fold-const: Avoid infinite recursion in +-*&|^minmax reassociation [PR114084]

  Hi!

In the following testcase we infinitely recurse during BIT_IOR_EXPR
reassociation.
One operand is (unsigned _BitInt(31)) a << 4 and another operand
2147483647 >> 1 | 80 where both the right shift and the | 80
trees have TREE_CONSTANT set, but weren't folded because of delayed
folding, where some foldings are apparently done even in that case
unfortunately.
Now, the fold_binary_loc reassocation code splits both operands into
variable part, minus variable part, constant part, minus constant part,
literal part and minus literal parts, to prevent infinite recursion
punts if there are just 2 parts altogether from the 2 operands and then goes
on with reassociation, merges first the corresponding parts from both
operands and then some further merges.
The problem with the above expressions is that we get 3 different objects,
var0 (the left shift), con1 (the right shift) and lit1 (80), so the infinite
recursion prevention doesn't trigger, and we eventually merge con1 with
lit1, which effectively reconstructs the original op1 and then associate
that with var0 which is original op0, and associate_trees for that case
calls fold_binary.  There are some casts involved there too (the T typedef
type and the underlying _BitInt type which are stripped with STRIP_NOPS).

The following patch attempts to prevent this infinite recursion by tracking
the origin (if certain var comes from nothing - 0, op0 - 1, op1 - 2 or both - 3)
and propagates it through all the associate_tree calls which merge the vars.
If near the end we'd try to merge what comes solely from op0 with what comes
solely from op1 (or vice versa), the patch punts, because then it isn't any
kind of reassociation between the two operands, if anything it should be
handled when folding the suboperands.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2024-02-26  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/114084
	* fold-const.cc (fold_binary_loc): Avoid the final associate_trees
	if all subtrees of var0 come from one of the op0 or op1 operands
	and all subtrees of con0 come from the other one.  Don't clear
	variables which are never used afterwards.

	* gcc.dg/bitint-94.c: New test.


	Jakub
  

On Mon, 26 Feb 2024, Jakub Jelinek wrote:

> Hi!
> 
> In the following testcase we infinitely recurse during BIT_IOR_EXPR
> reassociation.
> One operand is (unsigned _BitInt(31)) a << 4 and another operand
> 2147483647 >> 1 | 80 where both the right shift and the | 80
> trees have TREE_CONSTANT set, but weren't folded because of delayed
> folding, where some foldings are apparently done even in that case
> unfortunately.
> Now, the fold_binary_loc reassocation code splits both operands into
> variable part, minus variable part, constant part, minus constant part,
> literal part and minus literal parts, to prevent infinite recursion
> punts if there are just 2 parts altogether from the 2 operands and then goes
> on with reassociation, merges first the corresponding parts from both
> operands and then some further merges.
> The problem with the above expressions is that we get 3 different objects,
> var0 (the left shift), con1 (the right shift) and lit1 (80), so the infinite
> recursion prevention doesn't trigger, and we eventually merge con1 with
> lit1, which effectively reconstructs the original op1 and then associate
> that with var0 which is original op0, and associate_trees for that case
> calls fold_binary.  There are some casts involved there too (the T typedef
> type and the underlying _BitInt type which are stripped with STRIP_NOPS).
> 
> The following patch attempts to prevent this infinite recursion by tracking
> the origin (if certain var comes from nothing - 0, op0 - 1, op1 - 2 or both - 3)
> and propagates it through all the associate_tree calls which merge the vars.
> If near the end we'd try to merge what comes solely from op0 with what comes
> solely from op1 (or vice versa), the patch punts, because then it isn't any
> kind of reassociation between the two operands, if anything it should be
> handled when folding the suboperands.

That sounds like a nice thing to do.

> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.

Thanks,
Richard.

> 2024-02-26  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR middle-end/114084
> 	* fold-const.cc (fold_binary_loc): Avoid the final associate_trees
> 	if all subtrees of var0 come from one of the op0 or op1 operands
> 	and all subtrees of con0 come from the other one.  Don't clear
> 	variables which are never used afterwards.
> 
> 	* gcc.dg/bitint-94.c: New test.
> 
> --- gcc/fold-const.cc.jj	2024-02-24 09:49:09.098815803 +0100
> +++ gcc/fold-const.cc	2024-02-24 11:08:56.036223491 +0100
> @@ -11779,6 +11779,15 @@ fold_binary_loc (location_t loc, enum tr
>  		  + (lit0 != 0) + (lit1 != 0)
>  		  + (minus_lit0 != 0) + (minus_lit1 != 0)) > 2)
>  	    {
> +	      int var0_origin = (var0 != 0) + 2 * (var1 != 0);
> +	      int minus_var0_origin
> +		= (minus_var0 != 0) + 2 * (minus_var1 != 0);
> +	      int con0_origin = (con0 != 0) + 2 * (con1 != 0);
> +	      int minus_con0_origin
> +		= (minus_con0 != 0) + 2 * (minus_con1 != 0);
> +	      int lit0_origin = (lit0 != 0) + 2 * (lit1 != 0);
> +	      int minus_lit0_origin
> +		= (minus_lit0 != 0) + 2 * (minus_lit1 != 0);
>  	      var0 = associate_trees (loc, var0, var1, code, atype);
>  	      minus_var0 = associate_trees (loc, minus_var0, minus_var1,
>  					    code, atype);
> @@ -11791,15 +11800,19 @@ fold_binary_loc (location_t loc, enum tr
>  
>  	      if (minus_var0 && var0)
>  		{
> +		  var0_origin |= minus_var0_origin;
>  		  var0 = associate_trees (loc, var0, minus_var0,
>  					  MINUS_EXPR, atype);
>  		  minus_var0 = 0;
> +		  minus_var0_origin = 0;
>  		}
>  	      if (minus_con0 && con0)
>  		{
> +		  con0_origin |= minus_con0_origin;
>  		  con0 = associate_trees (loc, con0, minus_con0,
>  					  MINUS_EXPR, atype);
>  		  minus_con0 = 0;
> +		  minus_con0_origin = 0;
>  		}
>  
>  	      /* Preserve the MINUS_EXPR if the negative part of the literal is
> @@ -11815,15 +11828,19 @@ fold_binary_loc (location_t loc, enum tr
>  		      /* But avoid ending up with only negated parts.  */
>  		      && (var0 || con0))
>  		    {
> +		      minus_lit0_origin |= lit0_origin;
>  		      minus_lit0 = associate_trees (loc, minus_lit0, lit0,
>  						    MINUS_EXPR, atype);
>  		      lit0 = 0;
> +		      lit0_origin = 0;
>  		    }
>  		  else
>  		    {
> +		      lit0_origin |= minus_lit0_origin;
>  		      lit0 = associate_trees (loc, lit0, minus_lit0,
>  					      MINUS_EXPR, atype);
>  		      minus_lit0 = 0;
> +		      minus_lit0_origin = 0;
>  		    }
>  		}
>  
> @@ -11833,37 +11850,51 @@ fold_binary_loc (location_t loc, enum tr
>  		return NULL_TREE;
>  
>  	      /* Eliminate lit0 and minus_lit0 to con0 and minus_con0. */
> +	      con0_origin |= lit0_origin;
>  	      con0 = associate_trees (loc, con0, lit0, code, atype);
> -	      lit0 = 0;
> +	      minus_con0_origin |= minus_lit0_origin;
>  	      minus_con0 = associate_trees (loc, minus_con0, minus_lit0,
>  					    code, atype);
> -	      minus_lit0 = 0;
>  
>  	      /* Eliminate minus_con0.  */
>  	      if (minus_con0)
>  		{
>  		  if (con0)
> -		    con0 = associate_trees (loc, con0, minus_con0,
> -					    MINUS_EXPR, atype);
> +		    {
> +		      con0_origin |= minus_con0_origin;
> +		      con0 = associate_trees (loc, con0, minus_con0,
> +					      MINUS_EXPR, atype);
> +		    }
>  		  else if (var0)
> -		    var0 = associate_trees (loc, var0, minus_con0,
> -					    MINUS_EXPR, atype);
> +		    {
> +		      var0_origin |= minus_con0_origin;
> +		      var0 = associate_trees (loc, var0, minus_con0,
> +					      MINUS_EXPR, atype);
> +		    }
>  		  else
>  		    gcc_unreachable ();
> -		  minus_con0 = 0;
>  		}
>  
>  	      /* Eliminate minus_var0.  */
>  	      if (minus_var0)
>  		{
>  		  if (con0)
> -		    con0 = associate_trees (loc, con0, minus_var0,
> -					    MINUS_EXPR, atype);
> +		    {
> +		      con0_origin |= minus_var0_origin;
> +		      con0 = associate_trees (loc, con0, minus_var0,
> +					      MINUS_EXPR, atype);
> +		    }
>  		  else
>  		    gcc_unreachable ();
> -		  minus_var0 = 0;
>  		}
>  
> +	      /* Reassociate only if there has been any actual association
> +		 between subtrees from op0 and subtrees from op1 in at
> +		 least one of the operands, otherwise we risk infinite
> +		 recursion.  See PR114084.  */
> +	      if (var0_origin != 3 && con0_origin != 3)
> +		return NULL_TREE;
> +
>  	      return
>  		fold_convert_loc (loc, type, associate_trees (loc, var0, con0,
>  							      code, atype));
> --- gcc/testsuite/gcc.dg/bitint-94.c.jj	2024-02-24 11:18:32.607018363 +0100
> +++ gcc/testsuite/gcc.dg/bitint-94.c	2024-02-24 11:19:09.023500121 +0100
> @@ -0,0 +1,12 @@
> +/* PR middle-end/114084 */
> +/* { dg-do compile { target bitint } } */
> +/* { dg-options "-std=c23 -pedantic-errors" } */
> +
> +typedef unsigned _BitInt(31) T;
> +T a, b;
> +
> +void
> +foo (void)
> +{
> +  b = (T) ((a | (-1U >> 1)) >> 1 | (a | 5) << 4);
> +}
> 
> 	Jakub
> 
>