Date: Wed, 9 Mar 2016 12:24:04 -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> Subject: Re: Ulrich Weigand has confirmed 3 of my 4 llvm bug submittals for clang 3.8.0 targeting powerpc/powerpc64. . . Message-ID: <C0525A0A-8D4A-4E0B-B024-1A655C281FDC@dsl-only.net> In-Reply-To: <EF077AD9-2F3E-493F-802D-D1CCD99461DF@dsl-only.net> References: <EF077AD9-2F3E-493F-802D-D1CCD99461DF@dsl-only.net>
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On 2016-Mar-9, at 11:16 AM, Mark Millard <markmi at dsl-only.net> wrote: >=20 > [He also includes a note about ELFV2 ABI for powerpc64le.] >=20 > Quoting llvm 26856 Comment 6 (he put the text in the 26856 submittal = but the content also covers 26519 and most of 26844): >=20 >> Ulrich Weigand 2016-03-09 11:53:17 CST >>=20 >> Yes, there's indeed a couple of problems here, which affect different = areas. >>=20 >> 1) On 32-bit ppc, LLVM violates the ABI by storing below the stack = pointer even though the ABI does not provide a "red zone". This affects = every function with a stack frame, and could in theory lead to spurious = crashes when an asynchronous signal overwrites this area. This seems to = be a known issue; the source code contains FIXME lines: >> // FIXME: On PPC32 SVR4, we must not spill before claiming the = stackframe. >>=20 >> 2) In some scenarios, registers may be spilled/restored twice to the = stack. This happens because while most of the spilling happens in = PPCFrameLowering::spillCalleeSavedRegisters, a few selected registers = are also spilled in PPCFrameLowering::emitPrologue. Those registers are = the frame pointer, base pointer, PIC base pointer, link register, and = condition code register. For the latter two, code ensures that they can = never be spilled in both places (for CR, there is extra code in = spillCalleeSavedRegisters; for LR, the register is removed from = SavedRegs in determineCalleeSaves). >>=20 >> However, for FP, BP, and PBP, nothing ensures the registers are not = spilled twice. It is probably *rare* for this to happen, because the = register allocator will not use those registers within the function if = they're needed for their special purpose, but it can happen in rare = cases. This includes the case of a system unwinder routine that uses = __builtin_unwind_init, but could also include other routines that = clobber one of those registers, e.g. the following case: >>=20 >> void func (void); >>=20 >> void test (void) >> { >> func (); >> asm ("nop" : : : "31"); >> } >>=20 >> When it happens that a register is spilled twice, the code as such = still works correctly, but the DWARF CFI unwind info associated with the = routine will be broken, which can mess up both C++ exception handling = and debugging. >>=20 >> 3) For the specific case of system unwinder routines that use = __builtin_unwind_init and/or __builtin_eh_return, special things need to = happen in the prolog and epilog that are not required for any other = routine. This in particular includes setting up save areas and CFI = records for the EH data registers (r3 ... r6). [See bug #26844. ] For = the ELFv2 ABI (powerpc64le), it also include using three separate save = areas for the three caller-saved condition register fields, so that the = EH logic can overwrite their values independently. >>=20 >> None of this is currently implemented in LLVM, since on Linux = generally GCC is used to build the system unwind libraries, and no other = code in the system ever needs those special constructs. >=20 >=20 > =3D=3D=3D > Mark Millard > markmi at dsl-only.net One point of his note is wrong: when the 2nd "spill" of a register is = after it had been changed it makes a bigger difference. I commented = back: > However, for FP, BP, and PBP, nothing ensures the registers are not = spilled > twice.. . . >=20 > When it happens that a register is spilled twice, the code as such = still > works correctly, but the DWARF CFI unwind info associated with the = routine > will be broken, which can mess up both C++ exception handling and = debugging. I will note that the Frame Pointer Register (r31) being saved again to = the same location but after it was adjusted to match the adjusted stack = pointer in the callee does not work correctly in that the restore of the = Frame Pointer for the return to the caller will restore the wrong = pointer value. If the caller then uses r31 without separately also restoring r31 first = then it will be addressing the wrong memory on the stack. The observed/reported code sequence had the 2nd r31 store in the callee = after r31 had been adjusted to match the adjusted stack pointer in the = callee. So more than C++ exception handling and debugging is broken for the = reported code sequence. =3D=3D=3D Mark Millard markmi at dsl-only.net
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