| Message ID | 20210105152624.GR725145@tucnak |
|---|---|
| State | New |
| Headers | show |
| Series | c++, v2: Fix ICE with __builtin_bit_cast [PR98469] | expand |
On 1/5/21 10:26 AM, Jakub Jelinek wrote: > On Mon, Jan 04, 2021 at 04:01:25PM -0500, Jason Merrill via Gcc-patches wrote: >> On 1/4/21 3:48 PM, Jakub Jelinek wrote: >>> On Mon, Jan 04, 2021 at 03:44:46PM -0500, Jason Merrill wrote: >>>> This change is OK, but part of the problem is that we're trying to do >>>> overload resolution for an S copy/move constructor, which we shouldn't be >>>> because bit_cast is a prvalue, so in C++17 and up we should use it to >>>> directly initialize the target without any implied constructor call. >>>> >>>> It seems we're mishandling this because the code in >>>> build_special_member_call specifically looks for TARGET_EXPR or CONSTRUCTOR, >>>> and BIT_CAST_EXPR is neither of those. >>>> >>>> Wrapping a BIT_CAST_EXPR of aggregate type in a TARGET_EXPR would address >>>> this, and any other places that expect a class prvalue to come in the form >>>> of a TARGET_EXPR. >>> >>> I can try that tomorrow. Won't that cause copying through extra temporary >>> in some cases though, or is that guaranteed to be optimized? >> >> It won't cause any extra copying when it's used to initialize another object >> (like the return value of std::bit_cast). Class prvalues are always >> expressed with a TARGET_EXPR in the front end; the TARGET_EXPR melts away >> when used as an initializer, it only creates a temporary when it's used in >> another way. > > Ok, this version wraps it into a TARGET_EXPR then, it alone fixes the bug, > but I've kept the constexpr.c change too. > > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? OK. > 2021-01-05 Jakub Jelinek <jakub@redhat.com> > > PR c++/98469 > * constexpr.c (cxx_eval_constant_expression) <case BIT_CAST_EXPR>: > Punt if lval is true. > * semantics.c (cp_build_bit_cast): Call get_target_expr_sfinae on > the result if it has a class type. > > * g++.dg/cpp2a/bit-cast8.C: New test. > * g++.dg/cpp2a/bit-cast9.C: New test. > > --- gcc/cp/constexpr.c.jj 2021-01-04 10:25:48.750121531 +0100 > +++ gcc/cp/constexpr.c 2021-01-05 11:41:38.315032636 +0100 > @@ -6900,6 +6900,15 @@ cxx_eval_constant_expression (const cons > return t; > > case BIT_CAST_EXPR: > + if (lval) > + { > + if (!ctx->quiet) > + error_at (EXPR_LOCATION (t), > + "address of a call to %qs is not a constant expression", > + "__builtin_bit_cast"); > + *non_constant_p = true; > + return t; > + } > r = cxx_eval_bit_cast (ctx, t, non_constant_p, overflow_p); > break; > > --- gcc/cp/semantics.c.jj 2021-01-04 10:25:48.489124486 +0100 > +++ gcc/cp/semantics.c 2021-01-05 11:27:49.327372582 +0100 > @@ -10761,6 +10761,10 @@ cp_build_bit_cast (location_t loc, tree > > tree ret = build_min (BIT_CAST_EXPR, type, arg); > SET_EXPR_LOCATION (ret, loc); > + > + if (!processing_template_decl && CLASS_TYPE_P (type)) > + ret = get_target_expr_sfinae (ret, complain); > + > return ret; > } > > --- gcc/testsuite/g++.dg/cpp2a/bit-cast8.C.jj 2021-01-05 11:41:38.315032636 +0100 > +++ gcc/testsuite/g++.dg/cpp2a/bit-cast8.C 2021-01-05 11:41:38.315032636 +0100 > @@ -0,0 +1,11 @@ > +// PR c++/98469 > +// { dg-do compile { target c++20 } } > +// { dg-options "-Wall" } > + > +struct S { int s; }; > + > +S > +foo () > +{ > + return __builtin_bit_cast (S, 0); > +} > --- gcc/testsuite/g++.dg/cpp2a/bit-cast9.C.jj 2021-01-05 11:41:38.315032636 +0100 > +++ gcc/testsuite/g++.dg/cpp2a/bit-cast9.C 2021-01-05 11:41:38.315032636 +0100 > @@ -0,0 +1,15 @@ > +// PR c++/98469 > +// { dg-do compile { target c++20 } } > +// { dg-options "-Wall" } > + > +template<typename T, typename F> > +constexpr T > +bit_cast (const F &f) noexcept > +{ > + return __builtin_bit_cast (T, f); > +} > +struct S { int s; }; > +constexpr int foo (const S &x) { return x.s; } > +constexpr int bar () { return foo (bit_cast<S> (0)); } > +constexpr int x = bar (); > +static_assert (!x); > > > Jakub >
On 1/5/21 10:26 AM, Jakub Jelinek wrote: > On Mon, Jan 04, 2021 at 04:01:25PM -0500, Jason Merrill via Gcc-patches wrote: >> On 1/4/21 3:48 PM, Jakub Jelinek wrote: >>> On Mon, Jan 04, 2021 at 03:44:46PM -0500, Jason Merrill wrote: >>>> This change is OK, but part of the problem is that we're trying to do >>>> overload resolution for an S copy/move constructor, which we shouldn't be >>>> because bit_cast is a prvalue, so in C++17 and up we should use it to >>>> directly initialize the target without any implied constructor call. >>>> >>>> It seems we're mishandling this because the code in >>>> build_special_member_call specifically looks for TARGET_EXPR or CONSTRUCTOR, >>>> and BIT_CAST_EXPR is neither of those. >>>> >>>> Wrapping a BIT_CAST_EXPR of aggregate type in a TARGET_EXPR would address >>>> this, and any other places that expect a class prvalue to come in the form >>>> of a TARGET_EXPR. >>> >>> I can try that tomorrow. Won't that cause copying through extra temporary >>> in some cases though, or is that guaranteed to be optimized? >> >> It won't cause any extra copying when it's used to initialize another object >> (like the return value of std::bit_cast). Class prvalues are always >> expressed with a TARGET_EXPR in the front end; the TARGET_EXPR melts away >> when used as an initializer, it only creates a temporary when it's used in >> another way. > > Ok, this version wraps it into a TARGET_EXPR then, it alone fixes the bug, > but I've kept the constexpr.c change too. This patch corrects this and one other place to not be as dependent on TARGET_EXPR, but I think I'm going to save it for stage 1. Jason
--- gcc/cp/constexpr.c.jj 2021-01-04 10:25:48.750121531 +0100 +++ gcc/cp/constexpr.c 2021-01-05 11:41:38.315032636 +0100 @@ -6900,6 +6900,15 @@ cxx_eval_constant_expression (const cons return t; case BIT_CAST_EXPR: + if (lval) + { + if (!ctx->quiet) + error_at (EXPR_LOCATION (t), + "address of a call to %qs is not a constant expression", + "__builtin_bit_cast"); + *non_constant_p = true; + return t; + } r = cxx_eval_bit_cast (ctx, t, non_constant_p, overflow_p); break; --- gcc/cp/semantics.c.jj 2021-01-04 10:25:48.489124486 +0100 +++ gcc/cp/semantics.c 2021-01-05 11:27:49.327372582 +0100 @@ -10761,6 +10761,10 @@ cp_build_bit_cast (location_t loc, tree tree ret = build_min (BIT_CAST_EXPR, type, arg); SET_EXPR_LOCATION (ret, loc); + + if (!processing_template_decl && CLASS_TYPE_P (type)) + ret = get_target_expr_sfinae (ret, complain); + return ret; } --- gcc/testsuite/g++.dg/cpp2a/bit-cast8.C.jj 2021-01-05 11:41:38.315032636 +0100 +++ gcc/testsuite/g++.dg/cpp2a/bit-cast8.C 2021-01-05 11:41:38.315032636 +0100 @@ -0,0 +1,11 @@ +// PR c++/98469 +// { dg-do compile { target c++20 } } +// { dg-options "-Wall" } + +struct S { int s; }; + +S +foo () +{ + return __builtin_bit_cast (S, 0); +} --- gcc/testsuite/g++.dg/cpp2a/bit-cast9.C.jj 2021-01-05 11:41:38.315032636 +0100 +++ gcc/testsuite/g++.dg/cpp2a/bit-cast9.C 2021-01-05 11:41:38.315032636 +0100 @@ -0,0 +1,15 @@ +// PR c++/98469 +// { dg-do compile { target c++20 } } +// { dg-options "-Wall" } + +template<typename T, typename F> +constexpr T +bit_cast (const F &f) noexcept +{ + return __builtin_bit_cast (T, f); +} +struct S { int s; }; +constexpr int foo (const S &x) { return x.s; } +constexpr int bar () { return foo (bit_cast<S> (0)); } +constexpr int x = bar (); +static_assert (!x);