Re: [PATCH V1] HIGH part of symbol ref is invalid for constant pool

[Jiufu Guo <guojiufu@linux.ibm.com>]

Segher Boessenkool <segher@kernel.crashing.org> writes:

Thanks a lot for your review!

> Hi!
>
> On Tue, Jul 19, 2022 at 10:30:54PM +0800, Jiufu Guo wrote:
>> In patch https://gcc.gnu.org/pipermail/gcc-patches/2022-July/597712.html,
>> test case was not added.  After more check, a testcase is added for it.
>> 
>> The high part of the symbol address is invalid for the constant pool.
>
> Invalid, how so?  Is there a PR related here?
Thanks, I just opened PR106460 for this issue.
>
> But it is not particularly useful ever, either: we do not know two
> different addresses will have the same HIGH unless we know the exact
> address, and then we don't need HIGH anyway.
>
>> 	* config/rs6000/rs6000.cc (rs6000_cannot_force_const_mem):
>> 	Return true for HIGH code rtx.
>
> 	* config/rs6000/rs6000.cc (rs6000_cannot_force_const_mem): Return true
> 	for HIGH code rtx.
>
> Please don't wrap lines early: changelog lines are 80 positions long,
> including the leading tab (which counts as eight positions).
Thanks for your suggestion!
>
>>  static bool
>>  rs6000_cannot_force_const_mem (machine_mode mode ATTRIBUTE_UNUSED, rtx x)
>>  {
>> -  if (GET_CODE (x) == HIGH
>> -      && GET_CODE (XEXP (x, 0)) == UNSPEC)
>> +  /* High part of a symbol ref/address can not be put into constant pool. e.g.
>> +     (high:DI (symbol_ref:DI ("var")..)) or
>> +     (high:DI (unspec:DI [(symbol_ref/u:DI ("*.LC0")..)
>> +     (high:DI (const:DI (plus:DI (symbol_ref:DI ("xx")) (const_int 12)))).  */
>> +  if (GET_CODE (x) == HIGH)
>>      return true;
>
> I'm not sure the new comment is helpful at all?  Are these examples of
> where the compiler (or assembler perhaps) will choke?
I debugged this function with the source code from GCC bootstrap and
regtest, and then figured out these examples.
In the next version patch, I updated the comments a little, hope that
is more meaningful. :-)
>
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.target/powerpc/constpoolcheck.c
>> @@ -0,0 +1,11 @@
>> +/* { dg-do compile { target powerpc*-*-* } } */
>
> Everything in gcc.target/powerpc is target powerpc* always.
Thanks! I would remove this line.
>
>> +/* { dg-options "-O1 -mdejagnu-cpu=power10" } */
>> +/* (high:DI (symbol_ref:DI ("var_48")..))) should not cause ICE. */
>
> Ah, so there is an ICE, I see.  Please open a PR, and mention that in
> the testcase as well as in the commit message and changelog.
Thanks! I should open PR ealry :)
In the updated patch,  a testcase is named as pr106460.c, and memtioned
in commit message and changelog.
>
> I agree with what the patch does, it just needs a little more work :-)
I submitted a new version patch:
https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598980.html

Thanks in advance for any comments!

BR,
Jeff(Jiufu)

>
>
> Segher