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

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

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> -----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?

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..65b25be3399b75d5e9cab0f78aa2340418571a33 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;

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diff --git a/gcc/tree-ssa-phiopt.c b/gcc/tree-ssa-phiopt.c
index 0e339c46afa29fa97f90d9bc4394370cd9b4b396..65b25be3399b75d5e9cab0f78aa2340418571a33 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;