Re: [PATCH] Ver.2: Add compile option "-msmall-data-limit=0" to avoid using .srodata section for riscv.

[Palmer Dabbelt <palmer@dabbelt.com>]

On Thu, 17 Nov 2022 19:30:23 PST (-0800), oriachiuan@gmail.com wrote:
> Got it, I used to regard this test case as targeting at test if the const
> data would use the ".rodata" section.

Sorry, I'm not quite sure what you're trying to say here.  Here's a dump 
of how I see things:

In some targets (RISC-V and MIPS) there's multiple copies of the 
data/rodata sections, with the small data/rodata ending up in the small 
sections (`.sdata` and `.srodata`).  I've never actually been 100% on 
that being allowed by any spec, but MIPS did it long before RISC-V so I 
figure software is expected to tolerate the oddness.

In RISC-V we use it to try and place as many symbols as possible close 
to GP, so we're more likely to relax to GP-relative addressing 
sequences.  IIRC that's pretty much the same as MIPS, though they have 
slightly different addressing requirements.

For targets that function this way `.srodata` and `.rodata` are 
functionally equivalent (assuming you're not playing any GP tricks to 
relocate, but those are way out of what's supported).  So unless the 
test is trying to dig into performance issues differences between these 
sections, it should just allow code to target either.

>
> Palmer Dabbelt <palmer@dabbelt.com> 于2022年11月18日周五 07:59写道：
>
>> On Thu, 17 Nov 2022 13:50:00 PST (-0800), gcc-patches@gcc.gnu.org wrote:
>> >
>> > On 11/17/22 02:53, Yixuan Chen wrote:
>> >> 2022-11-17  Yixuan Chen  <chenyixuan@iscas.ac.cn>
>> >>
>> >>          * gcc/testsuite/gcc.dg/pr25521.c: Add compile option
>> "-msmall-data-limit=0" to avoid using .srodata section for riscv.
>> >> ---
>> >>   gcc/testsuite/gcc.dg/pr25521.c | 3 ++-
>> >>   1 file changed, 2 insertions(+), 1 deletion(-)
>> >>
>> >> diff --git a/gcc/testsuite/gcc.dg/pr25521.c
>> b/gcc/testsuite/gcc.dg/pr25521.c
>> >> index 74fe2ae6626..628ddf1a761 100644
>> >> --- a/gcc/testsuite/gcc.dg/pr25521.c
>> >> +++ b/gcc/testsuite/gcc.dg/pr25521.c
>> >> @@ -2,7 +2,8 @@
>> >>      sections.
>> >>
>> >>      { dg-require-effective-target elf }
>> >> -   { dg-do compile } */
>> >> +   { dg-do compile }
>> >> +   { dg-options "-msmall-data-limit=0" { target { riscv*-*-* } } } */
>> >>
>> >>   const volatile int foo = 30;
>> >>
>> >
>> > Wouldn't this be better?  It avoids a target specific conditional by
>> > instead extending what we look for to cover [s]rodata sections.
>> >
>> >
>> > Thoughts?
>> >
>> > Jeff
>> > diff --git a/gcc/testsuite/gcc.dg/pr25521.c
>> b/gcc/testsuite/gcc.dg/pr25521.c
>> > index 74fe2ae6626..63363a03b9f 100644
>> > --- a/gcc/testsuite/gcc.dg/pr25521.c
>> > +++ b/gcc/testsuite/gcc.dg/pr25521.c
>> > @@ -7,4 +7,4 @@
>> >  const volatile int foo = 30;
>> >
>> >
>> > -/* { dg-final { scan-assembler "\\.rodata" } } */
>> > +/* { dg-final { scan-assembler "\\.s\?rodata" } } */
>>
>> That's how I usually do it for these tests, there's some other targets
>> with sdata too so it fixes the test for everyone.  IIRC I said something
>> like that in the v1, but sorry if I'm just getting it confused with some
>> other patch.
>>
>> There's a few of these that need to get chased down for every release,
>> maybe we should add some sort of DG hepler?  Not sure that'd keep folks
>> from matching on .data, though...
>>