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

[David Malcolm <dmalcolm@redhat.com>]

On Thu, 2017-11-30 at 18:02 +0100, Michael Matz wrote:
> Hi,
> 
> On Thu, 30 Nov 2017, Martin Sebor wrote:
> 
> > >         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
> > 
> > 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.
> 
> Um, I think that's a bit overactive.  How would you know what 
> adddi3_imm_carry_m1 really means without knowing the particular GCC 
> backend?  Or what the above magic number after # means?
> 
> Or, for that matter, what "length" means?  Could be byte-length,
> sure.  
> But OTOH, for a RISC target it's always four, so why print it?  The
> GCC 
> developers surely meant cycle-length with that, nothing else makes
> sense.
> 
> My point is, to interpret the asm dumps there's no way around having
> some 
> knowledge and getting used to it.  In addition I doubt they're used 
> heavily to debug programs.  Rather they're used to study the
> interaction 
> between compiler and program (and potentially to find miscompilations
> or 
> strangenesses the compiler emits).  As such even -fverbose-asm is a 
> low-level compiler debugging tool, not something for an end-user
> that 
> needs stability or documentation.

-fverbose-asm is on the border of compiler-debugging vs end-user usage.

FWIW an improvement to -fverbose-asm was explicitly mentioned in the
gcc 7 release notes:
  https://gcc.gnu.org/gcc-7/changes.html
and I've seen at least some end-users comment favorably on that change;
this was:
  https://gcc.gnu.org/ml/gcc-patches/2016-08/msg01051.html
which was originally a "-fasm-show-source" option.


Dave