This code is miscompiled on aarch64 with -std=c++17, apparently due to the empty base class: struct base { }; struct pair : base { float first; float second; pair(float f, float s) : first(f), second(s) { } }; void f(pair); int main() { f({3.14, 666}); } It is wrong with any optimization level, but with -std=gnu++14 -O1 we get: main: stp x29, x30, [sp, -16]! mov x29, sp mov x0, 0 mov x1, 62915 movk x1, 0x4048, lsl 16 bfi x0, x1, 0, 32 mov x1, 32768 movk x1, 0x4426, lsl 16 bfi x0, x1, 32, 32 lsr w1, w0, 0 fmov d0, x0 ushr d1, d0, 32 fmov s0, w1 bl f(pair) mov w0, 0 ldp x29, x30, [sp], 16 ret With -std=gnu++17 -O1 we get: main: stp x29, x30, [sp, -16]! mov x29, sp mov x0, 0 mov x1, 62915 movk x1, 0x4048, lsl 16 bfi x0, x1, 0, 32 mov x1, 32768 movk x1, 0x4426, lsl 16 bfi x0, x1, 32, 32 bl f(pair) mov w0, 0 ldp x29, x30, [sp], 16 ret If the translation unit containing the called function is: struct base { }; struct pair : base { float first; float second; pair(float f, float s) : first(f), second(s) { } }; void f(pair lr) { __builtin_printf("%f %f\n", lr.first, lr.second); } and the caller is compiled with -std=gnu++14 and the callee is compiled with -std=gnu++17 then the callee prints garabage: -13874.335938 0.000000
I haven't actually tested this with gcc-10 yet but I can't find an existing bug, so I assume it's also still present on master.
https://godbolt.org/z/BnTEsn is an example with both functions in the same translation unit, showing the generated code is different for both caller and callee. If the caller and callee are not in the same TU the differences produce garbage.
Is the difference maybe related to the empty field that is added for c++17 mode, mentioned in Bug 89358 comment 12? Is the aarch64 back end not ignoring that field?
That would be consistent with the new field being introduced in gcc-7 (by r241187).
Yep, bisected it to r241187.
Seems it is in aapcs_vfp_sub_candidate, which for C++17 recurses on the DECL_FIELD_IS_BASE FIELD_DECL and because 16039 /* There must be no padding. */ 16040 if (maybe_ne (wi::to_poly_wide (TYPE_SIZE (type)), 16041 count * GET_MODE_BITSIZE (*modep))) 16042 return -1; check fails, as the type is base, with no elts (count == 0) but TYPE_SIZE of BITS_PER_UNIT, it returns -1 and thus returns it for the whole pair class too. The question is what exactly the AAPCS says. And if we want to ignore the empty base fields, the question is how to exactly recognize them in the backend, because DECL_FIELD_IS_BASE is a FE macro, and I doubt one can use it e.g. in LTO. So, shall aapcs_vfp_sub_candidate ignore DECL_ARTIFICIAL FIELD_DECLs that have DECL_SIZE (field) && integer_zerop (DECL_SIZE (field)) ? Something else? What do other compilers do here?
--- gcc/config/aarch64/aarch64.c.jj 2020-03-18 12:51:41.051640609 +0100 +++ gcc/config/aarch64/aarch64.c 2020-03-30 16:28:29.133717645 +0200 @@ -16030,6 +16030,16 @@ aapcs_vfp_sub_candidate (const_tree type if (TREE_CODE (field) != FIELD_DECL) continue; + /* Ignore C++17 empty base fields, while their type indicates + they do contain padding, they have zero size and thus don't + contain any padding. */ + if (DECL_ARTIFICIAL (field) + && DECL_NAME (field) == NULL_TREE + && RECORD_OR_UNION_TYPE_P (TREE_TYPE (field)) + && DECL_SIZE (field) + && integer_zerop (DECL_SIZE (field))) + continue; + sub_count = aapcs_vfp_sub_candidate (TREE_TYPE (field), modep); if (sub_count < 0) return -1; fixes that. Though, guess there might need to be a -Wpsabi warning and that would likely need propagate to callers whether such field has been seen and if it has been seen, let the code retry in a mode where it wouldn't ignore those, compare the result and if it is different, warn. Will defer to aarch64 maintainers. And, guess it would be nice to test other targets for similar issues. Do we want e.g. to adjust struct-layout-1_generate.c, so that it only tests structures with such an empty base and just check value passing of such classes (both caller and callee at various spots in argument list) or returning, compile everything in C++14 and C++17 and compare assembly, verify that it fails on aarch64 and then look for others?
I'd like to ping this, it would be nice to at least decide if this should be handled for GCC10 or postponed to GCC11 only.
(In reply to Jakub Jelinek from comment #8) > I'd like to ping this, it would be nice to at least decide if this should be > handled for GCC10 or postponed to GCC11 only. Hi Jakub -- I'm taking a look at this at the moment, so I'm hoping I can get it done for GCC10. So far I've only double checked what the AAPCS64 says and confirmed that we're producing the correct code for gnu++14 and not gnu++17 (the base class is empty and a language type that occupies zero bytes has no mapping on the ABI level, hence that `pair` structure is an HFA and should be passed in the vector registers). I'm just about to start looking at changes & testing.
Thanks.
Created attachment 48317 [details] gcc10-pr94383.patch Testsuite coverage. This passes make -j32 -k check-c++-all RUNTESTFLAGS=struct-layout-1.exp on x86_64-linux, but it would be nice to know if it does FAIL on aarch64-linux and/or arm*-linux-gnueabi.
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>: https://gcc.gnu.org/g:619602346aed9dae3f338d9f18767414446adf78 commit r10-7850-g619602346aed9dae3f338d9f18767414446adf78 Author: Jakub Jelinek <jakub@redhat.com> Date: Tue Apr 21 17:08:10 2020 +0200 testsuite: Extend C++ struct-layout-1.exp testing to test C++14 vs. C++17 interoperability of structs with empty bases [PR94383] Jonathan reported an ABI incompatibility between C++14 and C++17 in passing some aggregates with empty bases on aarch64 (and apparently on arm too). The following patch adds 3000 (by default) tests for such interoperability, using the struct-layout-1* framework. The current 3000 tests are generated as is (so unchanged from previous ones), and afterwards there is another set of 3000 ones, where always one of the tNNN_x.C and tNNN_y.C tests get added -std=c++14 -DCXX14_VS_CXX17 and another one -std=c++17 -DCXX14_VS_CXX17 options (which one which is chosen pseudo-randomly), which causes the structs to have an empty base. I haven't added (yet) checks if the alternate compiler does support these options (I think that can be done incrementally), so for now this testing is done only if the alternate compiler is not used. I had to fix a bug in the flexible array handling, because while we were lucky in the 3000 generated tests not to have toplevel fields after field with flexible array members, in the next 3000 we aren't lucky anymore. But even with that change, diff -upr between old and new testsuite/g++/g++.dg/g++.dg-struct-layout-1/ doesn't show any differences except for the ^Only in... messages for the new tests in there. Bootstrapped/regtested on x86_64-linux and i686-linux and additionally tested on aarch64-linux, where FAIL: tmpdir-g++.dg-struct-layout-1/t032 cp_compat_x_tst.o-cp_compat_y_tst.o execute FAIL: tmpdir-g++.dg-struct-layout-1/t056 cp_compat_x_tst.o-cp_compat_y_tst.o execute FAIL: tmpdir-g++.dg-struct-layout-1/t057 cp_compat_x_tst.o-cp_compat_y_tst.o execute FAIL: tmpdir-g++.dg-struct-layout-1/t058 cp_compat_x_tst.o-cp_compat_y_tst.o execute FAIL: tmpdir-g++.dg-struct-layout-1/t059 cp_compat_x_tst.o-cp_compat_y_tst.o execute because of the backend bug, and with that bug fixed it succeeds. Matthew has kindly tested it also on aarch64-linux and arm*-*. The primary goal of the patch is catch if some targets other than aarch64 or arm aren't affected too. 2020-04-21 Jakub Jelinek <jakub@redhat.com> PR c++/94383 * g++.dg/compat/struct-layout-1.exp: If !$use_alt, add -c to generator args. * g++.dg/compat/struct-layout-1_generate.c (dg_options): Add another %s to the start of dg-options arg. (cxx14_vs_cxx17, do_cxx14_vs_cxx17): New variables. (switchfiles): If cxx14_vs_cxx17, prepend -std=c++14 -DCXX14_VS_CXX17 or -std=c++17 -DCXX17_VS_CXX14 - randomly - to dg-options. (output): Don't append further fields once one with flexible array member is added. (generate_random_tests): Don't use toplevel unions if cxx14_vs_cxx17. (main): If -c, emit second set of tests for -std=c++14 vs. -std=c++17 testing. * g++.dg/compat/struct-layout-1_x1.h (empty_base): New type. (EMPTY_BASE): Define. (TX): Use EMPTY_BASE. * g++.dg/compat/struct-layout-1_y1.h (empty_base): New type. (EMPTY_BASE): Define. (TX): Use EMPTY_BASE.
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>: https://gcc.gnu.org/g:c3a34659036b55fa36a5355fdd67133f427eeb2d commit r10-7881-gc3a34659036b55fa36a5355fdd67133f427eeb2d Author: Jakub Jelinek <jakub@redhat.com> Date: Wed Apr 22 16:44:42 2020 +0200 calls: Introduce cxx17_empty_base_field_p [PR94383] As multiple targets are affected apparently, I believe at least aarch64, arm, powerpc64le, s390{,x} and ia64, I think we should have a middle-end predicate for this, so that if we need to tweak it, we can do it in one spot. 2020-04-22 Jakub Jelinek <jakub@redhat.com> PR target/94383 * calls.h (cxx17_empty_base_field_p): Declare. * calls.c (cxx17_empty_base_field_p): Define.
The master branch has been updated by Matthew Malcomson <matmal01@gcc.gnu.org>: https://gcc.gnu.org/g:e73a32d6d47ec7c5fb5a5fe7eb896c0e1258ea68 commit r10-7914-ge73a32d6d47ec7c5fb5a5fe7eb896c0e1258ea68 Author: Matthew Malcomson <matthew.malcomson@arm.com> Date: Thu Apr 23 15:33:55 2020 +0100 [AArch64] (PR94383) Avoid C++17 empty base field checking for HVA/HFA In C++17, an empty class deriving from an empty base is not an aggregate, while in C++14 it is. In order to implement this, GCC adds an artificial field to such classes. This artificial field has no mapping to Fundamental Data Types in the AArch64 PCS ABI and hence should not count towards determining whether an object can be passed using the vector registers as per section "6.4.2 Parameter Passing Rules" in the AArch64 PCS. https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst#the-base-procedure-call-standard This patch avoids counting this artificial field in aapcs_vfp_sub_candidate, and hence calculates whether such objects should be passed in vector registers in the same manner as C++14 (where the artificial field does not exist). Before this change, the test below would pass the arguments to `f` in general registers. After this change, the test passes the arguments to `f` using the vector registers. The new behaviour matches the behaviour of `armclang`, and also matches the behaviour when run with `-std=gnu++14`. > gcc -std=gnu++17 test.cpp ``` test.cpp struct base {}; struct pair : base { float first; float second; pair (float f, float s) : first(f), second(s) {} }; void f (pair); int main() { f({3.14, 666}); return 1; } ``` We add a `-Wpsabi` warning to catch cases where this fix has changed the ABI for some functions. Unfortunately this warning is not emitted twice for multiple calls to the same function, but I feel this is not much of a problem and can be fixed later if needs be. (i.e. if `main` called `f` twice in a row we only emit a diagnostic for the first). Testing: Bootstrap and regression test on aarch64-linux. All struct-layout-1 tests now pass. gcc/ChangeLog: 2020-04-23 Matthew Malcomson <matthew.malcomson@arm.com> Jakub Jelinek <jakub@redhat.com> PR target/94383 * config/aarch64/aarch64.c (aapcs_vfp_sub_candidate): Account for C++17 empty base class artificial fields. (aarch64_vfp_is_call_or_return_candidate): Warn when ABI PCS decision is different after this fix.
Fixed.
(In reply to CVS Commits from comment #14) > [AArch64] (PR94383) Avoid C++17 empty base field checking for HVA/HFA > > In C++17, an empty class deriving from an empty base is not an > aggregate, while in C++14 it is. FWIW that's backwards.
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>: https://gcc.gnu.org/g:9407f0c32b215d55d3474a234b0043bddc185b1c commit r10-7948-g9407f0c32b215d55d3474a234b0043bddc185b1c Author: Jakub Jelinek <jakub@redhat.com> Date: Fri Apr 24 19:14:27 2020 +0200 testsuite: C++14 vs. C++17 struct-layout-1.exp testing with ALT_CXX_UNDER_TEST [PR94383] > I haven't added (yet) checks if the alternate compiler does support these > options (I think that can be done incrementally), so for now this testing is > done only if the alternate compiler is not used. This patch does that, so now when testing against not too old compiler it can do the -std=c++14 vs. -std=c++17 testing also between under test and alt compilers. 2020-04-24 Jakub Jelinek <jakub@redhat.com> PR c++/94383 * g++.dg/compat/struct-layout-1.exp: Use the -std=c++14 vs. -std=c++17 ABI compatibility testing even with ALT_CXX_UNDER_TEST, as long as that compiler accepts -std=c++14 and -std=c++17 options.
$ cat test.c struct foo { int len; https://www.webb-dev.co.uk/category/computers/ int items[]; }; struct foo *p; http://www.compilatori.com/category/technology/ int main() { return 0; } $ gcc test.c -g -O0 -o test http://www.acpirateradio.co.uk/category/computers/ $ ./gdb -q -nx --data-directory=data-directory ./test -ex 'python gdb.parse_and_eval("p").type.target()["items"].type.range()' Reading symbols from ./test... http://www-look-4.com/category/computers/ /home/simark/src/binutils-gdb/gdb/gdbtypes.h:435: internal-error: LONGEST dynamic_prop::const_val() const: Assertion `m_kind == PROP_CONST' failed. http://www.mconstantine.co.uk/category/services/ A problem internal to GDB has been detected, further debugging may prove unreliable. Quit this debugging session? (y or n) http://www.logoarts.co.uk/category/computers/ This is because the Python code (typy_range) blindly reads the high bound of the type of `items` as a constant value. Since it is a http://www.iu-bloomington.com/category/computers/ flexible array member, it has no high bound, the property is undefined. Since commit 8c2e4e0689 https://komiya-dental.com/category/computers/ ("gdb: add accessors to struct dynamic_prop"), the getters check that you are not http://www.go-mk-websites.co.uk/category/services/ getting a property value of the wrong kind, so this causes a failed assertion. http://www.slipstone.co.uk/category/computers/ Fix it by checking if the property is indeed a constant value before http://embermanchester.uk/category/computers/ accessing it as such. Otherwise, use 0. This restores the previous GDB http://fishingnewsletters.co.uk/category/services/ behavior: because the structure was zero-initialized, http://connstr.net/category/computers/ this is what was returned before. But now this behavior is explicit and not accidental. But now this behavior is explicit and not accidental. But now this behavior is explicit and not http://the-hunters.org/technology/new-robot/ accidental. But now this behavior is explicit and not accidental. But now this behavior is explicit and is explicit and not accidental. http://joerg.li/category/computers/ Add a test, gdb.python/flexible-array-member.exp, that is derived from gdb.base/flexible-array-member.exp. http://www.jopspeech.com/category/computers/ It tests the same things, but through the Python API. It also specifically tests getting the range from the various kinds http://www.wearelondonmade.com/category/computers/ of flexible array member types (AFAIK it wasn't possible to do the equivalent through the CLI). https://waytowhatsnext.com/category/computers/ gdb/ChangeLog: