c++, c: Implement C++23 P1774R8 - Portable assumptions [PR106654]

The following patch implements C++23 P1774R8 - Portable assumptions
paper, by introducing support for [[assume (cond)]]; attribute for C++.
In addition to that the patch adds [[gnu::assume (cond)]]; and
__attribute__((assume (cond))); support to both C and C++.
As described in C++23, the attribute argument is conditional-expression
rather than the usual assignment-expression for attribute arguments,
the condition is contextually converted to bool (for C truthvalue conversion
is done on it) and is never evaluated at runtime.
For C++ constant expression evaluation, I only check the simplest conditions
for undefined behavior, because otherwise I'd need to undo changes to
*ctx->global which happened during the evaluation (but I believe the spec
allows that and we can further improve later).
The patch uses a new internal function, .ASSUME, to hold the condition
in the FEs.  At gimplification time, if the condition is simple/without
side-effects, it is gimplified as if (cond) ; else __builtin_unreachable ();
and otherwise for now dropped on the floor.  The intent is to incrementally
outline the conditions into separate artificial functions and use
.ASSUME further to tell the ranger and perhaps other optimization passes
about the assumptions, as detailed in the PR.

When implementing it, I found that assume entry hasn't been added to
https://eel.is/c++draft/cpp.cond#6
Jonathan said he'll file a NB comment about it, this patch assumes it
has been added into the table as 202207L when the paper has been voted in.

With the attributes for both C/C++, I'd say we don't need to add
__builtin_assume with similar purpose, especially when __builtin_assume
in LLVM is just weird.  It is strange for side-effects in function call's
argument not to be evaluated, and LLVM in that case (annoyingly) warns
and ignores the side-effects (but doesn't do then anything with it),
if there are no side-effects, it will work like our
if (!cond) __builtin_unreachable ();

2022-10-06  Jakub Jelinek  <jakub@redhat.com>

	PR c++/106654
gcc/
	* internal-fn.def (ASSUME): New internal function.
	* internal-fn.h (expand_ASSUME): Declare.
	* internal-fn.cc (expand_ASSUME): Define.
	* gimplify.cc (gimplify_call_expr): Gimplify IFN_ASSUME.
	* fold-const.h (simple_condition_p): Declare.
	* fold-const.cc (simple_operand_p_2): Rename to ...
	(simple_condition_p): ... this.  Remove forward declaration.
	No longer static.  Adjust function comment and fix a typo in it.
	Adjust recursive call.
	(simple_operand_p): Adjust function comment.
	(fold_truth_andor): Adjust simple_operand_p_2 callers to call
	simple_condition_p.
	* doc/extend.texi: Document assume attribute.  Move fallthrough
	attribute example to its section.
gcc/c-family/
	* c-attribs.cc (handle_assume_attribute): New function.
	(c_common_attribute_table): Add entry for assume attribute.
	* c-lex.cc (c_common_has_attribute): Handle
	__have_cpp_attribute (assume).
gcc/c/
	* c-parser.cc (handle_assume_attribute): New function.
	(c_parser_declaration_or_fndef): Handle assume attribute.
	(c_parser_attribute_arguments): Add assume_attr argument,
	if true, parse first argument as conditional expression.
	(c_parser_gnu_attribute, c_parser_std_attribute): Adjust
	c_parser_attribute_arguments callers.
	(c_parser_statement_after_labels) <case RID_ATTRIBUTE>: Handle
	assume attribute.
gcc/cp/
	* cp-tree.h (process_stmt_assume_attribute): Implement C++23
	P1774R8 - Portable assumptions.  Declare.
	(diagnose_failing_condition): Declare.
	(find_failing_clause): Likewise.
	* parser.cc (assume_attr): New enumerator.
	(cp_parser_parenthesized_expression_list): Handle assume_attr.
	Remove identifier variable, for id_attr push the identifier into
	expression_list right away instead of inserting it before all the
	others at the end.
	(cp_parser_conditional_expression): New function.
	(cp_parser_constant_expression): Use it.
	(cp_parser_statement): Handle assume attribute.
	(cp_parser_expression_statement): Likewise.
	(cp_parser_gnu_attribute_list): Use assume_attr for assume
	attribute.
	(cp_parser_std_attribute): Likewise.  Handle standard assume
	attribute like gnu::assume.
	* cp-gimplify.cc (process_stmt_assume_attribute): New function.
	* constexpr.cc: Include fold-const.h.
	(find_failing_clause_r, find_failing_clause): New functions,
	moved from semantics.cc with ctx argument added and if non-NULL,
	call cxx_eval_constant_expression rather than fold_non_dependent_expr.
	(cxx_eval_internal_function): Handle IFN_ASSUME.
	(potential_constant_expression_1): Likewise.
	* pt.cc (tsubst_copy_and_build): Likewise.
	* semantics.cc (diagnose_failing_condition): New function.
	(find_failing_clause_r, find_failing_clause): Moved to constexpr.cc.
	(finish_static_assert): Use it.  Add auto_diagnostic_group.
gcc/testsuite/
	* gcc.dg/attr-assume-1.c: New test.
	* gcc.dg/attr-assume-2.c: New test.
	* gcc.dg/attr-assume-3.c: New test.
	* g++.dg/cpp2a/feat-cxx2a.C: Add colon to C++20 features
	comment, add C++20 attributes comment and move C++20
	new features after the attributes before them.
	* g++.dg/cpp23/feat-cxx2b.C: Likewise.  Test
	__has_cpp_attribute(assume).
	* g++.dg/cpp23/attr-assume1.C: New test.
	* g++.dg/cpp23/attr-assume2.C: New test.
	* g++.dg/cpp23/attr-assume3.C: New test.
	* g++.dg/cpp23/attr-assume4.C: New test.

1 files changed, 36 insertions, 13 deletions

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index a5afb46..9ddfcf7 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -9187,6 +9187,20 @@ available for functions (@pxref{Function Attributes}), variables
 (@pxref{Variable Attributes}), labels (@pxref{Label Attributes}), enumerators
 (@pxref{Enumerator Attributes}), and for types (@pxref{Type Attributes}).
 
+@table @code
+@item fallthrough
+@cindex @code{fallthrough} statement attribute
+The @code{fallthrough} attribute with a null statement serves as a
+fallthrough statement.  It hints to the compiler that a statement
+that falls through to another case label, or user-defined label
+in a switch statement is intentional and thus the
+@option{-Wimplicit-fallthrough} warning must not trigger.  The
+fallthrough attribute may appear at most once in each attribute
+list, and may not be mixed with other attributes.  It can only
+be used in a switch statement (the compiler will issue an error
+otherwise), after a preceding statement and before a logically
+succeeding case label, or user-defined label.
+
 This example uses the @code{fallthrough} statement attribute to indicate that
 the @option{-Wimplicit-fallthrough} warning should not be emitted:
 
@@ -9201,19 +9215,28 @@ switch (cond)
   @}
 @end smallexample
 
-@table @code
-@item fallthrough
-@cindex @code{fallthrough} statement attribute
-The @code{fallthrough} attribute with a null statement serves as a
-fallthrough statement.  It hints to the compiler that a statement
-that falls through to another case label, or user-defined label
-in a switch statement is intentional and thus the
-@option{-Wimplicit-fallthrough} warning must not trigger.  The
-fallthrough attribute may appear at most once in each attribute
-list, and may not be mixed with other attributes.  It can only
-be used in a switch statement (the compiler will issue an error
-otherwise), after a preceding statement and before a logically
-succeeding case label, or user-defined label.
+@item assume
+@cindex @code{assume} statement attribute
+The @code{assume} attribute with a null statement serves as portable
+assumption.  It should have a single argument, a conditional expression,
+which is not evaluated.  If the argument would evaluate to true
+at the point where it appears, it has no effect, otherwise there
+is undefined behavior.  This is a GNU variant of the ISO C++23
+standard @code{assume} attribute, but it can be used in any version of
+both C and C++.
+
+@smallexample
+int
+foo (int x, int y)
+@{
+  __attribute__((assume(x == 42)));
+  __attribute__((assume(++y == 43)));
+  return x + y;
+@}
+@end smallexample
+
+@code{y} is not actually incremented and the compiler can but does not
+have to optimize it to just @code{return 42 + 42;}.
 
 @end table