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RISC-V: Relax PCREL to GPREL while doing other relaxations is dangerous.

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I get the feedback recently that enable linker relaxations may fail to
build some program.  Consider the following case,

	.text
foo:
	addi	a0, a0, %pcrel_lo(.L2)
	call	foo
.L1:	auipc	a1, %pcrel_hi(data_g)
	addi	a1, a1, %pcrel_lo(.L1)
	lui	a2, %hi(data_g)
	addi	a2, a2, %lo(data_g)
	lui	a3, %tprel_hi(data_t)
	add	a3, a3, tp, %tprel_add(data_t)
	addi	a3, a3, %tprel_lo(data_t)
.L2:	auipc	a0, %pcrel_hi(data_g)

	.data
	.word 0x0
	.global data_g
data_g:	.word 0x1

	.section .tbss
data_t:	.word 0x0

The current ld reports `dangerous relocation error` when doing the
pcgp relaxation,
test.o: in function `foo':
(.text+0x0): dangerous relocation: %pcrel_lo missing matching %pcrel_hi

The .L2 auipc should not be removed since it is behind the corresponding
addi, so we record the information in the pcgp_relocs table to avoid
removing the auipc later.  But current ld still remove it since we do not
update the pcgp_relocs table while doing other relaxations.  I have two
solutions to fix the problem,

1. Update the pcgp_relocs table once we actually delete the code.
2. Add new relax pass to do the pcgp relaxations

At first I tried to do the first solution, and we need to update at
least three information - hi_sec_off of riscv_pcgp_lo_reloc, hi_sec_off
and hi_addr (symbol value) of riscv_pcgp_hi_reloc.  Update the hi_sec_off
is simple, but it is more complicate to update the symbol value, since we
almost have to do parts the same works of _bfd_riscv_relax_call again in
the riscv_relax_delete_bytes to get the correct symbol value.

Compared with the first solution, the second one is more intuitive and
simple.  We add a new relax pass to do the pcgp relaxations later, so
we will get all the information correctly in the _bfd_riscv_relax_call,
including the symbol value, without changing so much code.  I do not see
any penalty by adding a new relax pass for now, so it should be fine
to delay the pcgp relaxations.

Besides, I have pass all riscv-gnu-toolchain regressions for this patch.

	bfd/
	* elfnn-riscv.c (_bfd_riscv_relax_section):  Add a new relax pass
	to do the pcgp relaxation later, after the lui and call relaxations,
	but before the delete and alignment relaxations.

	ld/
	* emultempl/riscvelf.em (riscv_elf_before_allocation): Change
	link_info.relax_pass from 3 to 4.
	* testsuite/ld-riscv-elf/pcgp-relax.d: New testcase.
	* testsuite/ld-riscv-elf/pcgp-relax.s: Likewise.
	* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Updated.
This commit is contained in:
Nelson Chu 2020-11-17 19:39:52 -08:00
parent d4087e8150
commit abd20cb637
7 changed files with 79 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
bfd/ChangeLog
View File

@ -1,3 +1,9 @@
2020-11-21 Nelson Chu <nelson.chu@sifive.com>
* elfnn-riscv.c (_bfd_riscv_relax_section): Add a new relax pass
to do the pcgp relaxation later, after the lui and call relaxations,
but before the delete and alignment relaxations.
2020-11-20 Nick Alcock <nick.alcock@oracle.com>
* elflink.c (elf_finalize_dynstr): Call examine_strtab after

31
bfd/elfnn-riscv.c
View File

@ -4607,7 +4607,7 @@ _bfd_riscv_relax_section (bfd *abfd, asection *sec,
|| (sec->flags & SEC_RELOC) == 0
|| sec->reloc_count == 0
|| (info->disable_target_specific_optimizations
&& info->relax_pass == 0))
&& info->relax_pass < 2))
return TRUE;
riscv_init_pcgp_relocs (&pcgp_relocs);
@ -4645,17 +4645,13 @@ _bfd_riscv_relax_section (bfd *abfd, asection *sec,
relax_func = NULL;
if (info->relax_pass == 0)
{
if (type == R_RISCV_CALL || type == R_RISCV_CALL_PLT)
if (type == R_RISCV_CALL
|| type == R_RISCV_CALL_PLT)
relax_func = _bfd_riscv_relax_call;
else if (type == R_RISCV_HI20
|| type == R_RISCV_LO12_I
|| type == R_RISCV_LO12_S)
relax_func = _bfd_riscv_relax_lui;
else if (!bfd_link_pic(info)
&& (type == R_RISCV_PCREL_HI20
|| type == R_RISCV_PCREL_LO12_I
|| type == R_RISCV_PCREL_LO12_S))
relax_func = _bfd_riscv_relax_pc;
else if (type == R_RISCV_TPREL_HI20
|| type == R_RISCV_TPREL_ADD
|| type == R_RISCV_TPREL_LO12_I
@ -4663,7 +4659,22 @@ _bfd_riscv_relax_section (bfd *abfd, asection *sec,
relax_func = _bfd_riscv_relax_tls_le;
else
continue;
}
else if (info->relax_pass == 1
&& !bfd_link_pic(info)
&& (type == R_RISCV_PCREL_HI20
|| type == R_RISCV_PCREL_LO12_I
|| type == R_RISCV_PCREL_LO12_S))
relax_func = _bfd_riscv_relax_pc;
else if (info->relax_pass == 2 && type == R_RISCV_DELETE)
relax_func = _bfd_riscv_relax_delete;
else if (info->relax_pass == 3 && type == R_RISCV_ALIGN)
relax_func = _bfd_riscv_relax_align;
else
continue;
if (info->relax_pass < 2)
{
/* Only relax this reloc if it is paired with R_RISCV_RELAX. */
if (i == sec->reloc_count - 1
|| ELFNN_R_TYPE ((rel + 1)->r_info) != R_RISCV_RELAX
@ -4673,12 +4684,6 @@ _bfd_riscv_relax_section (bfd *abfd, asection *sec,
/* Skip over the R_RISCV_RELAX. */
i++;
}
else if (info->relax_pass == 1 && type == R_RISCV_DELETE)
relax_func = _bfd_riscv_relax_delete;
else if (info->relax_pass == 2 && type == R_RISCV_ALIGN)
relax_func = _bfd_riscv_relax_align;
else
continue;
data->relocs = relocs;

8
ld/ChangeLog
View File

@ -1,3 +1,11 @@
2020-11-21 Nelson Chu <nelson.chu@sifive.com>
* emultempl/riscvelf.em (riscv_elf_before_allocation): Change
link_info.relax_pass from 3 to 4.
* testsuite/ld-riscv-elf/pcgp-relax.d: New testcase.
* testsuite/ld-riscv-elf/pcgp-relax.s: Likewise.
* testsuite/ld-riscv-elf/ld-riscv-elf.exp: Updated.
2020-11-20 Nick Alcock <nick.alcock@oracle.com>
* testsuite/ld-ctf/data-func-conflicted.d: Shrink the expected

2
ld/emultempl/riscvelf.em
View File

@ -42,7 +42,7 @@ riscv_elf_before_allocation (void)
ENABLE_RELAXATION;
}
link_info.relax_pass = 3;
link_info.relax_pass = 4;
}
static void

1
ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
View File

@ -62,6 +62,7 @@ proc run_dump_test_ifunc { name target output} {
if [istarget "riscv*-*-*"] {
run_dump_test "call-relax"
run_dump_test "pcgp-relax"
run_dump_test "c-lui"
run_dump_test "c-lui-2"
run_dump_test "disas-jalr"

16
ld/testsuite/ld-riscv-elf/pcgp-relax.d Normal file
View File

@ -0,0 +1,16 @@
#source: pcgp-relax.s
#ld: --relax
#objdump: -d -Mno-aliases
.*:[ ]+file format .*
Disassembly of section \.text:
0+[0-9a-f]+ <_start>:
.*:[ ]+[0-9a-f]+[ ]+addi[ ]+a0,a0,[0-9]+
.*:[ ]+[0-9a-f]+[ ]+jal[ ]+ra,[0-9a-f]+ <_start>
.*:[ ]+[0-9a-f]+[ ]+addi[ ]+a1,gp,\-[0-9]+ # [0-9a-f]+ <data_g>
.*:[ ]+[0-9a-f]+[ ]+addi[ ]+a2,gp,\-[0-9]+ # [0-9a-f]+ <data_g>
.*:[ ]+[0-9a-f]+[ ]+addi[ ]+a3,tp,0 # 0 <data_t>
.*:[ ]+[0-9a-f]+[ ]+auipc[ ]+a0,0x[0-9a-f]+

29
ld/testsuite/ld-riscv-elf/pcgp-relax.s Normal file
View File

@ -0,0 +1,29 @@
.text
.globl _start
_start:
addi a0, a0, %pcrel_lo(.L2)
call _start
.L1:
auipc a1, %pcrel_hi(data_g)
addi a1, a1, %pcrel_lo(.L1)
lui a2, %hi(data_g)
addi a2, a2, %lo(data_g)
lui a3, %tprel_hi(data_t)
add a3, a3, tp, %tprel_add(data_t)
addi a3, a3, %tprel_lo(data_t)
.L2:
auipc a0, %pcrel_hi(data_g)
.data
.word 0x0
.globl data_g
data_g:
.word 0x1
.section .tbss
data_t:
.word 0x0