Date: Wed, 24 Feb 2016 17:53:03 -0800 From: Mark Millard <markmi@dsl-only.net> To: FreeBSD PowerPC ML <freebsd-ppc@freebsd.org>, FreeBSD Toolchain <freebsd-toolchain@freebsd.org> Cc: Roman Divacky <rdivacky@vlakno.cz>, Dimitry Andric <dim@FreeBSD.org> Subject: Re: projects/clang380-import for TARGET_ARCH=powerpc64: /lib/libgcc_s.so.1 is incompatible with using the pair: /usr/lib/libc++.so.1 and /lib/libcxxrt.so.1 for C++ exception handling Message-ID: <440081D9-83A0-44B8-B209-C50DB4B52885@dsl-only.net> In-Reply-To: <B2EA9D1B-DCF0-4C98-AB83-F8C7B75F97BB@dsl-only.net> References: <5B9137BF-0E86-4C4B-A3FB-18F444A850AC@dsl-only.net> <B2EA9D1B-DCF0-4C98-AB83-F8C7B75F97BB@dsl-only.net>
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[In part: My references to libc++ should have been to libcxxrt. Also: = devel/powerpc64-gcc produced the incomplete .eh_frame information.] On 2016-Feb-24, at 5:07 PM, Mark Millard <markmi@dsl-only.net> wrote: >=20 > [Deliberate top posting an history removal for significant new = information.] >=20 > I've finally traced the low level details of the powerpc64 = _Unwind_RaiseException stuck looping failure. What I've submitted into = the defect is basically that for clang 3.8.0's .eh_frame information = generation in libcxxrt's code there is an error in the form of an = incompleteness. . . >=20 >=20 > Starting from the observed low-level evidence based on observation via = gdb and such: >=20 > A backtrace while stopped during the unbounded looping is: >=20 >> #0 uw_update_context (context=3Dcontext@entry=3D0xffffffffffffccf0, = fs=3Dfs@entry=3D0xffffffffffffc370) at = /usr/src/gnu/lib/libgcc/../../../contrib/gcc/unwind-dw2.c:1371 >> #1 0x00000000501cb95c in _Unwind_RaiseException (exc=3D0x50815058) = at /usr/src/gnu/lib/libgcc/../../../contrib/gcc/unwind.inc:126 >> #2 0x000000005016e3a0 in throw_exception (ex=3D0x50815000) at = /usr/src/lib/libcxxrt/../../contrib/libcxxrt/exception.cc:751 >> #3 0x0000000010000d50 in main () at exception_test.cpp:5 >=20 > _Unwind_RaiseException never returns for source code that looks like: >=20 >> #include <exception> >>=20 >> int main(void) >> { >> try { throw std::exception(); } >> catch (std::exception& e) {} >> return 0; >> } >=20 > where in /usr/lib/libc++.so.1 there is: >=20 >> 736 static void throw_exception(__cxa_exception *ex) >> 737 { >> . . . >> 751 _Unwind_Reason_Code err =3D = _Unwind_RaiseException(&ex->unwindHeader); >> . . . >> 756 } >=20 > The unbounded loop in _Unwind_RaiseException is in the code: >=20 >> 85 _Unwind_Reason_Code >> 86 _Unwind_RaiseException(struct _Unwind_Exception *exc) >> 87 { >> 88 struct _Unwind_Context this_context, cur_context; >> 89 _Unwind_Reason_Code code; >> 90=09 >> 91 /* Set up this_context to describe the current stack frame. = */ >> 92 uw_init_context (&this_context); >> 93 cur_context =3D this_context; >> 94=09 >> 95 /* Phase 1: Search. Unwind the stack, calling the personality = routine >> 96 with the _UA_SEARCH_PHASE flag set. Do not modify the = stack yet. */ >> 97 while (1) >> 98 { >> 99 _Unwind_FrameState fs; >> 100=09 >> 101 /* Set up fs to describe the FDE for the caller of = cur_context. The >> 102 first time through the loop, that means __cxa_throw. = */ >> 103 code =3D uw_frame_state_for (&cur_context, &fs); >> . . . >> 125 /* Update cur_context to describe the same frame as fs. = */ >> 126 uw_update_context (&cur_context, &fs); >> 127 } >> . . . >> 140 } >=20 > The uw_update_context call is doing the following before returning: >=20 >> 1367 /* Compute the return address now, since the return address = column >> 1368 can change from frame to frame. */ >> 1369 context->ra =3D __builtin_extract_return_addr >> 1370 (_Unwind_GetPtr (context, fs->retaddr_column)); >=20 > with context->ra before and after the call both being 0x5016e3a0 . In = fact it was 0x5016e3a0 for the prior uw_frame_state_for call as well and = continues to be so each loop iteration once the problem starts. >=20 > As for the code around 0x5016e3a0: >=20 >> 0x5016e398 <throw_exception(__cxxabiv1::__cxa_exception*)+136>: = stw r10,48(r9) >> 0x5016e39c <throw_exception(__cxxabiv1::__cxa_exception*)+140>: = bl 0x50162ae0 <00000017.plt_call._Unwind_RaiseException@@GCC_3.0> >> 0x5016e3a0 <throw_exception(__cxxabiv1::__cxa_exception*)+144>: = ld r2,40(r1) >> 0x5016e3a4 <throw_exception(__cxxabiv1::__cxa_exception*)+148>: = addi r1,r1,128 >> 0x5016e3a8 <throw_exception(__cxxabiv1::__cxa_exception*)+152>: = mr r4,r31 >> 0x5016e3ac <throw_exception(__cxxabiv1::__cxa_exception*)+156>: = ld r0,16(r1) >=20 > =46rom /usr/local/bin/objdump for FreeBSD projects/clang380-import = -r295902's /usr/lib/libc++.so.1 for the same code (to match up with the = .eh_frame dwarf information): >=20 >> 0000000000015398 <.__cxa_end_catch+0x4d8> stw r10,48(r9) >> 000000000001539c <.__cxa_end_catch+0x4dc> bl 0000000000009ae0 = <CXXABI_1.3+0x9ae0> >> 00000000000153a0 <.__cxa_end_catch+0x4e0> ld r2,40(r1) >> 00000000000153a4 <.__cxa_end_catch+0x4e4> addi r1,r1,128 >> 00000000000153a8 <.__cxa_end_catch+0x4e8> mr r4,r31 >> 00000000000153ac <.__cxa_end_catch+0x4ec> ld r0,16(r1) >=20 > The code block above from 153a0 up to 153a8 is being given 153a0 as = its "return address" (context->ra) by uw_update_context via interpreting = the dwarf .eh_frame information. So once in that range there it never = leaves that range. >=20 > The relevant dwarfdump output spanning that area is: > (Note that 153a0 up to 153a8 is part of the range that includes the bl = to _Unwind_RaiseException .) >=20 >> < 0><0x00015310:0x000153dc><><fde offset 0x000010d8 length: = 0x00000034><eh aug data len 0x0> >> 0x00015310: <off cfa=3D00(r1) >=20 >> 0x00015318: <off cfa=3D00(r1) > <off r31=3D-8(cfa) > <off = r65=3Dr0 >=20 >> 0x00015324: <off cfa=3D128(r1) > <off r31=3D-8(cfa) > <off = r65=3D16(cfa) >=20 >> 0x00015368: <off cfa=3D00(r1) > <off r31=3D-8(cfa) > <off = r65=3D16(cfa) >=20 >> 0x00015378: <off cfa=3D00(r1) >=20 >> 0x00015380: <off cfa=3D128(r1) > <off r31=3D-8(cfa) > <off = r65=3D16(cfa) >=20 >> 0x000153a8: <off cfa=3D00(r1) > <off r31=3D-8(cfa) > <off = r65=3D16(cfa) >=20 >> 0x000153b8: <off cfa=3D00(r1) >=20 >> 0x000153c0: <off cfa=3D128(r1) > <off r31=3D-8(cfa) > <off = r65=3D16(cfa) >=20 >> fde section offset 4312 0x000010d8 cie offset for fde: 4316 = 0x000010dc >> 0 DW_CFA_advance_loc 8 (2 * 4) >> 1 DW_CFA_register r65 =3D r0 >> 4 DW_CFA_offset r31 -8 (1 * -8) >> 6 DW_CFA_advance_loc 12 (3 * 4) >> 7 DW_CFA_def_cfa_offset 128 >> 10 DW_CFA_offset_extended_sf r65 16 (-2 * -8) >> 13 DW_CFA_advance_loc 68 (17 * 4) >> 14 DW_CFA_remember_state >> 15 DW_CFA_def_cfa_offset 0 >> 17 DW_CFA_advance_loc 16 (4 * 4) >> 18 DW_CFA_restore_extended r65 >> 20 DW_CFA_restore r31 >> 21 DW_CFA_advance_loc 8 (2 * 4) >> 22 DW_CFA_restore_state >> 23 DW_CFA_advance_loc 40 (10 * 4) >> 24 DW_CFA_remember_state >> 25 DW_CFA_def_cfa_offset 0 >> 27 DW_CFA_advance_loc 16 (4 * 4) >> 28 DW_CFA_restore_extended r65 >> 30 DW_CFA_restore r31 >> 31 DW_CFA_advance_loc 8 (2 * 4) >> 32 DW_CFA_restore_state >> 33 DW_CFA_nop >> 34 DW_CFA_nop >> 35 DW_CFA_nop >> 36 DW_CFA_nop >> 37 DW_CFA_nop >> 38 DW_CFA_nop >=20 > Note that the stack pointer manipulation (incrementing/popping) = *after* the return from the bl. 15339c, 153a0, and 153a4 should be 3 or = so distinct contexts in the .eh_frame information but are not. The code = is: >=20 >> 000000000001539c <.__cxa_end_catch+0x4dc> bl 0000000000009ae0 = <CXXABI_1.3+0x9ae0> >> 00000000000153a0 <.__cxa_end_catch+0x4e0> ld r2,40(r1) >> 00000000000153a4 <.__cxa_end_catch+0x4e4> addi r1,r1,128 >=20 > So there is a missing step or more of frame-context tracking in the = .eh_frame information. I should have noted that devel/powerpc64-gcc was used for buildworld of = projects/clang380-import -r295902 (and other versions) --and so for = libcxxrt as well. So it is g++ 5.3 that put out the incomplete .eh_frame frame-context = tracking, not clang 3.8.0. There is also the point that I typed "libc++" in places that I should = have typed "libcxxrt". =3D=3D=3D Mark Millard markmi at dsl-only.net
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