insn_trans/trans_rvb.c.inc
> > @@ -349,15 +349,17 @@ GEN_TRANS_SHADD(3)
> >
> > static void gen_clzw(TCGv ret, TCGv arg1)
> > {
> > - tcg_gen_clzi_tl(ret, arg1, 64);
> > - tcg_gen_subi_tl(ret, ret, 32);
> > + TCGv t = tcg_temp_new();
> > + tcg_gen_shli_tl(t, arg1, 32);
> > + tcg_gen_clzi_tl(ret, t, 32);
> > + tcg_temp_free(t);
> > }
> >
> > static bool trans_clzw(DisasContext *ctx, arg_clzw *a)
> > {
> > REQUIRE_64BIT(ctx);
> > REQUIRE_EXT(ctx, RVB);
> > - return gen_unary(ctx, a, EXT_ZERO, gen_clzw);
> > + return gen_unary(ctx, a, EXT_NONE, gen_clzw);
> > }
> >
> > static void gen_ctzw(TCGv ret, TCGv arg1)
>
