Re: [PATCH] [RISC-V] Optimize GP-relative addressing for linker.

[Palmer Dabbelt <palmer@dabbelt.com>]

On Tue, 27 Jun 2023 08:43:42 PDT (-0700), kito.cheng@sifive.com wrote:
> Testcase in your git commit has a base offset, but the testcase in the
> code only contains a base with no offset?
>
> I am a little concerned about the case with an offset like "lui
> a5,%hi(global_array+256) addi    a5,a5,%lo(global_array+256)" is still
> right for this optimization?

Ya, I think there's probably a bug somewhere around there: maybe this, 
or maybe reusing hi-parts, or something.  It's pretty close to the 
release, so let's hold off on this one until we have time -- Nelson and 
I are still trying to figure out this PC-relative GP SHN_ABS stuff...

>
> On Tue, Jun 27, 2023 at 8:47 PM Die Li <lidie@eswincomputing.com> wrote:
>>
>> Consider the following test:
>>
>> //test.c file
>>
>> struct Person {
>>     char name[20];
>>     int age;
>>     int nation;
>>     int hobby[100];
>> };
>>
>> int global_var;
>> int global_array[920];
>> struct Person person;
>>
>> int main() {
>>     global_var = 3;
>>     global_array[365] = 16;
>>     sprintf(person.name, "Lee");
>>     person.age = 27;
>>     person.nation = 77;
>>     return 0;
>> }
>>
>> Cflags:
>> -Xlinker --relax
>>
>> Link relaxation can be turned on by the above option, and in fact
>> is turned on by default. After compiling, linking, and disassembling
>> the test files, there are the following results:
>>
>> Before this patch:
>> Disassembly of section \.text:
>> 0+[0-9a-f]+ <main>:
>> .*:[    ]+[0-9a-f]+[    ]+sw[   ]+[0-9a-f]+,gp,\-[0-9]+ # [0-9a-f]+ <global_var>
>> .*:[    ]+[0-9a-f]+[    ]+c\.lui[       ]+.*
>> .*:[    ]+[0-9a-f]+[    ]+addi[         ]+[0-9a-f]+,[0-9a-f]+,[0-9]+ # [0-9a-f]+ <global_array>
>> .*:[    ]+[0-9a-f]+[    ]+c\.lui[       ]+.*
>> .*:[    ]+[0-9a-f]+[    ]+addi[         ]+[0-9a-f]+,[0-9a-f]+,[0-9]+ # [0-9a-f]+ <person>
>>
>> After this patch:
>> Disassembly of section \.text:
>> 0+[0-9a-f]+ <main>:
>> .*:[    ]+[0-9a-f]+[    ]+sw[   ]+[0-9a-f]+,gp,\-[0-9]+ # [0-9a-f]+ <global_var>
>> .*:[    ]+[0-9a-f]+[    ]+addi[         ]+[0-9a-f]+,gp,\-[0-9]+ # [0-9a-f]+ <global_array>
>> .*:[    ]+[0-9a-f]+[    ]+addi[         ]+[0-9a-f]+,gp,\-[0-9]+ # [0-9a-f]+ <person>
>>
>> After applying this patch, both array element and structure member
>> accesses have been optimized. In fact, for both array elements and
>> structure members, access is based on the data and the base address
>> of the structure, which is the address of the first member of the
>> array or structure. However, current linker takes into account the
>> size of arrays and structures when performing GP-relative addressing.
>> As a result, some array and structure bases within the GP-relative
>> addressing range cannot benefit from relaxation optimization. This
>> patch resolves this issue.
>>
>> Signed-off-by: Die Li <lidie@eswincomputing.com>
>>
>> ChangeLog:
>>
>>         * bfd/elfnn-riscv.c (_bfd_riscv_relax_section): Update reserve_size.
>>         * ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp: New test entry.
>>         * ld/testsuite/ld-riscv-elf/gp-relax.d: New test.
>>         * ld/testsuite/ld-riscv-elf/gp-relax.s: New test.
>> ---
>>  bfd/elfnn-riscv.c                          |  7 +++++
>>  ld/testsuite/ld-riscv-elf/gp-relax.d       | 12 +++++++++
>>  ld/testsuite/ld-riscv-elf/gp-relax.s       | 31 ++++++++++++++++++++++
>>  ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp |  1 +
>>  4 files changed, 51 insertions(+)
>>  create mode 100644 ld/testsuite/ld-riscv-elf/gp-relax.d
>>  create mode 100644 ld/testsuite/ld-riscv-elf/gp-relax.s
>>
>> diff --git a/bfd/elfnn-riscv.c b/bfd/elfnn-riscv.c
>> index 09aa7be225e..29fc5484e0f 100644
>> --- a/bfd/elfnn-riscv.c
>> +++ b/bfd/elfnn-riscv.c
>> @@ -5169,6 +5169,13 @@ _bfd_riscv_relax_section (bfd *abfd, asection *sec,
>>           if (h->type != STT_FUNC)
>>             reserve_size =
>>               (h->size - rel->r_addend) > h->size ? 0 : h->size - rel->r_addend;
>> +
>> +          /* For global pointer relative addressing, it is sufficient to ensure
>> +             that the symbol's base address falls within the range of global
>> +             pointer relative addressing.  */
>> +          if (h->type == STT_OBJECT)
>> +            reserve_size = 0;
>> +
>>           symtype = h->type;
>>         }
>>
>> diff --git a/ld/testsuite/ld-riscv-elf/gp-relax.d b/ld/testsuite/ld-riscv-elf/gp-relax.d
>> new file mode 100644
>> index 00000000000..ec2f59b1b19
>> --- /dev/null
>> +++ b/ld/testsuite/ld-riscv-elf/gp-relax.d
>> @@ -0,0 +1,12 @@
>> +#source: gp-relax.s
>> +#ld: --relax
>> +#objdump: -d -Mno-aliases
>> +
>> +.*:[   ]+file format .*
>> +
>> +
>> +Disassembly of section \.text:
>> +
>> +0+[0-9a-f]+ <_start>:
>> +.*:[   ]+[0-9a-f]+[    ]+addi[         ]+a5,gp,\-[0-9]+ # [0-9a-f]+ <global_array>
>> +.*:[   ]+[0-9a-f]+[    ]+addi[         ]+a4,gp,[0-9]+ # [0-9a-f]+ <person>
>> diff --git a/ld/testsuite/ld-riscv-elf/gp-relax.s b/ld/testsuite/ld-riscv-elf/gp-relax.s
>> new file mode 100644
>> index 00000000000..05548888ebf
>> --- /dev/null
>> +++ b/ld/testsuite/ld-riscv-elf/gp-relax.s
>> @@ -0,0 +1,31 @@
>> +       .text
>> +       .globl  global_var
>> +       .section        .sbss,"aw",@nobits
>> +       .align  2
>> +       .type   global_var, @object
>> +       .size   global_var, 4
>> +global_var:
>> +       .zero   4
>> +       .globl  global_array
>> +       .bss
>> +       .align  3
>> +       .type   global_array, @object
>> +       .size   global_array, 3680
>> +global_array:
>> +       .zero   3680
>> +       .globl  person
>> +       .align  3
>> +       .type   person, @object
>> +       .size   person, 428
>> +person:
>> +       .zero   428
>> +       .text
>> +       .align  1
>> +       .globl  _start
>> +       .type   _start, @function
>> +_start:
>> +       lui     a5,%hi(global_array)
>> +       addi    a5,a5,%lo(global_array)
>> +
>> +       lui     a4,%hi(person)
>> +       addi    a4,a4,%lo(person)
>> diff --git a/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp b/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
>> index 947a266ba72..6a04955b23b 100644
>> --- a/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
>> +++ b/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
>> @@ -124,6 +124,7 @@ if [istarget "riscv*-*-*"] {
>>      run_dump_test "pcgp-relax-01"
>>      run_dump_test "pcgp-relax-01-norelaxgp"
>>      run_dump_test "pcgp-relax-02"
>> +    run_dump_test "gp-relax"
>>      run_dump_test "c-lui"
>>      run_dump_test "c-lui-2"
>>      run_dump_test "disas-jalr"
>> --
>> 2.17.1
>>