c++: Avoid some false positive -Wfloat-conversion warnings with extended precision [PR108285]
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Commit Message
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
Comments
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
>
@@ -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;
@@ -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;
}
@@ -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" }
+}