Re: [PATCH] final: Improve output for -dp and -fverbose-asm

[Martin Sebor <msebor@gmail.com>]

On 11/30/2017 09:44 AM, Segher Boessenkool wrote:
> On Thu, Nov 30, 2017 at 09:28:44AM -0700, Martin Sebor wrote:
>>> The point is that [length = 12] takes up an awful lot of space.  The
>>> output of -fverbose-asm alread suffers from information overload.
>>
>> Amount of space vs amount of detail are two different concerns.
>
> Yes.
>
>> If you feel that the output is overly detailed then adding even
>> more to it won't help.  Other than that, I don't think trading
>> readability for space savings makes sense in a format whose main
>> purpose is to be readable.  If it's line length that's a concern
>> then using spaces instead of tabs would make them look shorter.
>
> Trunk:
>
> ===
> .L.yk:
>         cmpdi 0,4,0      # 8    *movdi_internal2/1      [length = 4]
>         beq 0,.L2        # 9    *rs6000.md:12754        [length = 4]
>         srdi 9,3,32      # 11   lshrdi3 [length = 4]
>         xor 9,9,3        # 12   *boolsi3        [length = 4]
>         rldicl 9,9,0,32  # 13   zero_extendsidi2/2      [length = 4]
>         cmpd 7,9,3       # 14   *cmpdi_signed   [length = 4]
>         beq 7,.L7        # 15   *rs6000.md:12754        [length = 4]
> .L5:
>         mr 3,4   # 32   *movdi_internal64/3     [length = 4]
>         blr      # 53   simple_return   [length = 4]
> .L2:
>         lwz 9,0(4)       # 28   zero_extendsidi2/1      [length = 4]
>         trap     # 29   trap    [length = 4]
> .L7:
>         addic 4,9,-1     # 47   *adddi3_imm_carry_m1    [length = 4]
>         subfe 4,4,9      # 48   *subfdi3_carry_in_internal      [length = 4]
>         b .L5    # 58   jump    [length = 4]
> ===
>
> Patched:
> ===
> .L.yk:
>         cmpdi 0,4,0      # 8    [c=4 l=4]  *movdi_internal2/0
>         beq 0,.L2        # 9    [c=4 l=4]  *rs6000.md:12774
>         srdi 9,3,32      # 11   [c=4 l=4]  lshrdi3
>         xor 9,9,3        # 12   [c=4 l=4]  *boolsi3
>         rldicl 9,9,0,32  # 13   [c=4 l=4]  zero_extendsidi2/1
>         cmpd 7,9,3       # 14   [c=4 l=4]  *cmpdi_signed
>         beq 7,.L7        # 15   [c=4 l=4]  *rs6000.md:12774
> .L5:
>         mr 3,4           # 32   [c=4 l=4]  *movdi_internal64/2
>         blr              # 76   [c=4 l=4]  simple_return
> .L2:
>         lfiwzx 0,0,4     # 28   [c=8 l=4]  zero_extendsidi2/2
>         trap             # 29   [c=4 l=4]  trap
> .L7:
>         addic 4,9,-1     # 70   [c=4 l=4]  *adddi3_imm_carry_m1
>         subfe 4,4,9      # 71   [c=4 l=4]  *subfdi3_carry_in_internal
>         b .L5            # 81   [c=4 l=4]  jump
> ===
>
> It is neither line length nor amt of info that makes the second one
> way better readable.

The justification certainly makes it easier to read.  But
the abbreviations make it harder to interpret.  [c=4 l=4]
makes no sense to anyone not already familiar with what
it means.

There's nothing wrong with using mnemonics as long as they're
well established and commonly understood.  Absent that, they
should be explained in some accessible document.

Not everyone who reads the verbose assembly is familiar with
GCC internals.  Users read it to help debug problems in their
code.  They shouldn't have to also study GCC source code to
understand what the contents mean.

Martin