On Wed, Dec 1, 2021 at 3:10 AM Jason Merrill via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> For PR61825, honza changed tree_single_nonzero_warnv_p to prevent a later
> declaration from marking a function as weak after we've determined that it
> wasn't weak before. But we shouldn't do that for speculative folding; we
> should only do it when we actually need a constant value. In C++, such a
> context is called "manifestly constant-evaluated". In fold, this seems to
> correspond to the folding_initializer flag, since in C this situation only
> occurs in static initializers.
>
> This change makes nonzero-1.c well-formed; I've added a nonzero-1a.c to
> verify that we delete the null check eventually if there is no weak
> redeclaration.
>
> The varasm.c change is so that if we do get the weak redeclaration error, we
> get it at the position of the weak declaration rather than the previous
> declaration.
>
> Using the FOLD_INIT paths also affects floating point arithmetic: notably,
> this makes floating point division by zero in a manifestly
> constant-evaluated context constant, as in a C static initializer. I've had
> some success convincing CWG that this is the right direction; C++ should
> follow C's floating point semantics more than we have been doing, and Joseph
> says that the C policy is that Annex F overrides other parts of the standard
> that say that some operations are undefined. But since we're in stage 3,
> I'm only making this change with the new flag -fconstexpr-fp-except. It may
> turn on by default in a future release.
>
> I think this distinction is only relevant for binary operations; arithmetic
> for the floating point case, comparison for possibly non-zero addresses.
>
> Tested x86_64-pc-linux-gnu, OK for trunk?
OK.
Thanks,
Richard.
> PR c++/103310
>
> gcc/ChangeLog:
>
> * fold-const.c (maybe_nonzero_address): Use get_create or get
> depending on folding_initializer.
> (fold_binary_initializer_loc): New.
> * fold-const.h (fold_binary_initializer_loc): Declare.
> * varasm.c (mark_weak): Don't use the decl location.
> * doc/invoke.texi: Document -fconstexpr-fp-except.
>
> gcc/c-family/ChangeLog:
>
> * c.opt: Add -fconstexpr-fp-except.
>
> gcc/cp/ChangeLog:
>
> * constexpr.c (cxx_eval_binary_expression): Use
> fold_binary_initializer_loc if manifestly cxeval.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/constexpr-fp-except1.C: New test.
> * g++.dg/cpp1z/constexpr-if36.C: New test.
> * gcc.dg/tree-ssa/nonzero-1.c: Now well-formed.
> * gcc.dg/tree-ssa/nonzero-1a.c: New test.
> ---
> gcc/doc/invoke.texi | 14 ++++++++++
> gcc/c-family/c.opt | 4 +++
> gcc/fold-const.h | 1 +
> gcc/cp/constexpr.c | 9 ++++++-
> gcc/fold-const.c | 26 ++++++++++++++++---
> .../g++.dg/cpp0x/constexpr-fp-except1.C | 4 +++
> gcc/testsuite/g++.dg/cpp1z/constexpr-if36.C | 19 ++++++++++++++
> gcc/testsuite/gcc.dg/tree-ssa/nonzero-1.c | 5 ++--
> gcc/testsuite/gcc.dg/tree-ssa/nonzero-1a.c | 11 ++++++++
> gcc/varasm.c | 2 +-
> 10 files changed, 88 insertions(+), 7 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-fp-except1.C
> create mode 100644 gcc/testsuite/g++.dg/cpp1z/constexpr-if36.C
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/nonzero-1a.c
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 3bddfbaae6a..d6858d834f9 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -3035,6 +3035,20 @@ users are likely to want to adjust it, but if your
> code does heavy
> constexpr calculations you might want to experiment to find which
> value works best for you.
>
> +@item -fconstexpr-fp-except
> +@opindex fconstexpr-fp-except
> +Annex F of the C standard specifies that IEC559 floating point
> +exceptions encountered at compile time should not stop compilation.
> +C++ compilers have historically not followed this guidance, instead
> +treating floating point division by zero as non-constant even though
> +it has a well defined value. This flag tells the compiler to give
> +Annex F priority over other rules saying that a particular operation
> +is undefined.
> +
> +@smallexample
> +constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except
> +@end smallexample
> +
> @item -fconstexpr-loop-limit=@var{n}
> @opindex fconstexpr-loop-limit
> Set the maximum number of iterations for a loop in C++14 constexpr functions
> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> index 4b8a094b206..5654f044ae4 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -1615,6 +1615,10 @@ fconstexpr-cache-depth=
> C++ ObjC++ Joined RejectNegative UInteger Var(constexpr_cache_depth) Init(8)
> -fconstexpr-cache-depth=<number> Specify maximum constexpr recursion
> cache depth.
>
> +fconstexpr-fp-except
> +C++ ObjC++ Var(flag_constexpr_fp_except) Init(0)
> +Allow IEC559 floating point exceptions in constant expressions
> +
> fconstexpr-loop-limit=
> C++ ObjC++ Joined RejectNegative UInteger Var(constexpr_loop_limit)
> Init(262144)
> -fconstexpr-loop-limit=<number> Specify maximum constexpr loop
> iteration count.
> diff --git a/gcc/fold-const.h b/gcc/fold-const.h
> index 56e9d399c0d..01bfe5df9b5 100644
> --- a/gcc/fold-const.h
> +++ b/gcc/fold-const.h
> @@ -77,6 +77,7 @@ extern tree fold_build_call_array_loc (location_t, tree,
> tree, int, tree *);
> #define fold_build_call_array_initializer(T1,T2,N,T4)\
> fold_build_call_array_initializer_loc (UNKNOWN_LOCATION, T1, T2, N, T4)
> extern tree fold_build_call_array_initializer_loc (location_t, tree, tree,
> int, tree *);
> +extern tree fold_binary_initializer_loc (location_t, tree_code, tree, tree,
> tree);
> extern tree get_array_ctor_element_at_index (tree, offset_int,
> unsigned * = NULL);
> extern bool fold_convertible_p (const_tree, const_tree);
> diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
> index d66a56583ae..b4b8a96c6af 100644
> --- a/gcc/cp/constexpr.c
> +++ b/gcc/cp/constexpr.c
> @@ -3366,7 +3366,14 @@ cxx_eval_binary_expression (const constexpr_ctx *ctx,
> tree t,
> }
>
> if (r == NULL_TREE)
> - r = fold_binary_loc (loc, code, type, lhs, rhs);
> + {
> + if (ctx->manifestly_const_eval
> + && (flag_constexpr_fp_except
> + || TREE_CODE (type) != REAL_TYPE))
> + r = fold_binary_initializer_loc (loc, code, type, lhs, rhs);
> + else
> + r = fold_binary_loc (loc, code, type, lhs, rhs);
> + }
>
> if (r == NULL_TREE
> && (code == LSHIFT_EXPR || code == RSHIFT_EXPR)
> diff --git a/gcc/fold-const.c b/gcc/fold-const.c
> index 90d82257ae7..0b9a42f764a 100644
> --- a/gcc/fold-const.c
> +++ b/gcc/fold-const.c
> @@ -85,8 +85,8 @@ along with GCC; see the file COPYING3. If not see
> #include "asan.h"
> #include "gimple-range.h"
>
> -/* Nonzero if we are folding constants inside an initializer; zero
> - otherwise. */
> +/* Nonzero if we are folding constants inside an initializer or a C++
> + manifestly-constant-evaluated context; zero otherwise. */
> int folding_initializer = 0;
>
> /* The following constants represent a bit based encoding of GCC's
> @@ -9924,8 +9924,15 @@ pointer_may_wrap_p (tree base, tree offset, poly_int64
> bitpos)
> static int
> maybe_nonzero_address (tree decl)
> {
> + /* Normally, don't do anything for variables and functions before symtab is
> + built; it is quite possible that DECL will be declared weak later.
> + But if folding_initializer, we need a constant answer now, so create
> + the symtab entry and prevent later weak declaration. */
> if (DECL_P (decl) && decl_in_symtab_p (decl))
> - if (struct symtab_node *symbol = symtab_node::get_create (decl))
> + if (struct symtab_node *symbol
> + = (folding_initializer
> + ? symtab_node::get_create (decl)
> + : symtab_node::get (decl)))
> return symbol->nonzero_address ();
>
> /* Function local objects are never NULL. */
> @@ -13991,6 +13998,19 @@ fold_build_call_array_initializer_loc (location_t
> loc, tree type, tree fn,
> return result;
> }
>
> +tree
> +fold_binary_initializer_loc (location_t loc, tree_code code, tree type,
> + tree lhs, tree rhs)
> +{
> + tree result;
> + START_FOLD_INIT;
> +
> + result = fold_binary_loc (loc, code, type, lhs, rhs);
> +
> + END_FOLD_INIT;
> + return result;
> +}
> +
> #undef START_FOLD_INIT
> #undef END_FOLD_INIT
>
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-fp-except1.C
> b/gcc/testsuite/g++.dg/cpp0x/constexpr-fp-except1.C
> new file mode 100644
> index 00000000000..3887a5ba743
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-fp-except1.C
> @@ -0,0 +1,4 @@
> +// { dg-do compile { target c++11 } }
> +// { dg-additional-options -fconstexpr-fp-except }
> +
> +constexpr double inf = 1./0.;
> diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-if36.C
> b/gcc/testsuite/g++.dg/cpp1z/constexpr-if36.C
> new file mode 100644
> index 00000000000..4a1b134cc19
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if36.C
> @@ -0,0 +1,19 @@
> +// PR c++/103310
> +// Test that only manifestly-constant-evaluated comparisons lock a symbol's
> +// weakness.
> +
> +// { dg-do compile { target c++17 } }
> +
> +extern void weakfn1 (void);
> +extern void weakfn2 (void);
> +
> +void call_weakfn (void)
> +{
> + if (weakfn1)
> + weakfn1 ();
> + if constexpr (weakfn2)
> + weakfn2 ();
> +}
> +
> +extern void weakfn1 (void) __attribute__((weak));
> +extern void weakfn2 (void) __attribute__((weak)); // { dg-error "declared
> weak after being used" }
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/nonzero-1.c
> b/gcc/testsuite/gcc.dg/tree-ssa/nonzero-1.c
> index c9d438e9374..aa21b71c730 100644
> --- a/gcc/testsuite/gcc.dg/tree-ssa/nonzero-1.c
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/nonzero-1.c
> @@ -1,12 +1,13 @@
> /* { dg-do compile } */
> -/* { dg-options "-O2 -fdelete-null-pointer-checks" } */
> +/* { dg-options "-O2 -fdelete-null-pointer-checks -fdump-tree-optimized" } */
> /* { dg-require-weak "" } */
>
> /* { dg-skip-if "" keeps_null_pointer_checks } */
> -extern int a; /* { dg-error "declared weak after being used" } */
> +extern int a;
> int
> t()
> {
> + /* { dg-final { scan-tree-dump "&a != 0" "optimized" } } */
> return &a!=0;
> }
> extern int a __attribute__ ((weak));
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/nonzero-1a.c
> b/gcc/testsuite/gcc.dg/tree-ssa/nonzero-1a.c
> new file mode 100644
> index 00000000000..3d1eb97f151
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/nonzero-1a.c
> @@ -0,0 +1,11 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdelete-null-pointer-checks -fdump-tree-optimized" } */
> +
> +/* { dg-skip-if "" keeps_null_pointer_checks } */
> +extern int a;
> +int
> +t()
> +{
> + /* { dg-final { scan-tree-dump-not "&a != 0" "optimized" } } */
> + return &a!=0;
> +}
> diff --git a/gcc/varasm.c b/gcc/varasm.c
> index d6031d6f1a2..9315e2c6936 100644
> --- a/gcc/varasm.c
> +++ b/gcc/varasm.c
> @@ -5869,7 +5869,7 @@ mark_weak (tree decl)
>
> struct symtab_node *n = symtab_node::get (decl);
> if (n && n->refuse_visibility_changes)
> - error ("%+qD declared weak after being used", decl);
> + error ("%qD declared weak after being used", decl);
> DECL_WEAK (decl) = 1;
>
> if (DECL_RTL_SET_P (decl)
>
> base-commit: a3e75c1491cd2d501081210925a89a65b1c1e5e5
> --
> 2.27.0
>
