| Message ID | 20160310183957.GB3017@tucnak.redhat.com |
|---|---|
| State | New |
| Headers | show |
On Thu, Mar 10, 2016 at 07:39:57PM +0100, Jakub Jelinek wrote: > Is this also ok, if it passes bootstrap/regtest? > > 2016-03-10 Jakub Jelinek <jakub@redhat.com> > > PR c++/70001 > * constexpr.c (cxx_eval_vec_init_1): Reuse CONSTRUCTOR initializers > for 1..max even for multi-dimensional arrays, and reuse not just > eltinit itself, but surrounding subobject CONSTRUCTOR too. > > * g++.dg/cpp0x/constexpr-70001-4.C: New test. Successfully bootstrapped/regtested on x86_64-linux and i686-linux. Jakub
On 03/10/2016 01:39 PM, Jakub Jelinek wrote: > + /* Don't reuse the result of cxx_eval_constant_expression > + call if it isn't a constant initializer or if it requires > + relocations. */ Let's phrase this positively ("Reuse the result if..."). > + if (new_ctx.ctor != ctx->ctor) > + eltinit = new_ctx.ctor; > + for (i = 1; i < max; ++i) > + { > + idx = build_int_cst (size_type_node, i); > + CONSTRUCTOR_APPEND_ELT (*p, idx, eltinit); > + } This is going to use the same CONSTRUCTOR for all the elements, which will lead to problems if we then store into a subobject of one of the elements and see that reflected in all the others as well. We need to unshare_expr when reusing the initializer. Jason
On Fri, Mar 11, 2016 at 09:27:54AM -0500, Jason Merrill wrote: > On 03/10/2016 01:39 PM, Jakub Jelinek wrote: > >+ /* Don't reuse the result of cxx_eval_constant_expression > >+ call if it isn't a constant initializer or if it requires > >+ relocations. */ > > Let's phrase this positively ("Reuse the result if..."). > > >+ if (new_ctx.ctor != ctx->ctor) > >+ eltinit = new_ctx.ctor; > >+ for (i = 1; i < max; ++i) > >+ { > >+ idx = build_int_cst (size_type_node, i); > >+ CONSTRUCTOR_APPEND_ELT (*p, idx, eltinit); > >+ } > > This is going to use the same CONSTRUCTOR for all the elements, which will > lead to problems if we then store into a subobject of one of the elements > and see that reflected in all the others as well. We need to unshare_expr > when reusing the initializer. Well, but then even what has been already committed is unsafe, initializer_constant_valid_p can return null_pointer_node even on CONSTRUCTOR, or CONSTRUCTOR holding CONSTRUCTORs etc. So, either we need to unshare_expr it in every case, so CONSTRUCTOR_APPEND_ELT (*p, idx, unshare_expr (eltinit)); or alternatively we could use some flag on CONSTRUCTOR to mark (possibly) shared ctors and unshare them upon constexpr store to them, or unshare whenever we store. What would be a testcase for the unsharing? Following still works: // PR c++/70001 // { dg-do compile { target c++14 } } struct B { int a; constexpr B () : a (0) { } constexpr B (int x) : a (x) { } }; struct C { B c; constexpr C () : c (0) { } }; struct A { B b[1 << 4]; }; struct D { C d[1 << 4]; }; constexpr int foo (int a, int b) { A c; c.b[a].a += b; c.b[b].a += a; return c.b[0].a + c.b[a].a + c.b[b].a; } constexpr int d = foo (1, 2); constexpr int e = foo (0, 3); constexpr int f = foo (2, 4); static_assert (d == 3 && e == 6 && f == 6, ""); constexpr int bar (int a, int b) { D c; c.d[a].c.a += b; c.d[b].c.a += a; return c.d[0].c.a + c.d[a].c.a + c.d[b].c.a; } constexpr int g = bar (1, 2); constexpr int h = bar (0, 3); constexpr int i = bar (2, 4); static_assert (g == 3 && h == 6 && i == 6, ""); Jakub
--- gcc/cp/constexpr.c.jj 2016-03-10 12:52:04.000000000 +0100 +++ gcc/cp/constexpr.c 2016-03-10 19:24:28.435537864 +0100 @@ -2340,7 +2340,6 @@ cxx_eval_vec_init_1 (const constexpr_ctx vec<constructor_elt, va_gc> **p = &CONSTRUCTOR_ELTS (ctx->ctor); vec_alloc (*p, max + 1); bool pre_init = false; - tree pre_init_elt = NULL_TREE; unsigned HOST_WIDE_INT i; /* For the default constructor, build up a call to the default @@ -2370,6 +2369,7 @@ cxx_eval_vec_init_1 (const constexpr_ctx { tree idx = build_int_cst (size_type_node, i); tree eltinit; + bool reuse = false; constexpr_ctx new_ctx; init_subob_ctx (ctx, new_ctx, idx, pre_init ? init : elttype); if (new_ctx.ctor != ctx->ctor) @@ -2378,7 +2378,10 @@ cxx_eval_vec_init_1 (const constexpr_ctx { /* A multidimensional array; recurse. */ if (value_init || init == NULL_TREE) - eltinit = NULL_TREE; + { + eltinit = NULL_TREE; + reuse = i == 0; + } else eltinit = cp_build_array_ref (input_location, init, idx, tf_warning_or_error); @@ -2390,18 +2393,9 @@ cxx_eval_vec_init_1 (const constexpr_ctx { /* Initializing an element using value or default initialization we just pre-built above. */ - if (pre_init_elt == NULL_TREE) - pre_init_elt - = cxx_eval_constant_expression (&new_ctx, init, lval, - non_constant_p, overflow_p); - eltinit = pre_init_elt; - /* Don't reuse the result of cxx_eval_constant_expression - call if it isn't a constant initializer or if it requires - relocations. */ - if (initializer_constant_valid_p (pre_init_elt, - TREE_TYPE (pre_init_elt)) - != null_pointer_node) - pre_init_elt = NULL_TREE; + eltinit = cxx_eval_constant_expression (&new_ctx, init, lval, + non_constant_p, overflow_p); + reuse = i == 0; } else { @@ -2427,6 +2421,23 @@ cxx_eval_vec_init_1 (const constexpr_ctx } else CONSTRUCTOR_APPEND_ELT (*p, idx, eltinit); + /* Don't reuse the result of cxx_eval_constant_expression + call if it isn't a constant initializer or if it requires + relocations. */ + if (reuse + && max > 1 + && (initializer_constant_valid_p (eltinit, TREE_TYPE (eltinit)) + == null_pointer_node)) + { + if (new_ctx.ctor != ctx->ctor) + eltinit = new_ctx.ctor; + for (i = 1; i < max; ++i) + { + idx = build_int_cst (size_type_node, i); + CONSTRUCTOR_APPEND_ELT (*p, idx, eltinit); + } + break; + } } if (!*non_constant_p) --- gcc/testsuite/g++.dg/cpp0x/constexpr-70001-4.C.jj 2016-03-10 19:28:13.386481311 +0100 +++ gcc/testsuite/g++.dg/cpp0x/constexpr-70001-4.C 2016-03-10 19:28:43.295074924 +0100 @@ -0,0 +1,13 @@ +// PR c++/70001 +// { dg-do compile { target c++11 } } + +struct B +{ + int a; + constexpr B () : a (0) { } +}; + +struct A +{ + B b[1 << 19][1][1][1]; +} c;
On Thu, Mar 10, 2016 at 01:32:10PM -0500, Patrick Palka wrote: > Looks fine to me :) On a closer look, this doesn't handle the multi-dimensional array cases, and even for single-dimensional ones will not share the CONSTRUCTOR if init_subob_ctx created one, and call init_subob_ctx many times and in there in some cases e.g. build new new_ctx.object every time etc. Is this also ok, if it passes bootstrap/regtest? 2016-03-10 Jakub Jelinek <jakub@redhat.com> PR c++/70001 * constexpr.c (cxx_eval_vec_init_1): Reuse CONSTRUCTOR initializers for 1..max even for multi-dimensional arrays, and reuse not just eltinit itself, but surrounding subobject CONSTRUCTOR too. * g++.dg/cpp0x/constexpr-70001-4.C: New test. Jakub