suggest assert wide_int larger than hashval_t

suggest assert wide_int larger than hashval_t

[Jay K]

I get this in 4.3.5:

../../gcc/gcc/varasm.c: In function `const_rtx_hash_1':
../../gcc/gcc/varasm.c:3387: warning: right shift count >= width of type

./include/hashtab.h:typedef unsigned int hashval_t;

  unsigned HOST_WIDE_INT hwi;
  hashval_t h, *hp;
 ...
    const int shift = sizeof (hashval_t) * CHAR_BIT;
    const int n = sizeof (HOST_WIDE_INT) / sizeof (hashval_t);
    int i;

    h ^= (hashval_t) hwi;
    for (i = 1; i < n; ++i)
      {
        hwi >>= shift;  here


It looks about the same in 4.5.0 except without const:


    int shift = (sizeof (hashval_t) * CHAR_BIT);


Something is amiss here locally, for the types to be the same size.


But maybe add gcc_assert(sizeof(hashval_t) < sizeof(HOST_WIDE_INT),
outside the loop? It should be optimized away anyway.


Maybe I'd get -Werror but I use -disable-bootstrap.
Native compiler is gcc, but old.


Thanks,
 - Jay

 		 	   		  

RE: suggest assert wide_int larger than hashval_t (32bit hwint?)

[Jay K]

Hm, it seems on some 32bit systems, where there is no -m64, wideint can be a mere 32bits.

In which case the code should probably say:

hwi = ((hwi >> (shift - 1)) >> 1);

This was targeting OpenBSD/x86.
Maybe I should just stick need_64bit_hwint = yes on config.gcc for that and move along?
Assume there is always long long or __int64?
Coverage of this case is pretty rare now from my skimming.

 - Jay

----------------------------------------
> From: jay.krell@cornell.edu
> To: gcc@gcc.gnu.org
> Subject: suggest assert wide_int larger than hashval_t
> Date: Mon, 19 Jul 2010 06:44:33 +0000
>
>
> I get this in 4.3.5:
>
> ../../gcc/gcc/varasm.c: In function `const_rtx_hash_1':
> ../../gcc/gcc/varasm.c:3387: warning: right shift count >= width of type
>
> ./include/hashtab.h:typedef unsigned int hashval_t;
>
>   unsigned HOST_WIDE_INT hwi;
>   hashval_t h, *hp;
>  ...
>     const int shift = sizeof (hashval_t) * CHAR_BIT;
>     const int n = sizeof (HOST_WIDE_INT) / sizeof (hashval_t);
>     int i;
>
>     h ^= (hashval_t) hwi;
>     for (i = 1; i < n; ++i)
>       {
>         hwi >>= shift;  here
>
>
> It looks about the same in 4.5.0 except without const:
>
>
>     int shift = (sizeof (hashval_t) * CHAR_BIT);
>
>
> Something is amiss here locally, for the types to be the same size.
>
>
> But maybe add gcc_assert(sizeof(hashval_t) < sizeof(HOST_WIDE_INT),
> outside the loop? It should be optimized away anyway.
>
>
> Maybe I'd get -Werror but I use -disable-bootstrap.
> Native compiler is gcc, but old.
>
>
> Thanks,
>  - Jay
>
>
 		 	   		  

Re: suggest assert wide_int larger than hashval_t

[Ian Lance Taylor]

Jay K <jay.krell@cornell.edu> writes:

> I get this in 4.3.5:
>
> ../../gcc/gcc/varasm.c: In function `const_rtx_hash_1':
> ../../gcc/gcc/varasm.c:3387: warning: right shift count >= width of type
>
> ./include/hashtab.h:typedef unsigned int hashval_t;
>
>   unsigned HOST_WIDE_INT hwi;
>   hashval_t h, *hp;
>  ...
>     const int shift = sizeof (hashval_t) * CHAR_BIT;
>     const int n = sizeof (HOST_WIDE_INT) / sizeof (hashval_t);
>     int i;
>
>     h ^= (hashval_t) hwi;
>     for (i = 1; i < n; ++i)
>       {
>         hwi >>= shift;  here

It's not an actual problem, because the code is never executed in the
case where right shift count >= width of type.  Note that the loop
starts at 1.  If this is breaking bootstrap that it needs to be
addressed one way or another, otherwise it's not too serious.

Ian

RE: suggest assert wide_int larger than hashval_t

[Jay K]

It's "just" a warning, no "real" affects seen.
I patched my copy to say
  hwi = ((hwi >> (shift - 1)) >> 1);

Thanks,
 - Jay

----------------------------------------
> From: iant@google.com
> To: jay.krell@cornell.edu
> CC: gcc@gcc.gnu.org
> Subject: Re: suggest assert wide_int larger than hashval_t
> Date: Mon, 19 Jul 2010 00:36:06 -0700
>
> Jay K  writes:
>
> > I get this in 4.3.5:
> >
> > ../../gcc/gcc/varasm.c: In function `const_rtx_hash_1':
> > ../../gcc/gcc/varasm.c:3387: warning: right shift count >= width of type
> >
> > ./include/hashtab.h:typedef unsigned int hashval_t;
> >
> >   unsigned HOST_WIDE_INT hwi;
> >   hashval_t h, *hp;
> >  ...
> >     const int shift = sizeof (hashval_t) * CHAR_BIT;
> >     const int n = sizeof (HOST_WIDE_INT) / sizeof (hashval_t);
> >     int i;
> >
> >     h ^= (hashval_t) hwi;
> >     for (i = 1; i < n; ++i)
> >       {
> >         hwi >>= shift;  here
>
> It's not an actual problem, because the code is never executed in the
> case where right shift count >= width of type. Note that the loop
> starts at 1. If this is breaking bootstrap that it needs to be
> addressed one way or another, otherwise it's not too serious.
>
> Ian