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Re: [PATCH] opcodes/riscv: Hide '.L0 ' fake symbols


* Jim Wilson <jimw@sifive.com> [2018-12-03 12:24:03 -0800]:

> On Mon, Dec 3, 2018 at 7:23 AM Andrew Burgess
> <andrew.burgess@embecosm.com> wrote:
> > The RISC-V assembler generates fake labels with the name '.L0 ' as
> > part of the debug information (see
> > gas/config/tc-riscv.h:FAKE_LABEL_NAME).
> 
> They are also used for relaxable relocations, such as a medany auipc
> instruction.
> 
> > include/ChangeLog:
> >         * dis-asm.h (riscv_symbol_is_valid): Declare.
> >
> > opcodes/ChangeLog:
> >         * disassembler.c (disassemble_init_for_target): Add RISC-V
> >         initialisation.
> >         * riscv-dis.c (riscv_symbol_is_valid): New function.
> 
> This looks mostly OK to me, with the fix for the problem that Andreas
> pointed out except that ...
> 
> > +/* Prevent use of the fake labels that are generated as part of the DWARF
> > +   debug in the assembler.  The fake labels are edefined in
> > +   gas/config/tc-riscv.h as FAKE_LABEL_NAME.  */
> > ...
> > +  return (strcmp (name, ".L0 ") != 0);
> 
> You have a comment here pointing at GAS, but there is no comment in
> GAS pointing here.  The value for FAKE_LABEL_NAME has already changed
> at least once in gas, so we should have a better connection here to
> make sure that a future gas change won't accidentally break this
> disassembler support.  Putting a comment in gas pointing here would be
> OK.  Another possible solution is to move the definition of
> FAKE_LABEL_NAME into include/opcodes/riscv.h, and then you can use it
> directly here.  gas/config/tc-riscv.h already includes this header
> file so I think this should work OK.
> 
> You also have a typo in the comment, edefined -> defined.

This revision addresses the feedback from yourself and Andreas.

OK to apply?

Thanks,
Andrew

---

opcodes/riscv: Hide '.L0 ' fake symbols

The RISC-V assembler generates fake labels with the name '.L0 ' as
part of the debug information (see
gas/config/tc-riscv.h:FAKE_LABEL_NAME).

The problem is that currently, when disassembling an object file, the
output looks like this (this is an example from the GDB testsuite, but
is pretty representative of anything with debug information):

  000000000000001e <main>:
    1e:   7179                    addi    sp,sp,-48
    20:   f406                    sd      ra,40(sp)
    22:   f022                    sd      s0,32(sp)
    24:   1800                    addi    s0,sp,48

  0000000000000026 <.L0 >:
    26:   87aa                    mv      a5,a0
    28:   feb43023                sd      a1,-32(s0)
    2c:   fcc43c23                sd      a2,-40(s0)
    30:   fef42623                sw      a5,-20(s0)

  0000000000000034 <.L0 >:
    34:   fec42783                lw      a5,-20(s0)
    38:   0007871b                sext.w  a4,a5
    3c:   678d                    lui     a5,0x3
    3e:   03978793                addi    a5,a5,57 # 3039 <.LASF30+0x2a9d>
    42:   02f71463                bne     a4,a5,6a <.L0 >

  0000000000000046 <.L0 >:
    46:   000007b7                lui     a5,0x0
    4a:   0007b783                ld      a5,0(a5) # 0 <need_malloc>
    4e:   6f9c                    ld      a5,24(a5)

  0000000000000050 <.L0 >:
    50:   86be                    mv      a3,a5
    52:   466d                    li      a2,27
    54:   4585                    li      a1,1
    56:   000007b7                lui     a5,0x0
    5a:   00078513                mv      a0,a5
    5e:   00000097                auipc   ra,0x0
    62:   000080e7                jalr    ra # 5e <.L0 +0xe>

  0000000000000066 <.L0 >:
    66:   4785                    li      a5,1
    68:   a869                    j       102 <.L0 >

  000000000000006a <.L0 >:
    6a:   000007b7                lui     a5,0x0
    6e:   00078513                mv      a0,a5
    72:   00000097                auipc   ra,0x0
    76:   000080e7                jalr    ra # 72 <.L0 +0x8>

The frequent repeated '.L0 ' labels are pointless, as they are
non-unique there's no way to match a use of '.L0 ' to its appearence
in the output, so we'd be better off just not printing it at all.
That's what this patch does by defining a 'symbol_is_valid' method for
RISC-V.  With this commit, the same disassembly now looks like this:

  000000000000001e <main>:
    1e:   7179                    addi    sp,sp,-48
    20:   f406                    sd      ra,40(sp)
    22:   f022                    sd      s0,32(sp)
    24:   1800                    addi    s0,sp,48
    26:   87aa                    mv      a5,a0
    28:   feb43023                sd      a1,-32(s0)
    2c:   fcc43c23                sd      a2,-40(s0)
    30:   fef42623                sw      a5,-20(s0)
    34:   fec42783                lw      a5,-20(s0)
    38:   0007871b                sext.w  a4,a5
    3c:   678d                    lui     a5,0x3
    3e:   03978793                addi    a5,a5,57 # 3039 <.LASF30+0x2a9d>
    42:   02f71463                bne     a4,a5,6a <.L4>
    46:   000007b7                lui     a5,0x0
    4a:   0007b783                ld      a5,0(a5) # 0 <need_malloc>
    4e:   6f9c                    ld      a5,24(a5)
    50:   86be                    mv      a3,a5
    52:   466d                    li      a2,27
    54:   4585                    li      a1,1
    56:   000007b7                lui     a5,0x0
    5a:   00078513                mv      a0,a5
    5e:   00000097                auipc   ra,0x0
    62:   000080e7                jalr    ra # 5e <main+0x40>
    66:   4785                    li      a5,1
    68:   a869                    j       102 <.L5>

  000000000000006a <.L4>:
    6a:   000007b7                lui     a5,0x0
    6e:   00078513                mv      a0,a5
    72:   00000097                auipc   ra,0x0
    76:   000080e7                jalr    ra # 72 <.L4+0x8>

In order to share the fake label between the assembler and the
libopcodes library, I've added some new defines RISCV_FAKE_LABEL_NAME
and RISCV_FAKE_LABEL_CHAR in include/opcode/riscv.h.  I could have
just moved FAKE_LABEL_NAME to the include file, however, I thnk this
would be confusing, someone working on the assembler would likely not
expect to find FAKE_LABEL_NAME defined outside of the assembler source
tree.  By introducing the RISCV_FAKE_LABEL_* defines I can leave the
assembler standard FAKE_LABEL_ defines in the assembler source, but
still share the RISCV_FAKE_LABEL_* with libopcodes.

gas/ChangeLog:

	* config/tc-riscv.h (FAKE_LABEL_NAME): Define as
	RISCV_FAKE_LABEL_NAME.
	(FAKE_LABEL_CHAR): Define as RISCV_FAKE_LABEL_CHAR.

include/ChangeLog:

	* dis-asm.h (riscv_symbol_is_valid): Declare.
	* opcode/riscv.h (RISCV_FAKE_LABEL_NAME): Define.
	(RISCV_FAKE_LABEL_CHAR): Define.

opcodes/ChangeLog:

        * disassembler.c (disassemble_init_for_target): Add RISC-V
        initialisation.
        * riscv-dis.c (riscv_symbol_is_valid): New function.
---
 gas/ChangeLog          |  6 ++++++
 gas/config/tc-riscv.h  |  4 ++--
 include/ChangeLog      |  6 ++++++
 include/dis-asm.h      |  1 +
 include/opcode/riscv.h |  6 ++++++
 opcodes/ChangeLog      |  6 ++++++
 opcodes/disassemble.c  |  5 +++++
 opcodes/riscv-dis.c    | 17 +++++++++++++++++
 8 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/gas/config/tc-riscv.h b/gas/config/tc-riscv.h
index 5e59740e384..41a18be934a 100644
--- a/gas/config/tc-riscv.h
+++ b/gas/config/tc-riscv.h
@@ -38,10 +38,10 @@ struct expressionS;
 /* Symbols named FAKE_LABEL_NAME are emitted when generating DWARF, so make
    sure FAKE_LABEL_NAME is printable.  It still must be distinct from any
    real label name.  So, append a space, which other labels can't contain.  */
-#define FAKE_LABEL_NAME ".L0 "
+#define FAKE_LABEL_NAME RISCV_FAKE_LABEL_NAME
 /* Changing the special character in FAKE_LABEL_NAME requires changing
    FAKE_LABEL_CHAR too.  */
-#define FAKE_LABEL_CHAR ' '
+#define FAKE_LABEL_CHAR RISCV_FAKE_LABEL_CHAR
 
 #define md_relax_frag(segment, fragp, stretch) \
   riscv_relax_frag (segment, fragp, stretch)
diff --git a/include/dis-asm.h b/include/dis-asm.h
index 84627950c02..6a55f3c1e9a 100644
--- a/include/dis-asm.h
+++ b/include/dis-asm.h
@@ -302,6 +302,7 @@ extern void print_wasm32_disassembler_options (FILE *);
 extern bfd_boolean aarch64_symbol_is_valid (asymbol *, struct disassemble_info *);
 extern bfd_boolean arm_symbol_is_valid (asymbol *, struct disassemble_info *);
 extern bfd_boolean csky_symbol_is_valid (asymbol *, struct disassemble_info *);
+extern bfd_boolean riscv_symbol_is_valid (asymbol *, struct disassemble_info *);
 extern void disassemble_init_powerpc (struct disassemble_info *);
 extern void disassemble_init_s390 (struct disassemble_info *);
 extern void disassemble_init_wasm32 (struct disassemble_info *);
diff --git a/include/opcode/riscv.h b/include/opcode/riscv.h
index 12cb4ca4544..d57f45beb13 100644
--- a/include/opcode/riscv.h
+++ b/include/opcode/riscv.h
@@ -270,6 +270,12 @@ static const char * const riscv_pred_succ[16] =
 #define NGPR 32
 #define NFPR 32
 
+/* These fake label defines are use by both the assembler, and
+   libopcodes.  The assembler uses this when it needs to generate a fake
+   label, and libopcodes uses it to hide the fake labels in its output.  */
+#define RISCV_FAKE_LABEL_NAME ".L0 "
+#define RISCV_FAKE_LABEL_CHAR ' '
+
 /* Replace bits MASK << SHIFT of STRUCT with the equivalent bits in
    VALUE << SHIFT.  VALUE is evaluated exactly once.  */
 #define INSERT_BITS(STRUCT, VALUE, MASK, SHIFT) \
diff --git a/opcodes/disassemble.c b/opcodes/disassemble.c
index 750d76ad865..7370bd13593 100644
--- a/opcodes/disassemble.c
+++ b/opcodes/disassemble.c
@@ -656,6 +656,11 @@ disassemble_init_for_target (struct disassemble_info * info)
       disassemble_init_powerpc (info);
       break;
 #endif
+#ifdef ARCH_riscv
+    case bfd_arch_riscv:
+      info->symbol_is_valid = riscv_symbol_is_valid;
+      break;
+#endif
 #ifdef ARCH_wasm32
     case bfd_arch_wasm32:
       disassemble_init_wasm32 (info);
diff --git a/opcodes/riscv-dis.c b/opcodes/riscv-dis.c
index 97908209483..f1bbfdb1f14 100644
--- a/opcodes/riscv-dis.c
+++ b/opcodes/riscv-dis.c
@@ -518,6 +518,23 @@ print_insn_riscv (bfd_vma memaddr, struct disassemble_info *info)
   return riscv_disassemble_insn (memaddr, insn, info);
 }
 
+/* Prevent use of the fake labels that are generated as part of the DWARF
+   and for relaxable relocations in the assembler.  */
+
+bfd_boolean
+riscv_symbol_is_valid (asymbol * sym,
+                       struct disassemble_info * info ATTRIBUTE_UNUSED)
+{
+  const char * name;
+
+  if (sym == NULL)
+    return FALSE;
+
+  name = bfd_asymbol_name (sym);
+
+  return (strcmp (name, RISCV_FAKE_LABEL_NAME) != 0);
+}
+
 void
 print_riscv_disassembler_options (FILE *stream)
 {
-- 
2.14.5


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