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

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.

2 files changed, 24 insertions, 13 deletions

diff --git a/bfd/ChangeLog b/bfd/ChangeLog
index b2553a6..36a048a 100644
--- a/bfd/ChangeLog
+++ b/bfd/ChangeLog
@@ -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
diff --git a/bfd/elfnn-riscv.c b/bfd/elfnn-riscv.c
index a5b2c8a..4e87140 100644
--- a/bfd/elfnn-riscv.c
+++ b/bfd/elfnn-riscv.c
@@ -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;