RE: [PATCH] middle-end: fix de-optimizations with bitclear patterns on signed values

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

> -----Original Message-----
> From: Richard Biener <rguenther@suse.de>
> Sent: Tuesday, October 26, 2021 9:26 AM
> To: Tamar Christina <Tamar.Christina@arm.com>
> Cc: gcc-patches@gcc.gnu.org; Jakub Jelinek <jakub@redhat.com>; nd
> <nd@arm.com>
> Subject: RE: [PATCH] middle-end: fix de-optimizations with bitclear patterns
> on signed values
> 
> On Mon, 25 Oct 2021, Tamar Christina wrote:
> 
> > > -----Original Message-----
> > > From: Richard Biener <rguenther@suse.de>
> > > Sent: Friday, October 15, 2021 12:31 PM
> > > To: Tamar Christina <Tamar.Christina@arm.com>
> > > Cc: gcc-patches@gcc.gnu.org; Jakub Jelinek <jakub@redhat.com>; nd
> > > <nd@arm.com>
> > > Subject: Re: [PATCH] middle-end: fix de-optimizations with bitclear
> > > patterns on signed values
> > >
> > > On Fri, 15 Oct 2021, Tamar Christina wrote:
> > >
> > > > Hi All,
> > > >
> > > > During testing after rebasing to commit I noticed a failing
> > > > testcase with the bitmask compare patch.
> > > >
> > > > Consider the following C++ testcase:
> > > >
> > > > #include <compare>
> > > >
> > > > #define A __attribute__((noipa))
> > > > A bool f5 (double i, double j) { auto c = i <=> j; return c >= 0;
> > > > }
> > > >
> > > > This turns into a comparison against chars, on systems where chars
> > > > are signed the pattern inserts an unsigned convert such that it's
> > > > able to do the transformation.
> > > >
> > > > i.e.:
> > > >
> > > >   # RANGE [-1, 2]
> > > >   # c$_M_value_22 = PHI <-1(3), 0(2), 2(5), 1(4)>
> > > >   # RANGE ~[3, 254]
> > > >   _11 = (unsigned char) c$_M_value_22;
> > > >   _19 = _11 <= 1;
> > > >   # .MEM_24 = VDEF <.MEM_6(D)>
> > > >   D.10434 ={v} {CLOBBER};
> > > >   # .MEM_14 = VDEF <.MEM_24>
> > > >   D.10407 ={v} {CLOBBER};
> > > >   # VUSE <.MEM_14>
> > > >   return _19;
> > > >
> > > > instead of:
> > > >
> > > >   # RANGE [-1, 2]
> > > >   # c$_M_value_5 = PHI <-1(3), 0(2), 2(5), 1(4)>
> > > >   # RANGE [-2, 2]
> > > >   _3 = c$_M_value_5 & -2;
> > > >   _19 = _3 == 0;
> > > >   # .MEM_24 = VDEF <.MEM_6(D)>
> > > >   D.10440 ={v} {CLOBBER};
> > > >   # .MEM_14 = VDEF <.MEM_24>
> > > >   D.10413 ={v} {CLOBBER};
> > > >   # VUSE <.MEM_14>
> > > >   return _19;
> > > >
> > > > This causes much worse codegen under -ffast-math due to phiops no
> > > > longer recognizing the pattern.  It turns out that phiopts
> > > > spaceship_replacement is looking for the exact form that was just
> changed.
> > > >
> > > > Trying to get it to recognize the new form is not trivial as the
> > > > transformation doesn't look to work when the thing it's pointing
> > > > to is itself
> > > a phi-node.
> > >
> > > What do you mean?  Where it handles the BIT_AND it could also handle
> > > the conversion, no?  The later handling would probably more
> > > explicitely need to distinguish between the BIT_AND and the conversion
> forms.
> >
> > Looks like I misunderstood the code, it was looking at the uses not
> > the defs of the value.
> >
> > --- inline copy of patch ---
> >
> > The comments seems to suggest this code only checks for (res & ~1) ==
> > 0 but the implementation seems to suggest it's broader.
> >
> > As such I added a case to check to see if the value comparison we
> > found is a type cast.  and strips away the type cast and continues.
> >
> > In match.pd the typecasts are only added for signed comparisons to ==
> > 0 and != 0 which are then rewritten into comparisons with 1.
> >
> > As such I only check for 1 and LE and GT, which is what match.pd would
> > have rewritten it to.
> >
> > This fixes the regression but this is not code I 100% understand,
> > since I don't really know the semantics of the spaceship operator so
> > would appreciate an extra look.
> >
> > Bootstrapped Regtested on aarch64-none-linux-gnu, x86_64-pc-linux-gnu
> > and no regressions.
> >
> > Ok for master?
> 
> Please add a testcase.  I hope Jakub can review the spaceship_replacement
> patch since he's the one familiar with the code.

There's already a bunch of testcases that test the various variants: gcc/testsuite/g++.dg/opt/pr94589-1.C
and gcc/testsuite/g++.dg/opt/pr94589-2.C which is how I noticed the failure.  However they only trigger
the failure on signed chars.  I tried forcing `-fsigned-char` to see if I can make a general testcase but this
seems to have not done it.

Is there another flag I can use?

Regards,
Tamar
> 
> Thanks,
> Richard.
> 
> > Thanks,
> > Tamar
> >
> > gcc/ChangeLog:
> >
> > 	* tree-ssa-phiopt.c (spaceship_replacement): Handle new canonical
> > 	codegen.
> >
> > diff --git a/gcc/tree-ssa-phiopt.c b/gcc/tree-ssa-phiopt.c index
> >
> 0e339c46afa29fa97f90d9bc4394370cd9b4b396..65b25be3399b75d5e9cab0f78
> aa2
> > 340418571a33 100644
> > --- a/gcc/tree-ssa-phiopt.c
> > +++ b/gcc/tree-ssa-phiopt.c
> > @@ -2037,6 +2037,7 @@ spaceship_replacement (basic_block cond_bb,
> basic_block middle_bb,
> >    tree lhs, rhs;
> >    gimple *orig_use_stmt = use_stmt;
> >    tree orig_use_lhs = NULL_TREE;
> > +  bool is_canon = false;
> >    int prec = TYPE_PRECISION (TREE_TYPE (phires));
> >    if (is_gimple_assign (use_stmt)
> >        && gimple_assign_rhs_code (use_stmt) == BIT_AND_EXPR @@ -2063,6
> > +2064,26 @@ spaceship_replacement (basic_block cond_bb, basic_block
> middle_bb,
> >      }
> >    else if (is_gimple_assign (use_stmt))
> >      {
> > +      /* Deal with if match.pd has rewritten the (res & ~1) == 0
> > +	 into res <= 1 and has left a type-cast for signed types.  */
> > +      if (gimple_assign_cast_p (use_stmt))
> > +	{
> > +	  orig_use_lhs = gimple_assign_lhs (use_stmt);
> > +	  if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (orig_use_lhs))
> > +	    return false;
> > +	  if (EDGE_COUNT (phi_bb->preds) != 4)
> > +	    return false;
> > +	  if (!TYPE_UNSIGNED (TREE_TYPE (orig_use_lhs)))
> > +	    return false;
> > +	  if (!single_imm_use (orig_use_lhs, &use_p, &use_stmt))
> > +	    return false;
> > +	  tree_code cmp;
> > +	  if (is_gimple_assign (use_stmt)
> > +	      && (cmp = gimple_assign_rhs_code (use_stmt))
> > +	      && (cmp == LE_EXPR || cmp == GT_EXPR)
> > +	      && wi::eq_p (wi::to_wide (gimple_assign_rhs2 (use_stmt)), 1))
> > +	    is_canon = true;
> > +	}
> >        if (gimple_assign_rhs_class (use_stmt) == GIMPLE_BINARY_RHS)
> >  	{
> >  	  cmp = gimple_assign_rhs_code (use_stmt); @@ -2099,7 +2120,9
> @@
> > spaceship_replacement (basic_block cond_bb, basic_block middle_bb,
> >        || !tree_fits_shwi_p (rhs)
> >        || !IN_RANGE (tree_to_shwi (rhs), -1, 1))
> >      return false;
> > -  if (orig_use_lhs)
> > +  /* If we're already in the canonical form we need to keep the original
> > +     comparison.  */
> > +  if (orig_use_lhs && !is_canon)
> >      {
> >        if ((cmp != EQ_EXPR && cmp != NE_EXPR) || !integer_zerop (rhs))
> >  	return false;
> > @@ -2310,6 +2333,7 @@ spaceship_replacement (basic_block cond_bb,
> basic_block middle_bb,
> >      one_cmp = GT_EXPR;
> >
> >    enum tree_code res_cmp;
> > +
> >    switch (cmp)
> >      {
> >      case EQ_EXPR:
> > @@ -2345,6 +2369,8 @@ spaceship_replacement (basic_block cond_bb,
> basic_block middle_bb,
> >  	res_cmp = one_cmp == LT_EXPR ? GE_EXPR : LE_EXPR;
> >        else if (integer_minus_onep (rhs))
> >  	res_cmp = one_cmp == LT_EXPR ? GT_EXPR : LT_EXPR;
> > +      else if (integer_onep (rhs) && is_canon)
> > +	res_cmp = GE_EXPR;
> >        else
> >  	return false;
> >        break;
> > @@ -2353,6 +2379,8 @@ spaceship_replacement (basic_block cond_bb,
> basic_block middle_bb,
> >  	res_cmp = one_cmp == LT_EXPR ? LE_EXPR : GE_EXPR;
> >        else if (integer_zerop (rhs))
> >  	res_cmp = one_cmp;
> > +      else if (integer_onep (rhs) && is_canon)
> > +	res_cmp = LE_EXPR;
> >        else
> >  	return false;
> >        break;
> >
> 
> --
> Richard Biener <rguenther@suse.de>
> SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409
> Nuernberg, Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)