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c++: Only reject reinterpret casts from pointers to integers for manifestly_const_eval evaluation [PR99456]

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My PR82304/PR95307 fix moved reinterpret cast from pointer to integer
diagnostics from cxx_eval_outermost_constant_expr where it caught
invalid code only at the outermost level down into
cxx_eval_constant_expression.
Unfortunately, it regressed following testcase, we emit worse code
including dynamic initialization of some vars.
While the initializers are not constant expressions due to the
reinterpret_cast in there, there is no reason not to fold them as an
optimization.

I've tried to make this dependent on !ctx->quiet, but that regressed
two further tests, and on ctx->strict, which regressed other tests,
so this patch bases that on manifestly_const_eval.

The new testcase is now optimized as much as it used to be in GCC 10
and the only regression it causes is an extra -Wnarrowing warning
on vla22.C test on invalid code (which the patch adjusts).

2021-03-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99456
	* constexpr.c (cxx_eval_constant_expression): For CONVERT_EXPR from
	INDIRECT_TYPE_P to ARITHMETIC_TYPE_P, when !ctx->manifestly_const_eval
	don't diagnose it, set *non_constant_p nor return t.

	* g++.dg/opt/pr99456.C: New test.
	* g++.dg/ext/vla22.C: Expect a -Wnarrowing warning for c++11 and
	later.
This commit is contained in:
Jakub Jelinek 2021-03-19 18:36:56 +01:00
parent 02f305440f
commit 82bb66730b
3 changed files with 36 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
gcc/cp/constexpr.c
View File

@ -6656,7 +6656,8 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
if (TREE_CODE (t) == CONVERT_EXPR
&& ARITHMETIC_TYPE_P (type)
&& INDIRECT_TYPE_P (TREE_TYPE (op)))
&& INDIRECT_TYPE_P (TREE_TYPE (op))
&& ctx->manifestly_const_eval)
{
if (!ctx->quiet)
error_at (loc,

2
gcc/testsuite/g++.dg/ext/vla22.C
View File

@ -6,4 +6,4 @@ void
f ()
{
const int tbl[(long) "h"] = { 12 }; // { dg-error "size of array .tbl. is not an integral constant-expression" }
}
} // { dg-warning "narrowing conversion" "" { target c++11 } .-1 }

33
gcc/testsuite/g++.dg/opt/pr99456.C Normal file
View File

@ -0,0 +1,33 @@
// PR c++/99456
// { dg-do compile { target c++17 } }
// { dg-options "-g0" }
// { dg-final { scan-assembler-not "PR99456Var0\[1234]" } }
// { dg-final { scan-assembler-not "__static_initialization_and_destruction" } }
// { dg-final { scan-assembler-not "_GLOBAL__sub_I" } }
// { dg-final { scan-assembler-not "_ZGV12PR99456Var1\[1234]" } }
typedef __UINTPTR_TYPE__ uintptr_t;
class Container
{
public:
uintptr_t m;
};
extern unsigned desc;
static constexpr unsigned &descRef = desc;
inline Container PR99456Var01 {reinterpret_cast<uintptr_t> (&descRef)};
inline Container PR99456Var02 {reinterpret_cast<uintptr_t> (&desc)};
inline uintptr_t PR99456Var03 {reinterpret_cast<uintptr_t> (&descRef)};
inline uintptr_t PR99456Var04 {reinterpret_cast<uintptr_t> (&desc)};
inline Container PR99456Var11 {reinterpret_cast<uintptr_t> (&descRef)};
inline Container PR99456Var12 {reinterpret_cast<uintptr_t> (&desc)};
inline uintptr_t PR99456Var13 {reinterpret_cast<uintptr_t> (&descRef)};
inline uintptr_t PR99456Var14 {reinterpret_cast<uintptr_t> (&desc)};
auto *PR99456Ref11 = &PR99456Var11;
auto *PR99456Ref12 = &PR99456Var12;
auto *PR99456Ref13 = &PR99456Var13;
auto *PR99456Ref14 = &PR99456Var14;