RE: [PATCH] RFC: New compact syntax for insn and insn_split in Machine Descriptions

[Tamar Christina <Tamar.Christina@arm.com>]

> -----Original Message-----
> From: Richard Sandiford <richard.sandiford@arm.com>
> Sent: Friday, April 21, 2023 6:19 PM
> To: Tamar Christina <Tamar.Christina@arm.com>
> Cc: gcc-patches@gcc.gnu.org; nd <nd@arm.com>; Richard Earnshaw
> <Richard.Earnshaw@arm.com>
> Subject: Re: [PATCH] RFC: New compact syntax for insn and insn_split in
> Machine Descriptions
> 
> Tamar Christina <tamar.christina@arm.com> writes:
> > Hi All,
> >
> > This patch adds support for a compact syntax for specifying
> > constraints in instruction patterns. Credit for the idea goes to Richard
> Earnshaw.
> >
> > I am sending up this RFC to get feedback for it's inclusion in GCC 14.
> > With this new syntax we want a clean break from the current
> > limitations to make something that is hopefully easier to use and maintain.
> >
> > The idea behind this compact syntax is that often times it's quite
> > hard to correlate the entries in the constrains list, attributes and instruction
> lists.
> >
> > One has to count and this often is tedious.  Additionally when
> > changing a single line in the insn multiple lines in a diff change,
> > making it harder to see what's going on.
> >
> > This new syntax takes into account many of the common things that are
> done in MD
> > files.   It's also worth saying that this version is intended to deal with the
> > common case of a string based alternatives.   For C chunks we have some
> ideas
> > but those are not intended to be addressed here.
> >
> > It's easiest to explain with an example:
> >
> > normal syntax:
> >
> > (define_insn_and_split "*movsi_aarch64"
> >   [(set (match_operand:SI 0 "nonimmediate_operand" "=r,k,r,r,r,r, r,w, m, m,
> r,  r,  r, w,r,w, w")
> > 	(match_operand:SI 1 "aarch64_mov_operand"  "
> r,r,k,M,n,Usv,m,m,rZ,w,Usw,Usa,Ush,rZ,w,w,Ds"))]
> >   "(register_operand (operands[0], SImode)
> >     || aarch64_reg_or_zero (operands[1], SImode))"
> >   "@
> >    mov\\t%w0, %w1
> >    mov\\t%w0, %w1
> >    mov\\t%w0, %w1
> >    mov\\t%w0, %1
> >    #
> >    * return aarch64_output_sve_cnt_immediate (\"cnt\", \"%x0\",
> operands[1]);
> >    ldr\\t%w0, %1
> >    ldr\\t%s0, %1
> >    str\\t%w1, %0
> >    str\\t%s1, %0
> >    adrp\\t%x0, %A1\;ldr\\t%w0, [%x0, %L1]
> >    adr\\t%x0, %c1
> >    adrp\\t%x0, %A1
> >    fmov\\t%s0, %w1
> >    fmov\\t%w0, %s1
> >    fmov\\t%s0, %s1
> >    * return aarch64_output_scalar_simd_mov_immediate (operands[1],
> SImode);"
> >   "CONST_INT_P (operands[1]) && !aarch64_move_imm (INTVAL
> (operands[1]), SImode)
> >     && REG_P (operands[0]) && GP_REGNUM_P (REGNO (operands[0]))"
> >    [(const_int 0)]
> >    "{
> >        aarch64_expand_mov_immediate (operands[0], operands[1]);
> >        DONE;
> >     }"
> >   ;; The "mov_imm" type for CNT is just a placeholder.
> >   [(set_attr "type"
> "mov_reg,mov_reg,mov_reg,mov_imm,mov_imm,mov_imm,load_4,
> >
> load_4,store_4,store_4,load_4,adr,adr,f_mcr,f_mrc,fmov,neon_move")
> >    (set_attr "arch"   "*,*,*,*,*,sve,*,fp,*,fp,*,*,*,fp,fp,fp,simd")
> >    (set_attr "length" "4,4,4,4,*,  4,4, 4,4, 4,8,4,4, 4, 4, 4,   4")
> > ]
> > )
> >
> > New syntax:
> >
> > (define_insn_and_split "*movsi_aarch64"
> >   [(set (match_operand:SI 0 "nonimmediate_operand")
> > 	(match_operand:SI 1 "aarch64_mov_operand"))]
> >   "(register_operand (operands[0], SImode)
> >     || aarch64_reg_or_zero (operands[1], SImode))"
> >   "@@ (cons: 0 1; attrs: type arch length)
> >    [=r, r  ; mov_reg  , *   , 4] mov\t%w0, %w1
> >    [k , r  ; mov_reg  , *   , 4] ^
> >    [r , k  ; mov_reg  , *   , 4] ^
> >    [r , M  ; mov_imm  , *   , 4] mov\t%w0, %1
> >    [r , n  ; mov_imm  , *   , *] #
> >    [r , Usv; mov_imm  , sve , 4] << aarch64_output_sve_cnt_immediate ('cnt',
> '%x0', operands[1]);
> >    [r , m  ; load_4   , *   , 4] ldr\t%w0, %1
> >    [w , m  ; load_4   , fp  , 4] ldr\t%s0, %1
> >    [m , rZ ; store_4  , *   , 4] str\t%w1, %0
> >    [m , w  ; store_4  , fp  , 4] str\t%s1, %0
> >    [r , Usw; load_4   , *   , 8] adrp\t%x0, %A1;ldr\t%w0, [%x0, %L1]
> >    [r , Usa; adr      , *   , 4] adr\t%x0, %c1
> >    [r , Ush; adr      , *   , 4] adrp\t%x0, %A1
> >    [w , rZ ; f_mcr    , fp  , 4] fmov\t%s0, %w1
> >    [r , w  ; f_mrc    , fp  , 4] fmov\t%w0, %s1
> >    [w , w  ; fmov     , fp  , 4] fmov\t%s0, %s1
> >    [w , Ds ; neon_move, simd, 4] <<
> aarch64_output_scalar_simd_mov_immediate (operands[1], SImode);"
> >   "CONST_INT_P (operands[1]) && !aarch64_move_imm (INTVAL
> (operands[1]), SImode)
> >     && REG_P (operands[0]) && GP_REGNUM_P (REGNO (operands[0]))"
> >   [(const_int 0)]
> >   {
> >     aarch64_expand_mov_immediate (operands[0], operands[1]);
> >     DONE;
> >   }
> >   ;; The "mov_imm" type for CNT is just a placeholder.
> > )
> >
> > The patch contains some more rewritten examples for both Arm and
> > AArch64.  I have included them for examples in this RFC but the final
> > version posted in GCC 14 will have these split out.
> >
> > The main syntax rules are as follows (See docs for full rules):
> >   - Template must start with "@@" to use the new syntax.
> >   - "@@" is followed by a layout in parentheses which is "cons:" followed by
> >     a list of match_operand/match_scratch IDs, then a semicolon, then the
> >     same for attributes ("attrs:"). Both sections are optional (so you can
> >     use only cons, or only attrs, or both), and cons must come before attrs
> >     if present.
> >   - Each alternative begins with any amount of whitespace.
> >   - Following the whitespace is a comma-separated list of constraints and/or
> >     attributes within brackets [], with sections separated by a semicolon.
> >   - Following the closing ']' is any amount of whitespace, and then the actual
> >     asm output.
> >   - Spaces are allowed in the list (they will simply be removed).
> >   - All alternatives should be specified: a blank list should be
> >     "[,,]", "[,,;,]" etc., not "[]" or "" (however genattr may segfault if
> >     you leave certain attributes empty, I have found).
> >   - The actual constraint string in the match_operand or match_scratch, and
> >     the attribute string in the set_attr, must be blank or an empty string
> >     (you can't combine the old and new syntaxes).
> >   - The common idion * return can be shortened by using <<.
> >   - Any unexpanded iterators left during processing will result in an error at
> >     compile time.   If for some reason <> is needed in the output then these
> >     must be escaped using \.
> >   - Inside a @@ block '' is treated as "" when there are multiple characters
> >     inside the single quotes.  This version does not handle multi byte literals
> >     like specifying characters as their numerical encoding, like \003 nor does
> >     it handle unicode, especially multibyte encodings.  This feature may be
> more
> >     trouble than it's worth so have no finished it off, however this means one
> >     can use 'foo' instead of \"foo\" to denote a multicharacter string.
> >   - Inside an @@ block any unexpanded iterators will result in a compile time
> >     fault instead of incorrect assembly being generated at runtime.  If the
> >     literal <> is needed in the output this needs to be escaped with \<\>.
> >   - This check is not performed inside C blocks (lines starting with *).
> >   - Instead of copying the previous instruction again in the next pattern, one
> >     can use ^ to refer to the previous asm string.
> 
> Thanks for doing this.  The new syntax seems like a clear improvement for
> complex patterns like movs.
> 
> Some comments/suggestions:
> 
> - From a style perspective, out-of-order constraints should IMO be strongly
>   discouraged.  The asm string uses %0, %1, %2 etc. to refer to operands,
>   and having that directly after a list that puts the constraints in
>   a different order (such as [%2, %0, %1]) would IMO be very confusing.
> 
>   I agree there might be cases where dropping constraints makes sense.
>   But I think in general we should encourage all constraints to be
>   specified, and be specified in order.  And that's likely to be the
>   natural choice in an overwhelming majority of cases anyway.
> 
>   So how about having a simpler syntax for the first line when all
>   constraints are specified in order?  Maybe just "cons" (without the
>   colon or numbers).
> 
> - I'm not too keen on the '' thing.  It sounded from internal
>   discussion like backslashes and quoting were a problem generally.
> 
>   Would it work to quote the new form in {@ ... } instead?  There should
>   be no compatibility problem with that, since @ isn't a standard C++
>   lexing token.

Fair enough, did you mean {@<string>} or @'string' ? 

Just so I understand before implementing 😊

> 
> - Could we support a comment syntax?  E.g. ignore lines beginning with
>   ;; or // (or both)?  In the example above, it would be good to keep
>   the comment about the CNT type attribute nearer to the attribute itself.

Fair point. 

> 
> - Very minor, but using [...] rather than (...) for the first line
>   might make it more visually obvious that it's acting as a table
>   header for the [...] rows.

Thanks,
Will send new version out soon.

Thanks,
Tamar
> 
> Haven't done a detailed review of the gensupport bits, but:
> 
> > [...]
> > @@ -700,12 +702,37 @@ process_template (class data *d, const char
> *template_code)
> >  	  if (sp != ep)
> >  	    message_at (d->loc, "trailing whitespace in output template");
> >
> > -	  while (cp < sp)
> > +	  /* Check for any unexpanded iterators.  */
> > +	  std::string buff (cp, sp - cp);
> > +	  if (bp[0] != '*' && d->compact_syntax_p)
> >  	    {
> > -	      putchar (*cp);
> > -	      cp++;
> > +	      size_t start = buff.find ('<');
> > +	      size_t end = buff.find ('>', start + 1);
> > +	      if (end != std::string::npos || start != std::string::npos)
> > +		{
> > +		  if (end == std::string::npos || start == std::string::npos)
> > +		    fatal_at (d->loc, "unmatched angle brackets, likely an "
> > +			      "error in iterator syntax in %s", buff.c_str ());
> > +
> > +		  if (start != 0
> > +		      && buff[start-1] == '\\'
> > +		      && buff[end-1] == '\\')
> > +		    {
> > +		      /* Found a valid escape sequence, erase the characters for
> > +			 output.  */
> > +		      buff.erase (end-1, 1);
> > +		      buff.erase (start-1, 1);
> > +		    }
> > +		  else
> > +		    fatal_at (d->loc, "unresolved iterator '%s' in '%s'",
> > +			      buff.substr(start+1, end - start-1).c_str (),
> > +			      buff.c_str ());
> > +		}
> >  	    }
> 
> Asm strings that want unbalanced but quoted < or > should be able to use
> them, so the check for backslashes should probably come first.
> I suppose this also runs into the classic problem of whether the preceding
> backslash was itself quoted, etc.
> 
> So maybe it would make sense to walk through character-by-character,
> something like:
> 
>     const char *p = cp;
>     const char *last_bracket = nullptr;
>     while (p < sp)
>       {
>         if (*p == '\\' && p + 1 < sp)
>           {
>             p += 2;
>             continue;
>           }
>         if (*p == '>' && last_bracket && *last_bracket == '<')
>           ... unexpanded iterator ...
>         else if (*p == '<' || *p == '>')
>           last_bracket = p;
>         p += 1;
>       }
>     if (last_bracket)
>       ... error ...
> 
> That also copes with unlikely things like \<...\>...<foo>, where an unexpanded
> iterator (or incorrectly quoted <...>) comes after a correctly-quoted <...>.
> 
> Thanks,
> Richard