gdb: disassembler opcode display formatting

This commit changes the format of 'disassemble /r' to match GNU
objdump.  Specifically, GDB will now display the instruction bytes in
as 'objdump --wide --disassemble' does.

Here is an example for RISC-V before this patch:

  (gdb) disassemble /r 0x0001018e,0x0001019e
  Dump of assembler code from 0x1018e to 0x1019e:
     0x0001018e <call_me+66>:     03 26 84 fe     lw      a2,-24(s0)
     0x00010192 <call_me+70>:     83 25 c4 fe     lw      a1,-20(s0)
     0x00010196 <call_me+74>:     61 65   lui     a0,0x18
     0x00010198 <call_me+76>:     13 05 85 6a     addi    a0,a0,1704
     0x0001019c <call_me+80>:     f1 22   jal     0x10368 <printf>
  End of assembler dump.

And here's an example after this patch:

  (gdb) disassemble /r 0x0001018e,0x0001019e
  Dump of assembler code from 0x1018e to 0x1019e:
     0x0001018e <call_me+66>:     fe842603                lw      a2,-24(s0)
     0x00010192 <call_me+70>:     fec42583                lw      a1,-20(s0)
     0x00010196 <call_me+74>:     6561                    lui     a0,0x18
     0x00010198 <call_me+76>:     6a850513                addi    a0,a0,1704
     0x0001019c <call_me+80>:     22f1                    jal     0x10368 <printf>
  End of assembler dump.

There are two differences here.  First, the instruction bytes after
the patch are grouped based on the size of the instruction, and are
byte-swapped to little-endian order.

Second, after the patch, GDB now uses the bytes-per-line hint from
libopcodes to add whitespace padding after the opcode bytes, this
means that in most cases the instructions are nicely aligned.

It is still possible for a very long instruction to intrude into the
disassembled text space.  The next example is x86-64, before the
patch:

  (gdb) disassemble /r main
  Dump of assembler code for function main:
     0x0000000000401106 <+0>:     55      push   %rbp
     0x0000000000401107 <+1>:     48 89 e5        mov    %rsp,%rbp
     0x000000000040110a <+4>:     c7 87 d8 00 00 00 01 00 00 00   movl   $0x1,0xd8(%rdi)
     0x0000000000401114 <+14>:    b8 00 00 00 00  mov    $0x0,%eax
     0x0000000000401119 <+19>:    5d      pop    %rbp
     0x000000000040111a <+20>:    c3      ret
  End of assembler dump.

And after the patch:

  (gdb) disassemble /r main
  Dump of assembler code for function main:
     0x0000000000401106 <+0>:     55                      push   %rbp
     0x0000000000401107 <+1>:     48 89 e5                mov    %rsp,%rbp
     0x000000000040110a <+4>:     c7 87 d8 00 00 00 01 00 00 00   movl   $0x1,0xd8(%rdi)
     0x0000000000401114 <+14>:    b8 00 00 00 00          mov    $0x0,%eax
     0x0000000000401119 <+19>:    5d                      pop    %rbp
     0x000000000040111a <+20>:    c3                      ret
  End of assembler dump.

Most instructions are aligned, except for the very long instruction.
Notice too that for x86-64 libopcodes doesn't request that GDB group
the instruction bytes.  This matches the behaviour of objdump.

In case the user really wants the old behaviour, I have added a new
modifier 'disassemble /b', this displays the instruction byte at a
time.  For x86-64, which never groups instruction bytes, /b and /r are
equivalent, but for RISC-V, using /b gets the old layout back (except
that the whitespace for alignment is still present).  Consider our
original RISC-V example, this time using /b:

  (gdb) disassemble /b 0x0001018e,0x0001019e
  Dump of assembler code from 0x1018e to 0x1019e:
     0x0001018e <call_me+66>:     03 26 84 fe             lw      a2,-24(s0)
     0x00010192 <call_me+70>:     83 25 c4 fe             lw      a1,-20(s0)
     0x00010196 <call_me+74>:     61 65                   lui     a0,0x18
     0x00010198 <call_me+76>:     13 05 85 6a             addi    a0,a0,1704
     0x0001019c <call_me+80>:     f1 22                   jal     0x10368 <printf>
  End of assembler dump.

Obviously, this patch is a potentially significant change to the
behaviour or /r.  I could have added /b with the new behaviour and
left /r alone.  However, personally, I feel the new behaviour is
significantly better than the old, hence, I made /r be what I consider
the "better" behaviour.

The reason I prefer the new behaviour is that, when I use /r, I almost
always want to manually decode the instruction for some reason, and
having the bytes displayed in "instruction order" rather than memory
order, just makes this easier.

The 'record instruction-history' command also takes a /r modifier, and
has been modified in the same way as disassemble; /r gets the new
behaviour, and /b has been added to retain the old behaviour.

Finally, the MI command -data-disassemble, is unchanged in behaviour,
this command now requests the raw bytes of the instruction, which is
equivalent to the /b modifier.  This means that the MI output will
remain backward compatible.

1 files changed, 41 insertions, 7 deletions

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 238a49b..596e587 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -7945,7 +7945,10 @@ are printed in execution order.
 
 It can also print mixed source+disassembly if you specify the the
 @code{/m} or @code{/s} modifier, and print the raw instructions in hex
-as well as in symbolic form by specifying the @code{/r} modifier.
+as well as in symbolic form by specifying the @code{/r} or @code{/b}
+modifier.  The behaviour of the @code{/m}, @code{/s}, @code{/r}, and
+@code{/b} modifiers are the same as for the @kbd{disassemble} command
+(@pxref{disassemble,,@kbd{disassemble}}).
 
 The current position marker is printed for the instruction at the
 current program counter value.  This instruction can appear multiple
@@ -9859,6 +9862,7 @@ After @code{info line}, using @code{info line} again without
 specifying a location will display information about the next source
 line.
 
+@anchor{disassemble}
 @table @code
 @kindex disassemble
 @cindex assembly instructions
@@ -9869,16 +9873,17 @@ line.
 @itemx disassemble /m
 @itemx disassemble /s
 @itemx disassemble /r
+@itemx disassemble /b
 This specialized command dumps a range of memory as machine
 instructions.  It can also print mixed source+disassembly by specifying
-the @code{/m} or @code{/s} modifier and print the raw instructions in hex
-as well as in symbolic form by specifying the @code{/r} modifier.
-The default memory range is the function surrounding the
+the @code{/m} or @code{/s} modifier and print the raw instructions in
+hex as well as in symbolic form by specifying the @code{/r} or @code{/b}
+modifier.  The default memory range is the function surrounding the
 program counter of the selected frame.  A single argument to this
 command is a program counter value; @value{GDBN} dumps the function
-surrounding this value.  When two arguments are given, they should
-be separated by a comma, possibly surrounded by whitespace.  The
-arguments specify a range of addresses to dump, in one of two forms:
+surrounding this value.  When two arguments are given, they should be
+separated by a comma, possibly surrounded by whitespace.  The arguments
+specify a range of addresses to dump, in one of two forms:
 
 @table @code
 @item @var{start},@var{end}
@@ -9916,6 +9921,35 @@ Dump of assembler code from 0x32c4 to 0x32e4:
 End of assembler dump.
 @end smallexample
 
+The following two examples are for RISC-V, and demonstrates the
+difference between the @code{/r} and @code{/b} modifiers.  First with
+@code{/b}, the bytes of the instruction are printed, in hex, in memory
+order:
+
+@smallexample
+(@value{GDBP}) disassemble /b 0x00010150,0x0001015c
+Dump of assembler code from 0x10150 to 0x1015c:
+   0x00010150 <call_me+4>:      22 dc                 	sw	s0,56(sp)
+   0x00010152 <call_me+6>:      80 00                 	addi	s0,sp,64
+   0x00010154 <call_me+8>:      23 26 a4 fe           	sw	a0,-20(s0)
+   0x00010158 <call_me+12>:     23 24 b4 fe           	sw	a1,-24(s0)
+End of assembler dump.
+@end smallexample
+
+In contrast, with @code{/r} the bytes of the instruction are displayed
+in the instruction order, for RISC-V this means that the bytes have been
+swapped to little-endian order:
+
+@smallexample
+(@value{GDBP}) disassemble /r 0x00010150,0x0001015c
+Dump of assembler code from 0x10150 to 0x1015c:
+   0x00010150 <call_me+4>:      dc22              	sw	s0,56(sp)
+   0x00010152 <call_me+6>:      0080              	addi	s0,sp,64
+   0x00010154 <call_me+8>:      fea42623        	sw	a0,-20(s0)
+   0x00010158 <call_me+12>:     feb42423        	sw	a1,-24(s0)
+End of assembler dump.
+@end smallexample
+
 Here is an example showing mixed source+assembly for Intel x86
 with @code{/m} or @code{/s}, when the program is stopped just after
 function prologue in a non-optimized function with no inline code.