Re: [PATCH v2] target/arm: Merge arm_cpu_vq_map_next_smaller into sole caller

[Andrew Jones]

On Sat, Nov 16, 2019 at 12:06:42PM +0100, Richard Henderson wrote:
> Coverity reports, in sve_zcr_get_valid_len,
> 
> "Subtract operation overflows on operands
> arm_cpu_vq_map_next_smaller(cpu, start_vq + 1U) and 1U"
> 
> First, the aarch32 stub version of arm_cpu_vq_map_next_smaller,
> returning 0, does exactly what Coverity reports.  Remove it.
> 
> Second, the aarch64 version of arm_cpu_vq_map_next_smaller has
> a set of asserts, but they don't cover the case in question.
> Further, there is a fair amount of extra arithmetic needed to
> convert from the 0-based zcr register, to the 1-base vq form,
> to the 0-based bitmap, and back again.  This can be simplified
> by leaving the value in the 0-based form.
> 
> Finally, use test_bit to simplify the common case, where the
> length in the zcr registers is in fact a supported length.

I don't see test_bit() getting used in the changes below.

> 
> Reported-by: Coverity (CID 1407217)
> Signed-off-by: Richard Henderson <address@hidden>
> ---
> 
> v2: Merge arm_cpu_vq_map_next_smaller into sve_zcr_get_valid_len,
>     as suggested by Andrew Jones.
> 
>     Use test_bit to make the code even more obvious; the
>     current_length + 1 thing to let us find current_length in the
>     bitmap seemed unnecessarily clever.
> 
> ---
>  target/arm/cpu.h    |  3 ---
>  target/arm/cpu64.c  | 15 ---------------
>  target/arm/helper.c |  8 ++++++--
>  3 files changed, 6 insertions(+), 20 deletions(-)
> 
> diff --git a/target/arm/cpu.h b/target/arm/cpu.h
> index e1a66a2d1c..47d24a5375 100644
> --- a/target/arm/cpu.h
> +++ b/target/arm/cpu.h
> @@ -185,12 +185,9 @@ typedef struct {
>  #ifdef TARGET_AARCH64
>  # define ARM_MAX_VQ    16
>  void arm_cpu_sve_finalize(ARMCPU *cpu, Error **errp);
> -uint32_t arm_cpu_vq_map_next_smaller(ARMCPU *cpu, uint32_t vq);
>  #else
>  # define ARM_MAX_VQ    1
>  static inline void arm_cpu_sve_finalize(ARMCPU *cpu, Error **errp) { }
> -static inline uint32_t arm_cpu_vq_map_next_smaller(ARMCPU *cpu, uint32_t vq)
> -{ return 0; }
>  #endif
>  
>  typedef struct ARMVectorReg {
> diff --git a/target/arm/cpu64.c b/target/arm/cpu64.c
> index 68baf0482f..a39d6fcea3 100644
> --- a/target/arm/cpu64.c
> +++ b/target/arm/cpu64.c
> @@ -458,21 +458,6 @@ void arm_cpu_sve_finalize(ARMCPU *cpu, Error **errp)
>      cpu->sve_max_vq = max_vq;
>  }
>  
> -uint32_t arm_cpu_vq_map_next_smaller(ARMCPU *cpu, uint32_t vq)
> -{
> -    uint32_t bitnum;
> -
> -    /*
> -     * We allow vq == ARM_MAX_VQ + 1 to be input because the caller may want
> -     * to find the maximum vq enabled, which may be ARM_MAX_VQ, but this
> -     * function always returns the next smaller than the input.
> -     */
> -    assert(vq && vq <= ARM_MAX_VQ + 1);
> -
> -    bitnum = find_last_bit(cpu->sve_vq_map, vq - 1);
> -    return bitnum == vq - 1 ? 0 : bitnum + 1;
> -}
> -
>  static void cpu_max_get_sve_max_vq(Object *obj, Visitor *v, const char *name,
>                                     void *opaque, Error **errp)
>  {
> diff --git a/target/arm/helper.c b/target/arm/helper.c
> index be67e2c66d..b5ee35a174 100644
> --- a/target/arm/helper.c
> +++ b/target/arm/helper.c
> @@ -5363,9 +5363,13 @@ int sve_exception_el(CPUARMState *env, int el)
>  
>  static uint32_t sve_zcr_get_valid_len(ARMCPU *cpu, uint32_t start_len)
>  {
> -    uint32_t start_vq = (start_len & 0xf) + 1;
> +    uint32_t end_len;
>  
> -    return arm_cpu_vq_map_next_smaller(cpu, start_vq + 1) - 1;
> +    start_len &= 0xf;
> +    end_len = find_last_bit(cpu->sve_vq_map, start_len + 1);
> +
> +    assert(end_len <= start_len);
> +    return end_len;
>  }
>  
>  /*
> -- 
> 2.17.1
> 
>

Besides the commit message referencing test_bit, but no use of it, this
looks good to me

Reviewed-by: Andrew Jones <address@hidden>