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

[Die Li <lidie@eswincomputing.com>]

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