diff mbox

[PR66726] Factor conversion out of COND_EXPR

Message ID 559BCB15.9010209@linaro.org
State New
Headers show

Commit Message

Kugan Vivekanandarajah July 7, 2015, 12:50 p.m. UTC 
On 07/07/15 07:37, Jeff Law wrote:
> On 07/04/2015 06:32 AM, Kugan wrote:

> I would also verify that this turns into a MIN_EXPR.  I think the patch
> as-written won't detect the MIN_EXPR until the _next_ time phi-opt is
> called.  And one of the benefits we're really looking for here is to
> remove barriers to finding these min/max expressions.
> 

> 
>> +
>> diff --git a/gcc/tree-ssa-phiopt.c b/gcc/tree-ssa-phiopt.c
>> index d2a5cee..12ab9ee 100644
>> --- a/gcc/tree-ssa-phiopt.c
>> +++ b/gcc/tree-ssa-phiopt.c
>> @@ -73,6 +73,7 @@ along with GCC; see the file COPYING3.  If not see
>>   static unsigned int tree_ssa_phiopt_worker (bool, bool);
>>   static bool conditional_replacement (basic_block, basic_block,
>>                        edge, edge, gphi *, tree, tree);
>> +static bool factor_out_conditional_conversion (edge, edge, gphi *,
>> tree, tree);
>>   static int value_replacement (basic_block, basic_block,
>>                     edge, edge, gimple, tree, tree);
>>   static bool minmax_replacement (basic_block, basic_block,
>> @@ -342,6 +343,8 @@ tree_ssa_phiopt_worker (bool do_store_elim, bool
>> do_hoist_loads)
>>           cfgchanged = true;
>>         else if (minmax_replacement (bb, bb1, e1, e2, phi, arg0, arg1))
>>           cfgchanged = true;
>> +      else if (factor_out_conditional_conversion (e1, e2, phi, arg0,
>> arg1))
>> +        cfgchanged = true;
> So this transformation does not inherently change the CFG, so setting
> CFGCHANGED isn't really appropriate and may trigger unnecessary cleanups.
> 
> I think the transformation needs to occur prior this if-elseif-else
> block since the transformation should enable the code in the
> if-elseif-else block to find more optimization opportunities.
> 
> That will also imply we either restart after the transformation applies,
> or we update the local variables that are used as arguments to
> conditional_replacement, abs_replacement and minmax_replacement.
> 
> 
>>       }
>>       }
>>
>> @@ -410,6 +413,108 @@ replace_phi_edge_with_variable (basic_block
>> cond_block,
>>             bb->index);
>>   }
>>
>> +/* PR66726: Factor conversion out of COND_EXPR.  If the arguments of
>> the PHI
>> +   stmt are CONVERT_STMT, factor out the conversion and perform the
>> conversion
>> +   to the result of PHI stmt.  */
>> +
>> +static bool
>> +factor_out_conditional_conversion (edge e0, edge e1, gphi *phi,
>> +                   tree arg0, tree arg1)
>> +{
>> +  gimple def0 = NULL, def1 = NULL, new_stmt;
>> +  tree new_arg0 = NULL_TREE, new_arg1 = NULL_TREE;
>> +  tree temp, result;
>> +  gimple_stmt_iterator gsi;
>> +
>> +  /* One of the arguments has to be SSA_NAME and other argument can
>> +     be an SSA_NAME of INTEGER_CST.  */
>> +  if ((TREE_CODE (arg0) != SSA_NAME
>> +       && TREE_CODE (arg0) != INTEGER_CST)
>> +      || (TREE_CODE (arg1) != SSA_NAME
>> +      && TREE_CODE (arg1) != INTEGER_CST)
>> +      || (TREE_CODE (arg0) == INTEGER_CST
>> +      && TREE_CODE (arg1) == INTEGER_CST))
>> +    return false;
>> +
>> +  /* Handle only PHI statements with two arguments.  TODO: If all
>> +     other arguments to PHI are INTEGER_CST, we can handle more
>> +     than two arguments too.  */
>> +  if (gimple_phi_num_args (phi) != 2)
>> +    return false;
> If you're just handling two arguments, then it's probably easiest to
> just swap arg0/arg1 e0/e1 if arg0 is not an SSA_NAME like this:
> 
>  /* First canonicalize to simplify tests.  */
>   if (TREE_CODE (arg0) != SSA_NAME)
>     {
>       std::swap (arg0, arg1);
>       std::swap (e0, e1);
>     }
> 
>   if (TREE_CODE (arg0) != SSA_NAME)
>     return false;
> 
> That simplifies things a bit since you're going to know from thsi point
> forward that arg0 is an SSA_NAME.
> 
> 
> 
>> +
>> +  /* If arg0 is an SSA_NAME and the stmt which defines arg0 is
>> +     a CONVERT_STMT, use the LHS as new_arg0.  */
>> +  if (TREE_CODE (arg0) == SSA_NAME)
>> +    {
>> +      def0 = SSA_NAME_DEF_STMT (arg0);
>> +      if (!is_gimple_assign (def0)
>> +      || !CONVERT_EXPR_CODE_P (gimple_assign_rhs_code (def0)))
>> +    return false;
>> +      new_arg0 = gimple_assign_rhs1 (def0);
>> +    }
> Use gimple_assign_cast_p rather than checking CONVERT_EXPR_CODE_P
> directly, so something like:
> 
>   /* Now see if ARG0 was defined by a typecast.  */
>   gimple arg0_def = SSA_NAME_DEF_STMT (arg0);
> 
>   if (!is_gimple_assign (arg0_def) || !gimple_assign_cast_p (arg0_def))
>     return false;
> 
> Similarly for arg1 when it's an SSA_NAME.
> 
> 
>> +
>> +  /* If types of new_arg0 and new_arg1 are different bailout.  */
>> +  if (TREE_TYPE (new_arg0) != TREE_TYPE (new_arg1))
>> +    return false;
> Do we want to restrict this to just integral types?    I haven't though
> about it too deeply, so perhaps not.
> 
>> +
>> +  /* Replace the PHI stmt with the new_arg0 and new_arg1.  Also insert
>> +     a new CONVERT_STMT that converts the phi results.  */
>> +  gsi = gsi_after_labels (gimple_bb (phi));
>> +  result = PHI_RESULT (phi);
>> +  temp = make_ssa_name (TREE_TYPE (new_arg0), phi);
>> +
>> +  if (dump_file && (dump_flags & TDF_DETAILS))
>> +    {
>> +      fprintf (dump_file, "PHI ");
>> +      print_generic_expr (dump_file, gimple_phi_result (phi), 0);
>> +      fprintf (dump_file,
>> +           " changed to factor CONVERT_EXPR out from COND_EXPR.\n");
>> +      fprintf (dump_file, "New PHI_RESULT is ");
>> +      print_generic_expr (dump_file, temp, 0);
>> +      fprintf (dump_file, " and new stmt with CONVERT_EXPR defines ");
>> +      print_generic_expr (dump_file, result, 0);
>> +      fprintf (dump_file, ".\n");
>> +    }
>> +
>> +  gimple_phi_set_result (phi, temp);
>> +  SET_PHI_ARG_DEF (phi, e0->dest_idx, new_arg0);
>> +  SET_PHI_ARG_DEF (phi, e1->dest_idx, new_arg1);
>> +  new_stmt = gimple_build_assign (result, CONVERT_EXPR, temp);
>> +  gsi_insert_before (&gsi, new_stmt, GSI_SAME_STMT);
>> +  return true;
> So I think you want to also remove the old cast(s) so that the minmax
> optimization can apply without having to wait for another pass of phi-opt.
> 
> To safely remove the old cast(s) you have to verify the result of the
> cast has a single use (which is obviously in the PHI).  Otherwise your
> transformation will have introduced a runtime redundancy.
> 
> I also suspect it's better to create a new PHI rather than modify the
> original PHI.  The original PHI should be removed and the result re-used
> as the result of the new convert statement.
> 
> Extra points if you can easily make this transformation apply to a
> generic unary operator.  So for example, we might have a sinkable bit-not.
> 
> Overall it's heading the right direction.  But I think it needs another
> iteration.


Thanks for the review. I have addressed your comments above in the
attached patch.

I have one question with respect to unary operation. For generic unary
operation with INTEGER_CST, do we skip this or do we have to perform the
inverse operation so that the conversion after PHI will restore it? Is
there any easy way to do this safely?

Bootstrapped and regression tested the attached patch on
x86-64-none-linux-gnu with no new regressions.

Thanks,
Kugan

Comments

Jeff Law July 7, 2015, 2:41 p.m. UTC | #1 
On 07/07/2015 06:50 AM, Kugan wrote:
>
> Thanks for the review. I have addressed your comments above in the
> attached patch.
>
> I have one question with respect to unary operation. For generic unary
> operation with INTEGER_CST, do we skip this or do we have to perform the
> inverse operation so that the conversion after PHI will restore it? Is
> there any easy way to do this safely?
I think we'd have to invert the operation -- some of which are trivial, 
such as BIT_NOT_EXPR.

NEGATE_EXPR is trivial once you filter out the cases where inversion 
will create signed overflow (ie INT_MIN and like when arg1 is an 
INTEGER_CST).

Similarly ABS_EXPR is trivial once you filter out cases where arg1 is a 
negative INTEGER_CST.

If you want to try and handle those cases, I'm certainly comfortable 
with that as a follow-up.  Obviously we'll want to testcases for them, 
including the cases where we don't want to make the transformation for 
NEGATE_EXPR and ABS_EXPR.

There may be other special cases we need to handle for other unary 
operations.  I haven't walked through the full list.

>
> Bootstrapped and regression tested the attached patch on
> x86-64-none-linux-gnu with no new regressions.
>
> Thanks,
> Kugan
>
>
> p.txt
>
>
> diff --git a/gcc/tree-ssa-phiopt.c b/gcc/tree-ssa-phiopt.c
> index 92b4ab0..1d6de9b 100644
> --- a/gcc/tree-ssa-phiopt.c
> +++ b/gcc/tree-ssa-phiopt.c
> @@ -73,6 +73,7 @@ along with GCC; see the file COPYING3.  If not see
>   static unsigned int tree_ssa_phiopt_worker (bool, bool);
>   static bool conditional_replacement (basic_block, basic_block,
>   				     edge, edge, gphi *, tree, tree);
> +static bool factor_out_conditional_conversion (edge, edge, gphi *, tree, tree);
>   static int value_replacement (basic_block, basic_block,
>   			      edge, edge, gimple, tree, tree);
>   static bool minmax_replacement (basic_block, basic_block,
> @@ -335,6 +336,17 @@ tree_ssa_phiopt_worker (bool do_store_elim, bool do_hoist_loads)
>   	     node.  */
>   	  gcc_assert (arg0 != NULL && arg1 != NULL);
>
> +	  if (factor_out_conditional_conversion (e1, e2, phi, arg0, arg1))
> +	    {
> +	      /* Update arg0 and arg1.  */
> +	      phis = phi_nodes (bb2);
> +	      phi = single_non_singleton_phi_for_edges (phis, e1, e2);
> +	      gcc_assert (phi);
> +	      arg0 = gimple_phi_arg_def (phi, e1->dest_idx);
> +	      arg1 = gimple_phi_arg_def (phi, e2->dest_idx);
> +	      gcc_assert (arg0 != NULL && arg1 != NULL);
> +	    }
> +
So small comment before this block of code indicating why we're 
recomputing these values.  Something like this perhaps:

/* factor_out_conditional_conversion may create a new PHI in BB2 and
    eliminate an existing PHI in BB2.  Recompute values that may be
    affected by that change.  */


Or something along those lines.


>   	  /* Do the replacement of conditional if it can be done.  */
>   	  if (conditional_replacement (bb, bb1, e1, e2, phi, arg0, arg1))
>   	    cfgchanged = true;
> @@ -410,6 +422,133 @@ replace_phi_edge_with_variable (basic_block cond_block,
>   	      bb->index);
>   }
>
> +/* PR66726: Factor conversion out of COND_EXPR.  If the arguments of the PHI
> +   stmt are CONVERT_STMT, factor out the conversion and perform the conversion
> +   to the result of PHI stmt.  */
> +
> +static bool
> +factor_out_conditional_conversion (edge e0, edge e1, gphi *phi,
> +				   tree arg0, tree arg1)
> +{
> +  gimple arg0_def_stmt = NULL, arg1_def_stmt = NULL, new_stmt;
> +  tree new_arg0 = NULL_TREE, new_arg1 = NULL_TREE;
> +  tree temp, result;
> +  gphi *newphi;
> +  gimple_stmt_iterator gsi, gsi_for_def;
> +  source_location locus = gimple_location (phi);
> +  enum tree_code convert_code;
> +
> +  /* Handle only PHI statements with two arguments.  TODO: If all
> +     other arguments to PHI are INTEGER_CST, we can handle more
> +     than two arguments too.  */
> +  if (gimple_phi_num_args (phi) != 2)
> +    return false;
Similarly we can handle more than one SSA_NAME if their defining 
statement all have the same unary operation.  Might as well add that to 
the comment too.  While handling > 2 arguments is certainly possible, I 
really wonder how often those cases occur in practice.

> +
> +  /* If arg0 is an SSA_NAME and the stmt which defines arg0 is
> +     a conversion.  */
> +  arg0_def_stmt = SSA_NAME_DEF_STMT (arg0);
> +  if (!is_gimple_assign (arg0_def_stmt)
> +      || (!gimple_assign_cast_p (arg0_def_stmt)
> +	  && (get_gimple_rhs_class (gimple_assign_rhs_code (arg0_def_stmt))
> +	      != GIMPLE_UNARY_RHS)))
So what happens if arg0_def_stmt is a GIMPLE_UNARY_RHS other than a 
NOP_EXPR or CONVERT_EXPR?    I'd probably punt anything that is not a 
gimple_assign_cast_p for the first iteration and follow-up with handling 
of the other unary operations as a follow-up.



> +    return false;
> +
> +  /* Use the LHS as new_arg0.  */
s/LHS/RHS/

> +  convert_code = gimple_assign_rhs_code (arg0_def_stmt);
> +  new_arg0 = gimple_assign_rhs1 (arg0_def_stmt);
> +  if (convert_code == VIEW_CONVERT_EXPR)
> +    new_arg0 = TREE_OPERAND (new_arg0, 0);
Is this really right for VIEW_CONVERT_EXPR?  Also, do we do the right 
thing for FIX_TRUNC_EXPR?


> +
> +  /* If arg1 is an SSA_NAME and the stmt which defines arg0 is
> +     a conversion.  */
s/arg0/arg1/

It's also the case that if arg1 is an SSA_NAME, then it must do the same 
operation as we found in arg0's defining statement.  I see that's 
properly tested in the code, so just a minor comment updated needed.

> +
> +  /* Create a new PHI stmt.  */
> +  result = PHI_RESULT (phi);
> +  temp = make_ssa_name (TREE_TYPE (new_arg0), NULL);
> +  newphi = create_phi_node (temp, gimple_bb (phi));
> +
> +  if (dump_file && (dump_flags & TDF_DETAILS))
> +    {
> +      fprintf (dump_file, "PHI ");
> +      print_generic_expr (dump_file, gimple_phi_result (phi), 0);
> +      fprintf (dump_file,
> +	       " changed to factor conversion out from COND_EXPR.\n");
> +      fprintf (dump_file, "New stmt with CAST that defines ");
> +      print_generic_expr (dump_file, result, 0);
> +      fprintf (dump_file, ".\n");
> +    }
> +
> +  /* Remove the old cast(s) that has single use.  */
> +  if (arg0_def_stmt && has_single_use (arg0))
> +    {
> +      gsi_for_def = gsi_for_stmt (arg0_def_stmt);
> +      gsi_remove (&gsi_for_def, true);
> +    }
> +
> +  if (arg1_def_stmt && has_single_use (arg1))
> +    {
> +      gsi_for_def = gsi_for_stmt (arg1_def_stmt);
> +      gsi_remove (&gsi_for_def, true);
> +    }
So I would move the have_single_use tests up higher and reject the 
transformation if arg0/arg1 have > 1 use.  The reason is if arg0/arg1 
have > 1 use, then this transformation actually increases the number of 
expressions evaluated at runtime.

It'd probably be good to include a test when arg0 or arg1 are both 
SSA_NAMEs and new_arg0 and new_arg1 have > 1 use to verify the 
transformation does not apply in that case.

Very close.     I suspect one more iteration.

jeff
diff mbox

Patch

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr66726.c b/gcc/testsuite/gcc.dg/tree-ssa/pr66726.c
index e69de29..a4c7418 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/pr66726.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr66726.c
@@ -0,0 +1,15 @@ 
+
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-phiopt1-details" } */
+
+extern unsigned short mode_size[];
+
+int
+oof (int mode)
+{
+  return (64 < mode_size[mode] ? 64 : mode_size[mode]);
+}
+
+/* { dg-final { scan-tree-dump-times "factor conversion out" 1 "phiopt1" } } */
+/* { dg-final { scan-tree-dump-times "MIN_EXPR" 1 "phiopt1" } } */
+
diff --git a/gcc/tree-ssa-phiopt.c b/gcc/tree-ssa-phiopt.c
index 92b4ab0..1d6de9b 100644
--- a/gcc/tree-ssa-phiopt.c
+++ b/gcc/tree-ssa-phiopt.c
@@ -73,6 +73,7 @@  along with GCC; see the file COPYING3.  If not see
 static unsigned int tree_ssa_phiopt_worker (bool, bool);
 static bool conditional_replacement (basic_block, basic_block,
 				     edge, edge, gphi *, tree, tree);
+static bool factor_out_conditional_conversion (edge, edge, gphi *, tree, tree);
 static int value_replacement (basic_block, basic_block,
 			      edge, edge, gimple, tree, tree);
 static bool minmax_replacement (basic_block, basic_block,
@@ -335,6 +336,17 @@  tree_ssa_phiopt_worker (bool do_store_elim, bool do_hoist_loads)
 	     node.  */
 	  gcc_assert (arg0 != NULL && arg1 != NULL);
 
+	  if (factor_out_conditional_conversion (e1, e2, phi, arg0, arg1))
+	    {
+	      /* Update arg0 and arg1.  */
+	      phis = phi_nodes (bb2);
+	      phi = single_non_singleton_phi_for_edges (phis, e1, e2);
+	      gcc_assert (phi);
+	      arg0 = gimple_phi_arg_def (phi, e1->dest_idx);
+	      arg1 = gimple_phi_arg_def (phi, e2->dest_idx);
+	      gcc_assert (arg0 != NULL && arg1 != NULL);
+	    }
+
 	  /* Do the replacement of conditional if it can be done.  */
 	  if (conditional_replacement (bb, bb1, e1, e2, phi, arg0, arg1))
 	    cfgchanged = true;
@@ -410,6 +422,133 @@  replace_phi_edge_with_variable (basic_block cond_block,
 	      bb->index);
 }
 
+/* PR66726: Factor conversion out of COND_EXPR.  If the arguments of the PHI
+   stmt are CONVERT_STMT, factor out the conversion and perform the conversion
+   to the result of PHI stmt.  */
+
+static bool
+factor_out_conditional_conversion (edge e0, edge e1, gphi *phi,
+				   tree arg0, tree arg1)
+{
+  gimple arg0_def_stmt = NULL, arg1_def_stmt = NULL, new_stmt;
+  tree new_arg0 = NULL_TREE, new_arg1 = NULL_TREE;
+  tree temp, result;
+  gphi *newphi;
+  gimple_stmt_iterator gsi, gsi_for_def;
+  source_location locus = gimple_location (phi);
+  enum tree_code convert_code;
+
+  /* Handle only PHI statements with two arguments.  TODO: If all
+     other arguments to PHI are INTEGER_CST, we can handle more
+     than two arguments too.  */
+  if (gimple_phi_num_args (phi) != 2)
+    return false;
+
+  /* First canonicalize to simplify tests.  */
+  if (TREE_CODE (arg0) != SSA_NAME)
+    {
+      std::swap (arg0, arg1);
+      std::swap (e0, e1);
+    }
+
+  if (TREE_CODE (arg0) != SSA_NAME
+      || (TREE_CODE (arg1) != SSA_NAME
+	  && TREE_CODE (arg1) != INTEGER_CST))
+    return false;
+
+  /* If arg0 is an SSA_NAME and the stmt which defines arg0 is
+     a conversion.  */
+  arg0_def_stmt = SSA_NAME_DEF_STMT (arg0);
+  if (!is_gimple_assign (arg0_def_stmt)
+      || (!gimple_assign_cast_p (arg0_def_stmt)
+	  && (get_gimple_rhs_class (gimple_assign_rhs_code (arg0_def_stmt))
+	      != GIMPLE_UNARY_RHS)))
+    return false;
+
+  /* Use the LHS as new_arg0.  */
+  convert_code = gimple_assign_rhs_code (arg0_def_stmt);
+  new_arg0 = gimple_assign_rhs1 (arg0_def_stmt);
+  if (convert_code == VIEW_CONVERT_EXPR)
+    new_arg0 = TREE_OPERAND (new_arg0, 0);
+
+  /* If arg1 is an SSA_NAME and the stmt which defines arg0 is
+     a conversion.  */
+  if (TREE_CODE (arg1) == SSA_NAME)
+    {
+      arg1_def_stmt = SSA_NAME_DEF_STMT (arg1);
+      if (!is_gimple_assign (arg1_def_stmt)
+	  || gimple_assign_rhs_code (arg1_def_stmt) != convert_code)
+	return false;
+
+      /* Use the LHS as new_arg1.  */
+      new_arg1 = gimple_assign_rhs1 (arg1_def_stmt);
+      if (gimple_assign_rhs_code (arg1_def_stmt) == VIEW_CONVERT_EXPR)
+	new_arg1 = TREE_OPERAND (new_arg1, 0);
+    }
+  else
+    {
+      /* If arg1 is an INTEGER_CST, fold it to new type.  */
+      if (INTEGRAL_TYPE_P (TREE_TYPE (new_arg0))
+	  && int_fits_type_p (arg1, TREE_TYPE (new_arg0)))
+	{
+	  if (gimple_assign_cast_p (arg0_def_stmt))
+	    new_arg1 = fold_convert (TREE_TYPE (new_arg0), arg1);
+	  else
+	    return false;
+	}
+      else
+	return false;
+    }
+
+  /* If types of new_arg0 and new_arg1 are different bailout.  */
+  if (TREE_TYPE (new_arg0) != TREE_TYPE (new_arg1))
+    return false;
+
+  /* Create a new PHI stmt.  */
+  result = PHI_RESULT (phi);
+  temp = make_ssa_name (TREE_TYPE (new_arg0), NULL);
+  newphi = create_phi_node (temp, gimple_bb (phi));
+
+  if (dump_file && (dump_flags & TDF_DETAILS))
+    {
+      fprintf (dump_file, "PHI ");
+      print_generic_expr (dump_file, gimple_phi_result (phi), 0);
+      fprintf (dump_file,
+	       " changed to factor conversion out from COND_EXPR.\n");
+      fprintf (dump_file, "New stmt with CAST that defines ");
+      print_generic_expr (dump_file, result, 0);
+      fprintf (dump_file, ".\n");
+    }
+
+  /* Remove the old cast(s) that has single use.  */
+  if (arg0_def_stmt && has_single_use (arg0))
+    {
+      gsi_for_def = gsi_for_stmt (arg0_def_stmt);
+      gsi_remove (&gsi_for_def, true);
+    }
+
+  if (arg1_def_stmt && has_single_use (arg1))
+    {
+      gsi_for_def = gsi_for_stmt (arg1_def_stmt);
+      gsi_remove (&gsi_for_def, true);
+    }
+
+  add_phi_arg (newphi, new_arg0, e0, locus);
+  add_phi_arg (newphi, new_arg1, e1, locus);
+
+  /* Create the conversion stmt and insert it.  */
+  if (convert_code == VIEW_CONVERT_EXPR)
+    temp = fold_build1 (VIEW_CONVERT_EXPR, TREE_TYPE (result), temp);
+  new_stmt = gimple_build_assign (result,  convert_code, temp);
+  gsi = gsi_after_labels (gimple_bb (phi));
+  gsi_insert_before (&gsi, new_stmt, GSI_SAME_STMT);
+
+  /* Remove he original PHI stmt.  */
+  gsi = gsi_for_stmt (phi);
+  gsi_remove (&gsi, true);
+  return true;
+}
+
 /*  The function conditional_replacement does the main work of doing the
     conditional replacement.  Return true if the replacement is done.
     Otherwise return false.
@@ -2142,6 +2281,26 @@  gate_hoist_loads (void)
    This pass also performs a fifth transformation of a slightly different
    flavor.
 
+   Factor conversion in COND_EXPR
+   ------------------------------
+
+   This transformation factors the conversion out of COND_EXPR with
+   factor_out_conditional_conversion.
+
+   For example:
+   if (a <= CST) goto <bb 3>; else goto <bb 4>;
+   <bb 3>:
+   tmp = (int) a;
+   <bb 4>:
+   tmp = PHI <tmp, CST>
+
+   Into:
+   if (a <= CST) goto <bb 3>; else goto <bb 4>;
+   <bb 3>:
+   <bb 4>:
+   a = PHI <a, CST>
+   tmp = (int) a;
+
    Adjacent Load Hoisting
    ----------------------