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Re: [Qemu-devel] [PATCH v2] RISC-V: Fix riscv_isa_string, use popcount t
From: |
Philippe Mathieu-Daudé |
Subject: |
Re: [Qemu-devel] [PATCH v2] RISC-V: Fix riscv_isa_string, use popcount to count bits |
Date: |
Sun, 11 Mar 2018 12:46:31 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 |
On 03/10/2018 10:25 PM, Philippe Mathieu-Daudé wrote:
> On 03/09/2018 10:01 PM, Michael Clark wrote:
>> Logic bug caused the string size calculation for the RISC-V
>> format ISA string to be small. This fix allows slack for rv128.
>>
>> Cc: Palmer Dabbelt <address@hidden>
>> Cc: Peter Maydell <address@hidden>
>> Cc: Eric Blake <address@hidden>
>> Signed-off-by: Michael Clark <address@hidden>
>> ---
>> target/riscv/cpu.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
>> index 4851890..1456535 100644
>> --- a/target/riscv/cpu.c
>> +++ b/target/riscv/cpu.c
>> @@ -391,7 +391,7 @@ static const TypeInfo riscv_cpu_type_info = {
>> char *riscv_isa_string(RISCVCPU *cpu)
>> {
>> int i;
>> - size_t maxlen = 5 + ctz32(cpu->env.misa);
>> + size_t maxlen = 8 + ctpop64(cpu->env.misa);
>
> Can you document the magic 5/8?
>
> This looks nice, but this seems to me too much optimization to save few
> bytes, using sizeof(riscv_exts) is overflow-free.
>
> Maybe this is enough and self-explanatory:
>
> const size_t maxlen = sizeof("rv128") + sizeof(riscv_exts);
Oops:
const size_t maxlen = sizeof("rv128") + sizeof(riscv_exts) + 1;
>
>> char *isa_string = g_new0(char, maxlen);
>> snprintf(isa_string, maxlen, "rv%d", TARGET_LONG_BITS);
>
> Also, if you keep the snprintf() return value, you can (naming it 'n')
> simplify (also easier to review):
>
>> for (i = 0; i < sizeof(riscv_exts); i++) {
>>
> if (cpu->env.misa & RV(riscv_exts[i])) {
> - isa_string[strlen(isa_string)] = riscv_exts[i] - 'A' + 'a';
> + isa_string[n++] = tolower(riscv_exts[i]);
> }
> }
>
> and simply use g_new() with:
>
> + isa_string[n] = '\0';
>
> return isa_string;
> }
>
> Regards,
>
> Phil.
>