c++: decl_constant_value and unsharing [PR96197]

c++: decl_constant_value and unsharing [PR96197]

In the testcase from the PR we're seeing excessive memory use (> 5GB)
during constexpr evaluation, almost all of which is due to the call to
decl_constant_value in the VAR_DECL/CONST_DECL branch of
cxx_eval_constant_expression.  We reach here every time we evaluate an
ARRAY_REF of a constexpr VAR_DECL, and from there decl_constant_value
makes an unshared copy of the VAR_DECL's initializer.  But unsharing
here is unnecessary because callers of cxx_eval_constant_expression
already unshare its result when necessary.

To fix this excessive unsharing, this patch adds a new defaulted
parameter unshare_p to decl_really_constant_value and
decl_constant_value so that callers can control whether to unshare.

As a simplification, we can also move the call to unshare_expr in
constant_value_1 outside of the loop, since doing unshare_expr on a
DECL_P is a no-op.

Now that we no longer unshare the result of decl_constant_value and
decl_really_constant_value from cxx_eval_constant_expression, memory use
during constexpr evaluation for the testcase from the PR falls from ~5GB
to 15MB according to -ftime-report.

gcc/cp/ChangeLog:

	PR c++/96197
	* constexpr.c (cxx_eval_constant_expression) <case CONST_DECL>:
	Pass false to decl_constant_value and decl_really_constant_value
	so that they don't unshare their result.
	* cp-tree.h (decl_constant_value): New declaration with an added
	bool parameter.
	(decl_really_constant_value): Add bool parameter defaulting to
	true to existing declaration.
	* init.c (constant_value_1): Add bool parameter which controls
	whether to unshare the initializer before returning.  Call
	unshare_expr at most once.
	(scalar_constant_value): Pass true to constant_value_1's new
	bool parameter.
	(decl_really_constant_value): Add bool parameter and forward it
	to constant_value_1.
	(decl_constant_value): Likewise, but instead define a new
	overload with an added bool parameter.

gcc/testsuite/ChangeLog:

	PR c++/96197
	* g++.dg/cpp1y/constexpr-array8.C: New test.