c++: Avoid some false positive -Wfloat-conversion warnings with extended precision [PR108285]

  Hi!

On the following testcase trunk emits a false positive warning on ia32.
convert_like_internal is there called with type of double and
expr EXCESS_PRECISION_EXPR with float type with long double operand
2.L * (long double) x.
Now, for the code generation we do the right thing, cp_convert
to double from that 2.L * (long double) x, but we call even
cp_convert_and_check with that and that emits the -Wfloat-conversion
warning.  Looking at what the C FE does in this case, it calls
convert_and_check with the EXCESS_PRECISION_EXPR expression rather
than its operand, and essentially uses the operand for code generation
and EXCESS_PRECISION_EXPR itself for warnings.

The following patch does that too for the C++ FE.

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

2023-01-11  Jakub Jelinek  <jakub@redhat.com>

	PR c++/108285
	* cvt.cc (cp_convert_and_check): For EXCESS_PRECISION_EXPR
	use its operand except that for warning purposes use the original
	EXCESS_PRECISION_EXPR.
	* call.cc (convert_like_internal): Only look through
	EXCESS_PRECISION_EXPR when calling cp_convert, not when calling
	cp_convert_and_check.

	* g++.dg/warn/pr108285.C: New test.


	Jakub
  

On 1/11/23 04:52, Jakub Jelinek wrote:
> Hi!
> 
> On the following testcase trunk emits a false positive warning on ia32.
> convert_like_internal is there called with type of double and
> expr EXCESS_PRECISION_EXPR with float type with long double operand
> 2.L * (long double) x.
> Now, for the code generation we do the right thing, cp_convert
> to double from that 2.L * (long double) x, but we call even
> cp_convert_and_check with that and that emits the -Wfloat-conversion
> warning.  Looking at what the C FE does in this case, it calls
> convert_and_check with the EXCESS_PRECISION_EXPR expression rather
> than its operand, and essentially uses the operand for code generation
> and EXCESS_PRECISION_EXPR itself for warnings.
> 
> The following patch does that too for the C++ FE.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.

> 2023-01-11  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/108285
> 	* cvt.cc (cp_convert_and_check): For EXCESS_PRECISION_EXPR
> 	use its operand except that for warning purposes use the original
> 	EXCESS_PRECISION_EXPR.
> 	* call.cc (convert_like_internal): Only look through
> 	EXCESS_PRECISION_EXPR when calling cp_convert, not when calling
> 	cp_convert_and_check.
> 
> 	* g++.dg/warn/pr108285.C: New test.
> 
> --- gcc/cp/cvt.cc.jj	2022-10-14 09:32:32.403797521 +0200
> +++ gcc/cp/cvt.cc	2023-01-10 13:53:00.639130717 +0100
> @@ -652,8 +652,10 @@ cp_convert (tree type, tree expr, tsubst
>   tree
>   cp_convert_and_check (tree type, tree expr, tsubst_flags_t complain)
>   {
> -  tree result;
> +  tree result, expr_for_warning = expr;
>   
> +  if (TREE_CODE (expr) == EXCESS_PRECISION_EXPR)
> +    expr = TREE_OPERAND (expr, 0);
>     if (TREE_TYPE (expr) == type)
>       return expr;
>     if (expr == error_mark_node)
> @@ -663,7 +665,7 @@ cp_convert_and_check (tree type, tree ex
>     if ((complain & tf_warning)
>         && c_inhibit_evaluation_warnings == 0)
>       {
> -      tree folded = cp_fully_fold (expr);
> +      tree folded = cp_fully_fold (expr_for_warning);
>         tree folded_result;
>         if (folded == expr)
>   	folded_result = result;
> --- gcc/cp/call.cc.jj	2023-01-09 23:41:11.135159084 +0100
> +++ gcc/cp/call.cc	2023-01-10 13:50:09.277640628 +0100
> @@ -8863,12 +8863,14 @@ convert_like_internal (conversion *convs
>       return error_mark_node;
>   
>     warning_sentinel w (warn_zero_as_null_pointer_constant);
> -  if (TREE_CODE (expr) == EXCESS_PRECISION_EXPR)
> -    expr = TREE_OPERAND (expr, 0);
>     if (issue_conversion_warnings)
>       expr = cp_convert_and_check (totype, expr, complain);
>     else
> -    expr = cp_convert (totype, expr, complain);
> +    {
> +      if (TREE_CODE (expr) == EXCESS_PRECISION_EXPR)
> +	expr = TREE_OPERAND (expr, 0);
> +      expr = cp_convert (totype, expr, complain);
> +    }
>   
>     return expr;
>   }
> --- gcc/testsuite/g++.dg/warn/pr108285.C.jj	2023-01-10 16:52:06.115345345 +0100
> +++ gcc/testsuite/g++.dg/warn/pr108285.C	2023-01-10 16:39:26.646532929 +0100
> @@ -0,0 +1,11 @@
> +// PR c++/108285
> +// { dg-do compile }
> +// { dg-options "-fexcess-precision=standard -Wfloat-conversion" }
> +
> +void bar (double);
> +
> +void
> +foo (float x)
> +{
> +  bar (2 * x);	// { dg-bogus "conversion from '\[^\n\r]\*' to 'double' may change value" }
> +}
> 
> 	Jakub
>