Re: scratch/igc 3f46110b878: Fix an eassert

Re: scratch/igc 3f46110b878: Fix an eassert

[Pip Cet via Emacs development discussions.]

"Gerd Moellmann" <gerd@gnu.org> writes:

> branch: scratch/igc
> commit 3f46110b878ef0940ad7c0a9eacd6ef441baad7d
> Author: Gerd Möllmann <gerd@gnu.org>
> Commit: Gerd Möllmann <gerd@gnu.org>
>
>     Fix an eassert
> ---
>  src/igc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/src/igc.c b/src/igc.c
> index b1ef6939b5f..2ce508e055e 100644
> --- a/src/igc.c
> +++ b/src/igc.c
> @@ -2223,7 +2223,7 @@ fix_weak_hash_table_weak_part (mps_ss_t ss, struct Lisp_Weak_Hash_Table_Weak_Par
>  	for (ssize_t i = 0; i < limit; i++)
>  	  {
>  	    if (w->entries[i].intptr & 1)
> -	      eassert ((mps_word_t)w->entries[i].intptr ^ w->entries[i].intptr == 0);
> +	      eassert ((mps_word_t)w->entries[i].intptr ^ (w->entries[i].intptr == 0));
>  	    bool was_nil = (w->entries[i].intptr) == 0;
>  	    intptr_t off = 0;
>  #ifdef WORDS_BIGENDIAN

I'm guessing I'm failing a basic C knowledge test here, but can you
explain?  Did you run into an assertion error with the old code, or a
compiler warning?

The intended assertion was that if the .intptr component is truncated to
mps_word_t size, no information is lost, because I saw some problems
with "negative" 32-bit addresses filling the MSB 32 bits of the
Lisp_Objects with ones (the sign extension/zero extension problem, in
other words). That would lead to the "splatted" Lisp_Object reading as
0xffffffff00000000, which caused trouble.

So on i386, the assertion could also be written as

(w->entries[i].intptr & 0xffffffff00000000) == 0

I think the "fixed" assertion is trivially true?

Pip

Re: scratch/igc 3f46110b878: Fix an eassert

[Gerd Möllmann]

Pip Cet <pipcet@protonmail.com> writes:

> "Gerd Moellmann" <gerd@gnu.org> writes:
>
>> branch: scratch/igc
>> commit 3f46110b878ef0940ad7c0a9eacd6ef441baad7d
>> Author: Gerd Möllmann <gerd@gnu.org>
>> Commit: Gerd Möllmann <gerd@gnu.org>
>>
>>     Fix an eassert
>> ---
>>  src/igc.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/src/igc.c b/src/igc.c
>> index b1ef6939b5f..2ce508e055e 100644
>> --- a/src/igc.c
>> +++ b/src/igc.c
>> @@ -2223,7 +2223,7 @@ fix_weak_hash_table_weak_part (mps_ss_t ss, struct Lisp_Weak_Hash_Table_Weak_Par
>>  	for (ssize_t i = 0; i < limit; i++)
>>  	  {
>>  	    if (w->entries[i].intptr & 1)
>> -	      eassert ((mps_word_t)w->entries[i].intptr ^ w->entries[i].intptr == 0);
>> +	      eassert ((mps_word_t)w->entries[i].intptr ^ (w->entries[i].intptr == 0));
>>  	    bool was_nil = (w->entries[i].intptr) == 0;
>>  	    intptr_t off = 0;
>>  #ifdef WORDS_BIGENDIAN
>
> I'm guessing I'm failing a basic C knowledge test here, but can you
> explain?  Did you run into an assertion error with the old code, or a
> compiler warning?

It was a warning from clang. So the right thing would be (x ^ y) == 0?

Re: scratch/igc 3f46110b878: Fix an eassert

[Gerd Möllmann]

Gerd Möllmann <gerd.moellmann@gmail.com> writes:

> Pip Cet <pipcet@protonmail.com> writes:
>
>> "Gerd Moellmann" <gerd@gnu.org> writes:
>>
>>> branch: scratch/igc
>>> commit 3f46110b878ef0940ad7c0a9eacd6ef441baad7d
>>> Author: Gerd Möllmann <gerd@gnu.org>
>>> Commit: Gerd Möllmann <gerd@gnu.org>
>>>
>>>     Fix an eassert
>>> ---
>>>  src/igc.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/src/igc.c b/src/igc.c
>>> index b1ef6939b5f..2ce508e055e 100644
>>> --- a/src/igc.c
>>> +++ b/src/igc.c
>>> @@ -2223,7 +2223,7 @@ fix_weak_hash_table_weak_part (mps_ss_t ss, struct Lisp_Weak_Hash_Table_Weak_Par
>>>  	for (ssize_t i = 0; i < limit; i++)
>>>  	  {
>>>  	    if (w->entries[i].intptr & 1)
>>> -	      eassert ((mps_word_t)w->entries[i].intptr ^ w->entries[i].intptr == 0);
>>> +	      eassert ((mps_word_t)w->entries[i].intptr ^ (w->entries[i].intptr == 0));
>>>  	    bool was_nil = (w->entries[i].intptr) == 0;
>>>  	    intptr_t off = 0;
>>>  #ifdef WORDS_BIGENDIAN
>>
>> I'm guessing I'm failing a basic C knowledge test here, but can you
>> explain?  Did you run into an assertion error with the old code, or a
>> compiler warning?
>
> It was a warning from clang. So the right thing would be (x ^ y) == 0?

So thi sone?

odified   src/igc.c
@@ -2278,7 +2278,7 @@ fix_weak_hash_table_weak_part (mps_ss_t ss, struct Lisp_Weak_Hash_Table_Weak_Par
 	for (ssize_t i = 0; i < limit; i++)
 	  {
 	    if (w->entries[i].intptr & 1)
-	      eassert ((mps_word_t)w->entries[i].intptr ^ (w->entries[i].intptr == 0));
+	      eassert (((mps_word_t)w->entries[i].intptr ^ w->entries[i].intptr) == 0);
 	    bool was_nil = (w->entries[i].intptr) == 0;
 	    intptr_t off = 0;
 #ifdef WORDS_BIGENDIAN

Re: scratch/igc 3f46110b878: Fix an eassert

[Gerd Möllmann]

Gerd Möllmann <gerd.moellmann@gmail.com> writes:

> Gerd Möllmann <gerd.moellmann@gmail.com> writes:
>
>> Pip Cet <pipcet@protonmail.com> writes:
>>
>>> "Gerd Moellmann" <gerd@gnu.org> writes:
>>>
>>>> branch: scratch/igc
>>>> commit 3f46110b878ef0940ad7c0a9eacd6ef441baad7d
>>>> Author: Gerd Möllmann <gerd@gnu.org>
>>>> Commit: Gerd Möllmann <gerd@gnu.org>
>>>>
>>>>     Fix an eassert
>>>> ---
>>>>  src/igc.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/src/igc.c b/src/igc.c
>>>> index b1ef6939b5f..2ce508e055e 100644
>>>> --- a/src/igc.c
>>>> +++ b/src/igc.c
>>>> @@ -2223,7 +2223,7 @@ fix_weak_hash_table_weak_part (mps_ss_t ss, struct Lisp_Weak_Hash_Table_Weak_Par
>>>>  	for (ssize_t i = 0; i < limit; i++)
>>>>  	  {
>>>>  	    if (w->entries[i].intptr & 1)
>>>> -	      eassert ((mps_word_t)w->entries[i].intptr ^ w->entries[i].intptr == 0);
>>>> +	      eassert ((mps_word_t)w->entries[i].intptr ^ (w->entries[i].intptr == 0));
>>>>  	    bool was_nil = (w->entries[i].intptr) == 0;
>>>>  	    intptr_t off = 0;
>>>>  #ifdef WORDS_BIGENDIAN
>>>
>>> I'm guessing I'm failing a basic C knowledge test here, but can you
>>> explain?  Did you run into an assertion error with the old code, or a
>>> compiler warning?
>>
>> It was a warning from clang. So the right thing would be (x ^ y) == 0?
>
> So thi sone?
>
> odified   src/igc.c
> @@ -2278,7 +2278,7 @@ fix_weak_hash_table_weak_part (mps_ss_t ss, struct Lisp_Weak_Hash_Table_Weak_Par
>  	for (ssize_t i = 0; i < limit; i++)
>  	  {
>  	    if (w->entries[i].intptr & 1)
> -	      eassert ((mps_word_t)w->entries[i].intptr ^ (w->entries[i].intptr == 0));
> +	      eassert (((mps_word_t)w->entries[i].intptr ^ w->entries[i].intptr) == 0);
>  	    bool was_nil = (w->entries[i].intptr) == 0;
>  	    intptr_t off = 0;
>  #ifdef WORDS_BIGENDIAN

Please ignore, this was already fixed, I didn't see it.