[Lldb-commits] [PATCH] Profile Assembly Until Ret Instruction
Tong Shen
endlessroad at google.com
Mon Aug 25 10:36:26 PDT 2014
Hi Jason, Todd,
Updated patch with fields changed to uint32_t.
Thank you very much for reviewing this! I understand it's a big change to
unwind mechanism and quite risky.
Let's see if it works better than what's there now :-)
On Mon, Aug 25, 2014 at 8:55 AM, Todd Fiala <tfiala at google.com> wrote:
> Hey Tong - let me know when Jason's latest comments are reflected in the
> patch and I'll test and get it in.
>
> Thanks!
>
> -Todd
>
>
> On Mon, Aug 25, 2014 at 1:40 AM, Jason Molenda <jason at molenda.com> wrote:
>
>> Hi Tong, I read it over and I think we should commit it and try living on
>> it. With unwinding there's usually nothing like real world experience with
>> a change to find crazy edge cases. :)
>>
>> You've been looking at this code more than me lately but I think my
>> initial definitions in AssemblyParse_x86,
>>
>> int m_machine_ip_regnum;
>> int m_machine_sp_regnum;
>> int m_machine_fp_regnum;
>>
>> int m_lldb_ip_regnum;
>> int m_lldb_sp_regnum;
>> int m_lldb_fp_regnum;
>>
>>
>> should be changed to uint32_t's - what do you think? When I wrote the
>> code originally the int v. uint32_t mismatches weren't a big deal but this
>> class has picked up a lot of static_cast<uint32_t>'s trying to work around
>> this mismatch.
>>
>> Thanks for working on this!
>>
>>
>> Jason
>>
>>
>> On Aug 22, 2014, at 6:37 PM, Tong Shen <endlessroad at google.com> wrote:
>>
>> > Hi Jason,
>> >
>> > Updated patch attached.
>> >
>> > Augmentation code is in UnwindAssembly-x86.cpp.
>> >
>> > Current augmentation progress:
>> > - Get eh_frame for the function.
>> > - Inspect each instruction:
>> > * If there's an FDE for this instruction, use it.
>> > * Otherwise, see if this instruction will change how we locate CFA
>> and make corresponding changes.
>> >
>> > The code assume there's prologue description.
>> > It checks whether there's an FDE for the first instruction; if not,
>> augmentation is aborted and we fallback to assembly profiling.
>> >
>> > I only tested it with i386 epilogue and pc relative addressing.
>> > If you think it's OK, I will write a complete test.
>> >
>> > Thanks for your time!
>> >
>> >
>> > On Fri, Aug 22, 2014 at 9:58 AM, Todd Fiala <tfiala at google.com> wrote:
>> > Ok - holding off on checking this per communication with Tong. Will
>> see a new patch later today on this.
>> >
>> >
>> > On Fri, Aug 22, 2014 at 9:49 AM, Todd Fiala <tfiala at google.com> wrote:
>> > Er I'll "get it" in... eek..
>> >
>> >
>> > On Fri, Aug 22, 2014 at 9:49 AM, Todd Fiala <tfiala at google.com> wrote:
>> > I'm going to test this now. If it all looks good, I'll ge tit in.
>> >
>> >
>> > On Tue, Aug 19, 2014 at 5:01 PM, Tong Shen <endlessroad at google.com>
>> wrote:
>> > Thanks Jason!
>> > I will finish this patch and let's see how it goes.
>> >
>> > P.S. I know a little about eh_frame stuff; I added CFI to the new
>> Android ahead-of-time Java compiler so AOT'ed code can properly unwind :-)
>> >
>> >
>> >
>> > On Tue, Aug 19, 2014 at 4:51 PM, Jason Molenda <jmolenda at apple.com>
>> wrote:
>> > The CIE sets the initial unwind state -- the CIE may describe the
>> unwind state at the first instruction (as it always does with gcc, clang)
>> but in theory it could describe the unwind state once the prologue had
>> executed.
>> >
>> > The idea is that there is one CIE entry which describes a typical
>> at-first-instruction unwind state and then many FDEs that describe the
>> unwind instructions for specific functions - they all use that one CIE.
>> >
>> > Anyway, that's just an implementation detail of eh_frame. I honestly
>> don't think we should worry about incomplete eh_frame - let's try living on
>> them and see how it works in practice.
>> >
>> > It may be possible to categorize eh_frame to see how complete it is.
>> Compiler-generated x86 prologues are very regular, it would be possible to
>> look at the first few bytes of a function for some pushes or stack pointer
>> changes and see if the eh_frame describes that. We know what the unwind
>> state is on the first instruction of a function (it's determined by the
>> ABI) -- does the eh_frame have the same instructions? Can we can through
>> the function for an epilogue, and if we find one, does the eh_frame have
>> unwind instructions there?
>> >
>> > But I don't want to have the perfect be the enemy of the good. IMO
>> let's take the plunge and try, to use eh_frame and see how that goes. We
>> can refine it later, or back it out again (it will be a very small change
>> to RegisterContextLLDB) if necessary.
>> >
>> >
>> > > On Aug 19, 2014, at 4:41 PM, Tong Shen <endlessroad at google.com>
>> wrote:
>> > >
>> > > And for no prologue case:
>> > > We can detect this easily (any CFI for start address?) and bail out,
>> so we will fallback to assembly profiler.
>> > >
>> > >
>> > > On Tue, Aug 19, 2014 at 4:36 PM, Tong Shen <endlessroad at google.com>
>> wrote:
>> > > Ahh sorry I've been working on something else this week and didn't
>> get back to you in time.
>> > > And you've been very patient and informative. Thanks!
>> > >
>> > > I'm only suggesting it for x86 / x86_64. What I am doing here relies
>> on:
>> > > - Compiler describes prologue;
>> > > - We can figure our all mid function CFA changes by inspecting
>> instructions.
>> > >
>> > > For frame 0, the new progress for CFA locating will look like this:
>> > > - Find the nearest CFI available before current PC.
>> > > - If the CFI is for current PC, viola :-) If not, continue.
>> > > - Inspect all instructions in between, and make changes to CFA
>> accordingly. This can solve the PC relative addressing case.
>> > > - For epilogue, detect if we are in middle of an epilogue.
>> Considering that there are not many patterns and they are all simple, I
>> think we can enumerate them and handle accordingly.
>> > >
>> > > From what I've seen so far, this actually can solve most of gcc/clang
>> generated code.
>> > > For JIT'ed code or hand written assembly, if there's no asynchronous
>> CFI we are screwed anyway, so trying this won't hurt either (except some
>> extra running time).\
>> > >
>> > > I hope I explain my thoughts clearly.
>> > >
>> > > Thank you.
>> > >
>> > >
>> > >
>> > > On Tue, Aug 19, 2014 at 4:22 PM, Jason Molenda <jmolenda at apple.com>
>> wrote:
>> > > Hi Tong, my message was a little rambling. Let's be specific.
>> > >
>> > > We are changing lldb to trust eh_frame instructions on the
>> currently-executing aka 0th frame.
>> > >
>> > > In practice, gcc and clang eh_frame both describe the prologue, so
>> this is OK.
>> > >
>> > > Old gcc and clang eh_frame do not describe the epilogue. So we need
>> to add a pass for i386/x86_64 (at least) to augment the eh_frame-sourced
>> unwind instructions. I don't know if it would be best to augment eh_frame
>> UnwindPlans when we create them in DWARFCallFrameInfo or if it would be
>> better to do it lazily when we are actually using the unwind instructions
>> in RegisterContextLLDB (probably RegisterContextLLDB like you were doing).
>> We should only do it once for a given function, of course.
>> > >
>> > > I think it would cleanest if the augmentation function lived in the
>> UnwindAssembly class. But I haven't looked how easy it is to get an
>> UnwindAssembly object where we need it.
>> > >
>> > >
>> > > Thanks for taking this on. It will be interesting to try living
>> entirely off eh_frame and see how that works for all the
>> architectures/environments lldb supports.
>> > >
>> > > I worry a little that we're depending on the generous eh_frame from
>> clang/gcc and if we try to run on icc (Intel's compiler) or something like
>> that, we may have no prologue instructions and stepping will work very
>> poorly. But we'll cross that bridge when we get to it.
>> > >
>> > >
>> > >
>> > > > On Aug 15, 2014, at 8:07 PM, Jason Molenda <jmolenda at apple.com>
>> wrote:
>> > > >
>> > > > Hi Tong, sorry for the delay in replying.
>> > > >
>> > > > I have a couple thoughts about the patch. First, the change in
>> RegisterContextLLDB::GetFullUnwindPlanForFrame() forces the use of eh_frame
>> unwind instructions ("UnwindPlanAtCallSite" - which normally means the
>> eh_frame unwind instructions) for the currently-executing aka zeroth
>> frame. We've talked about this before, but it's worth noting that this
>> patch includes that change.
>> > > >
>> > > > There's still the problem of detecting how *asynchronous* those
>> eh_frame unwind instructions are. For instance, what do you get for an
>> i386 program that does
>> > > >
>> > > > #include <stdio.h>
>> > > > int main()
>> > > > {
>> > > > puts ("HI");
>> > > > }
>> > > >
>> > > > Most codegen will use a sequence like
>> > > >
>> > > > call LNextInstruction
>> > > > .LNextInstruction
>> > > > pop ebx
>> > > >
>> > > > this call & pop sequence is establishing the "pic base", it the
>> program will then use that address to find the "HI" constant data. If you
>> compile this -fomit-frame-pointer, so we have to use the stack pointer to
>> find the CFA, do the eh_frame instructions describe this?
>> > > >
>> > > > It's a bit of an extreme example but it's one of those tricky cases
>> where asynchronous ("accurate at every instruction") unwind instructions
>> and synchronous ("accurate at places where we can throw an exception, or a
>> callee can throw an exception") unwind instructions are different.
>> > > >
>> > > >
>> > > > I would use behaves_like_zeroth_frame instead of if (IsFrameZero())
>> because you can have a frame in the middle of the stack which was the
>> zeroth frame when an asynchronous signal came in -- in which case, the
>> "callee" stack frame will be sigtramp.
>> > > >
>> > > >
>> > > > You'd want to update the UnwindLogMsgVerbose() text, of course.
>> > > >
>> > > >
>> > > > What your DWARFCallFrameInfo::PatchUnwindPlanForX86() function is
>> doing is assuming that the unwind plan fails to include an epilogue
>> description, steps through all the instructions in the function looking for
>> the epilogue.
>> > > >
>> > > > DWARFCallFrameInfo doesn't seem like the right place for this.
>> There's an assumption that the instructions came from eh_frame and that
>> they are incomplete. It seems like it would more naturally live in the
>> UnwindAssembly plugin and it would have a name like
>> AugmentIncompleteUnwindPlanWithEpilogue or something like that.
>> > > >
>> > > > What if the CFI already does describe the epilogue? I imagine
>> we'll just end up with a doubling of UnwindPlan Rows that describe the
>> epilogue instructions.
>> > > >
>> > > > What if we have a mid-function epilogue? I've never seen gcc/clang
>> generate these for x86, but it's possible. It's a common code sequence on
>> arm/arm64. You can see a messy bit of code in
>> UnwindAssemblyInstEmulation::GetNonCallSiteUnwindPlanFromAssembly which
>> handles these -- saving the UnwindPlan's unwind instructions when we see
>> the beginning of an epilogue, and once the epilogue is complete, restoring
>> the unwind instructions.
>> > > >
>> > > >
>> > > > I'm not opposed to the patch - but it does make the assumption that
>> we're going to use eh_frame for the currently executing function and that
>> the eh_frame instructions do not include a description of the epilogue.
>> (and that there is only one epilogue in the function). Mostly I want to
>> call all of those aspects out so we're clear what we're talking about
>> here. Let's clean it up a bit, put it in and see how it goes.
>> > > >
>> > > > J
>> > > >
>> > > >
>> > > >> On Aug 14, 2014, at 6:31 PM, Tong Shen <endlessroad at google.com>
>> wrote:
>> > > >>
>> > > >> Hi Jason,
>> > > >>
>> > > >> Turns out we still need CFI for frame 0 in certain situations...
>> > > >>
>> > > >> A possible approach is to disassemble machine code, and manually
>> adjust CFI for frame 0. For example, if we see "pop ebp; => ret", we set
>> cfa to [esp]; if we see "call next-insn; => pop %ebp", we set cfa_offset+=4.
>> > > >>
>> > > >> Patch attached, now it just implements adjustment for "pop ebp;
>> ret".
>> > > >>
>> > > >> If you think this approach is OK, I will go ahead and add other
>> tricks(i386 pc relative addressing, more styles of epilogue, etc).
>> > > >>
>> > > >> Thank you for your time!
>> > > >>
>> > > >>
>> > > >> On Thu, Jul 31, 2014 at 12:50 PM, Tong Shen <
>> endlessroad at google.com> wrote:
>> > > >> I think gdb's rationale for using CFI for leaf function is:
>> > > >> - gcc always generate CFI for progolue, so at function entry, we
>> know the correct CFA;
>> > > >> - any stack pointer altering operation after that(mid-function &
>> epilogue), we can recognize and handle them.
>> > > >> So basically, it assumes 2, hacks its way through 3 & 4, and
>> pretends we are at 5.
>> > > >> Number of hacks we need seems to be small in x86 world, so this
>> tradition is still here.
>> > > >>
>> > > >> Here's what gdb does for epilogue: normally when you run 'n', it
>> will run one instruction a time till the next line/different stack id. But
>> when it sees "pop %rbp; ret", it won't step into these instructions.
>> Instead it will execute past them directly.
>> > > >> I didn't experiment with x86 pc-relative addressing; but I guess
>> it will also recognize and execute past this pattern directly.
>> > > >>
>> > > >> So for compiler generated functions, what we do now with assembly
>> parser now can be done with CFI + those gdb hacks.
>> > > >> And for hand-written assembly, i think CFI is almost always
>> precise at instruction level. In this case, utilizing CFI instead of
>> assembly parser will be a big help.
>> > > >>
>> > > >> So maybe we can apply those hacks, and trust CFI only for x86 &
>> x86_64 targets?
>> > > >>
>> > > >>
>> > > >> On Thu, Jul 31, 2014 at 12:02 AM, Jason Molenda <
>> jmolenda at apple.com> wrote:
>> > > >> I think we could think of five levels of eh_frame information:
>> > > >>
>> > > >>
>> > > >> 1 unwind instructions at exception throw locations & locations
>> where a callee may throw an exception
>> > > >>
>> > > >> 2 unwind instructions that describe the prologue
>> > > >>
>> > > >> 3 unwind instructions that describe the epilogue at the end of the
>> function
>> > > >>
>> > > >> 4 unwind instructions that describe mid-function epilogues (I see
>> these on arm all the time, don't see them on x86 with compiler generated
>> code - but we don't use eh_frame on arm at Apple, I'm just mentioning it
>> for completeness)
>> > > >>
>> > > >> 5 unwind instructions that describe any changes mid-function
>> needed to unwind at all instructions ("asynchronous unwind information")
>> > > >>
>> > > >>
>> > > >> The eh_frame section only guarantees #1. gcc and clang always do
>> #1 and #2. Modern gcc's do #3. I don't know if gcc would do #4 on arm but
>> it's not important, I just mention it for completeness. And no one does #5
>> (as far as I know), even in the DWARF debug_frame section.
>> > > >>
>> > > >> I think it maybe possible to detect if an eh_frame entry fulfills
>> #3 by looking if the CFA definition on the last row is the same as the
>> initial CFA definition. But I'm not sure how a debugger could use
>> heuristics to determine much else.
>> > > >>
>> > > >>
>> > > >> In fact, detecting #3 may be the easiest thing to detect. I'm not
>> sure if the debugger could really detect #2 except maybe if the function
>> had a standard prologue (push rbp, mov rsp rbp) and the eh_frame didn't
>> describe the effects of these instructions, the debugger could know that
>> the eh_frame does not describe the prologue.
>> > > >>
>> > > >>
>> > > >>
>> > > >>
>> > > >>> On Jul 30, 2014, at 6:58 PM, Tong Shen <endlessroad at google.com>
>> wrote:
>> > > >>>
>> > > >>> Ah I understand now.
>> > > >>>
>> > > >>> Now prologue seems always included in CFI fro gcc & clang; and
>> newer gcc includes epilogue as well.
>> > > >>> Maybe we can detect and use them when they are available?
>> > > >>>
>> > > >>>
>> > > >>> On Wed, Jul 30, 2014 at 6:44 PM, Jason Molenda <
>> jmolenda at apple.com> wrote:
>> > > >>> Ah, it looks like gcc changed since I last looked at its eh_frame
>> output.
>> > > >>>
>> > > >>> It's not a bug -- the eh_frame unwind instructions only need to
>> be accurate at instructions where an exception can be thrown, or where a
>> callee function can throw an exception. There's no requirement to include
>> prologue or epilogue instructions in the eh_frame.
>> > > >>>
>> > > >>> And unfortunately from lldb's perspective, when we see eh_frame
>> we'll never know how descriptive it is. If it's old-gcc or clang, it won't
>> include epilogue instructions. If it's from another compiler, it may not
>> include any prologue/epilogue instructions at all.
>> > > >>>
>> > > >>> Maybe we could look over the UnwindPlan rows and see if the CFA
>> definition of the last row matches the initial row's CFA definition. That
>> would show that the epilogue is described. Unless it is a tail-call (aka
>> noreturn) function - in which case the stack is never restored.
>> > > >>>
>> > > >>>
>> > > >>>
>> > > >>>
>> > > >>>> On Jul 30, 2014, at 6:32 PM, Tong Shen <endlessroad at google.com>
>> wrote:
>> > > >>>>
>> > > >>>> GCC seems to generate a row for epilogue.
>> > > >>>> Do you think this is a clang bug, or at least a discrepancy
>> between clang & gcc?
>> > > >>>>
>> > > >>>> Source:
>> > > >>>> int f() {
>> > > >>>> puts("HI\n");
>> > > >>>> return 5;
>> > > >>>> }
>> > > >>>>
>> > > >>>> Compile option: only -g
>> > > >>>>
>> > > >>>> gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1)
>> > > >>>> clang version 3.5.0 (213114)
>> > > >>>>
>> > > >>>> Env: Ubuntu 14.04, x86_64
>> > > >>>>
>> > > >>>> drawfdump -F of clang binary:
>> > > >>>> < 2><0x00400530:0x00400559><f><fde offset 0x00000088 length:
>> 0x0000001c><eh aug data len 0x0>
>> > > >>>> 0x00400530: <off cfa=08(r7) > <off r16=-8(cfa) >
>> > > >>>> 0x00400531: <off cfa=16(r7) > <off r6=-16(cfa) > <off
>> r16=-8(cfa) >
>> > > >>>> 0x00400534: <off cfa=16(r6) > <off r6=-16(cfa) > <off
>> r16=-8(cfa) >
>> > > >>>>
>> > > >>>> drawfdump -F of gcc binary:
>> > > >>>> < 1><0x0040052d:0x00400542><f><fde offset 0x00000070 length:
>> 0x0000001c><eh aug data len 0x0>
>> > > >>>> 0x0040052d: <off cfa=08(r7) > <off r16=-8(cfa) >
>> > > >>>> 0x0040052e: <off cfa=16(r7) > <off r6=-16(cfa) > <off
>> r16=-8(cfa) >
>> > > >>>> 0x00400531: <off cfa=16(r6) > <off r6=-16(cfa) > <off
>> r16=-8(cfa) >
>> > > >>>> 0x00400541: <off cfa=08(r7) > <off r6=-16(cfa) > <off
>> r16=-8(cfa) >
>> > > >>>>
>> > > >>>>
>> > > >>>> On Wed, Jul 30, 2014 at 5:43 PM, Jason Molenda <
>> jmolenda at apple.com> wrote:
>> > > >>>> I'm open to trying to trust eh_frame at frame 0 for x86_64. The
>> lack of epilogue descriptions in eh_frame is the biggest problem here.
>> > > >>>>
>> > > >>>> When you "step" or "next" in the debugger, the debugger
>> instruction steps across the source line until it gets to the next source
>> line. Every time it stops after an instruction step, it confirms that it
>> is (1) between the start and end pc values for the source line, and (2)
>> that the "stack id" (start address of the function + CFA address) is the
>> same. If it stops and the stack id has changed, for a "next" command, it
>> will backtrace one stack frame to see if it stepped into a function. If
>> so, it sets a breakpoint on the return address and continues.
>> > > >>>>
>> > > >>>> If you switch lldb to prefer eh_frame instructions for x86_64,
>> e.g.
>> > > >>>>
>> > > >>>> Index: source/Plugins/Process/Utility/RegisterContextLLDB.cpp
>> > > >>>>
>> ===================================================================
>> > > >>>> --- source/Plugins/Process/Utility/RegisterContextLLDB.cpp
>> (revision 214344)
>> > > >>>> +++ source/Plugins/Process/Utility/RegisterContextLLDB.cpp
>> (working copy)
>> > > >>>> @@ -791,6 +791,22 @@
>> > > >>>> }
>> > > >>>> }
>> > > >>>>
>> > > >>>> + // For x86_64 debugging, let's try using the eh_frame
>> instructions even if this is the currently
>> > > >>>> + // executing function (frame zero).
>> > > >>>> + Target *target = exe_ctx.GetTargetPtr();
>> > > >>>> + if (target
>> > > >>>> + && (target->GetArchitecture().GetCore() ==
>> ArchSpec::eCore_x86_64_x86_64h
>> > > >>>> + || target->GetArchitecture().GetCore() ==
>> ArchSpec::eCore_x86_64_x86_64))
>> > > >>>> + {
>> > > >>>> + unwind_plan_sp =
>> func_unwinders_sp->GetUnwindPlanAtCallSite (m_current_offset_backed_up_one);
>> > > >>>> + int valid_offset = -1;
>> > > >>>> + if (IsUnwindPlanValidForCurrentPC(unwind_plan_sp,
>> valid_offset))
>> > > >>>> + {
>> > > >>>> + UnwindLogMsgVerbose ("frame uses %s for full
>> UnwindPlan, preferred over assembly profiling on x86_64",
>> unwind_plan_sp->GetSourceName().GetCString());
>> > > >>>> + return unwind_plan_sp;
>> > > >>>> + }
>> > > >>>> + }
>> > > >>>> +
>> > > >>>> // Typically the NonCallSite UnwindPlan is the unwind
>> created by inspecting the assembly language instructions
>> > > >>>> if (behaves_like_zeroth_frame)
>> > > >>>> {
>> > > >>>>
>> > > >>>>
>> > > >>>> you'll find that you have to "next" twice to step out of a
>> function. Why? With a simple function like:
>> > > >>>>
>> > > >>>> * thread #1: tid = 0xaf31e, 0x0000000100000eb9 a.out`foo + 25 at
>> a.c:5, queue = 'com.apple.main-thread', stop reason = step over
>> > > >>>> #0: 0x0000000100000eb9 a.out`foo + 25 at a.c:5
>> > > >>>> 2 int foo ()
>> > > >>>> 3 {
>> > > >>>> 4 puts("HI");
>> > > >>>> -> 5 return 5;
>> > > >>>> 6 }
>> > > >>>> 7
>> > > >>>> 8 int bar ()
>> > > >>>> (lldb) disass
>> > > >>>> a.out`foo at a.c:3:
>> > > >>>> 0x100000ea0: pushq %rbp
>> > > >>>> 0x100000ea1: movq %rsp, %rbp
>> > > >>>> 0x100000ea4: subq $0x10, %rsp
>> > > >>>> 0x100000ea8: leaq 0x6b(%rip), %rdi ; "HI"
>> > > >>>> 0x100000eaf: callq 0x100000efa ; symbol stub
>> for: puts
>> > > >>>> 0x100000eb4: movl $0x5, %ecx
>> > > >>>> -> 0x100000eb9: movl %eax, -0x4(%rbp)
>> > > >>>> 0x100000ebc: movl %ecx, %eax
>> > > >>>> 0x100000ebe: addq $0x10, %rsp
>> > > >>>> 0x100000ec2: popq %rbp
>> > > >>>> 0x100000ec3: retq
>> > > >>>>
>> > > >>>>
>> > > >>>> if you do "next" lldb will instruction step, comparing the stack
>> ID at every stop, until it gets to 0x100000ec3 at which point the stack ID
>> will change. The CFA address (which the eh_frame tells us is rbp+16) just
>> changed to the caller's CFA address because we're about to return. The
>> eh_frame instructions really need to tell us that the CFA is now rsp+8 at
>> 0x100000ec3.
>> > > >>>>
>> > > >>>> The end result is that you need to "next" twice to step out of a
>> function.
>> > > >>>>
>> > > >>>> AssemblyParse_x86 has a special bit where it looks or the 'ret'
>> instruction sequence at the end of the function -
>> > > >>>>
>> > > >>>> // Now look at the byte at the end of the AddressRange for a
>> limited attempt at describing the
>> > > >>>> // epilogue. We're looking for the sequence
>> > > >>>>
>> > > >>>> // [ 0x5d ] mov %rbp, %rsp
>> > > >>>> // [ 0xc3 ] ret
>> > > >>>> // [ 0xe8 xx xx xx xx ] call __stack_chk_fail (this is
>> sometimes the final insn in the function)
>> > > >>>>
>> > > >>>> // We want to add a Row describing how to unwind when we're
>> stopped on the 'ret' instruction where the
>> > > >>>> // CFA is no longer defined in terms of rbp, but is now
>> defined in terms of rsp like on function entry.
>> > > >>>>
>> > > >>>>
>> > > >>>> and adds an extra row of unwind details for that instruction.
>> > > >>>>
>> > > >>>>
>> > > >>>> I mention x86_64 as being a possible good test case here because
>> I worry about the i386 picbase sequence (call next-instruction; pop $ebx)
>> which occurs a lot. But for x86_64, my main concern is the epilogues.
>> > > >>>>
>> > > >>>>
>> > > >>>>
>> > > >>>>> On Jul 30, 2014, at 2:52 PM, Tong Shen <endlessroad at google.com>
>> wrote:
>> > > >>>>>
>> > > >>>>> Thanks Jason! That's a very informative post, clarify things a
>> lot :-)
>> > > >>>>>
>> > > >>>>> Well I have to admit that my patch is specifically for certain
>> kind of functions, and now I see that's not the general case.
>> > > >>>>>
>> > > >>>>> I did some experiment with gdb. gdb uses CFI for frame 0,
>> either x86 or x86_64. It looks for FDE of frame 0, and do CFA calculations
>> according to that.
>> > > >>>>>
>> > > >>>>> - For compiler generated functions: I think there are 2 usage
>> scenarios for frame 0: breakpoint and signal.
>> > > >>>>> - Breakpoints are usually at source line boundary instead of
>> instruction boundary, and generally we won't be caught at stack pointer
>> changing locations, so CFI is still valid.
>> > > >>>>> - For signal, synchronous unwind table may not be sufficient
>> here. But only stack changing instructions will cause incorrect CFA
>> calculation, so it' not always the case.
>> > > >>>>> - For hand written assembly functions: from what I've seen,
>> most of the time CFI is present and actually asynchronous.
>> > > >>>>> So it seems that in most cases, even with only synchronous
>> unwind table, CFI is still correct.
>> > > >>>>>
>> > > >>>>> I believe we can trust eh_frame for frame 0 and use assembly
>> profiling as fallback. If both failed, maybe code owner should use
>> -fasynchronous-unwind-tables :-)
>> > > >>>>>
>> > > >>>>>
>> > > >>>>> On Tue, Jul 29, 2014 at 4:59 PM, Jason Molenda <
>> jmolenda at apple.com> wrote:
>> > > >>>>> It was a tricky one and got lost in the shuffle of a busy
>> week. I was always reluctant to try profiling all the instructions in a
>> function. On x86, compiler generated code (gcc/clang anyway) is very
>> simplistic about setting up the stack frame at the start and only having
>> one epilogue - so anything fancier risked making mistakes and could
>> possibly have a performance impact as we run functions through the
>> disassembler.
>> > > >>>>>
>> > > >>>>> For hand-written assembly functions (which can be very creative
>> with their prologue/epilogue and where it is placed), my position is that
>> they should write eh_frame instructions in their assembly source to tell
>> lldb where to find things. There is one or two libraries on Mac OS X where
>> we break the "ignore eh_frame for the currently executing function" because
>> there are many hand-written assembly functions in there and the eh_frame is
>> going to beat our own analysis.
>> > > >>>>>
>> > > >>>>>
>> > > >>>>> After I wrote the x86 unwinder, Greg and Caroline implemented
>> the arm unwinder where it emulates every instruction in the function
>> looking for prologue/epilogue instructions. We haven't seen it having a
>> particularly bad impact performance-wise (lldb only does this disassembly
>> for functions that it finds on stacks during an execution run, and it saves
>> the result so it won't re-compute it for a given function). The clang
>> armv7 codegen often has mid-function epilogues (early returns) which
>> definitely complicated things and made it necessary to step through the
>> entire function bodies. There's a bunch of code I added to support these
>> mid-function epilogues - I have to save the register save state when I see
>> an instruction which looks like an epilogue, and when I see the final ret
>> instruction (aka restoring the saved lr contents into pc), I re-install the
>> register save state from before the epilogue started.
>> > > >>>>>
>> > > >>>>> These things always make me a little nervous because the
>> instruction analyzer obviously is doing a static analysis so it knows
>> nothing about flow control. Tong's patch stops when it sees the first CALL
>> instruction - but that's not right, that's just solving the problem for his
>> particular function which doesn't have any CALL instructions before his
>> prologue. :) You could imagine a function which saves a couple of
>> registers, calls another function, then saves a couple more because it
>> needs more scratch registers.
>> > > >>>>>
>> > > >>>>> If we're going to change to profiling deep into the function --
>> and I'm not opposed to doing that, it's been fine on arm -- we should just
>> do the entire function I think.
>> > > >>>>>
>> > > >>>>>
>> > > >>>>> Another alternative would be to trust eh_frame on x86_64 at
>> frame 0. This is one of those things where there's not a great solution.
>> The unwind instructions in eh_frame are only guaranteed to be accurate for
>> synchronous unwinds -- that is, they are only guaranteed to be accurate at
>> places where an exception could be thrown - at call sites. So for
>> instances, there's no reason why the compiler has to describe the function
>> prologue instructions at all. There's no requirement that the eh_frame
>> instructions describe the epilogue instructions. The information about
>> spilled registers only needs to be emitted where we could throw an
>> exception, or where a callee could throw an exception.
>> > > >>>>>
>> > > >>>>> clang/gcc both emit detailed instructions for the prologue
>> setup. But for i386 codegen if the compiler needs to access some
>> pc-relative data, it will do a "call next-instruction; pop %eax" to get the
>> current pc value. (x86_64 has rip-relative addressing so this isn't
>> needed) If you're debugging -fomit-frame-pointer code, that means your CFA
>> is expressed in terms of the stack pointer and the stack pointer just
>> changed mid-function --- and eh_frame instructions don't describe this.
>> > > >>>>>
>> > > >>>>> The end result: If you want accurate unwinds 100% of the time,
>> you can't rely on the unwind instructions from eh_frame. But they'll get
>> you accurate unwinds 99.9% of the time ... also, last I checked, neither
>> clang nor gcc describe the epilogue instructions.
>> > > >>>>>
>> > > >>>>>
>> > > >>>>> In *theory* the unwind instructions from the DWARF debug_frame
>> section should be asynchronous -- they should describe how to find the CFA
>> address for every instruction in the function. Which makes sense - you
>> want eh_frame to be compact because it's bundled into the executable, so it
>> should only have the information necessary for exception handling and you
>> can put the verbose stuff in debug_frame DWARF for debuggers. But instead
>> (again, last time I checked), the compilers put the exact same thing in
>> debug_frame even if you use the -fasynchronous-unwind-tables (or whatever
>> that switch was) option.
>> > > >>>>>
>> > > >>>>>
>> > > >>>>> So I don't know, maybe we should just start trusting eh_frame
>> at frame 0 and write off those .1% cases where it isn't correct instead of
>> trying to get too fancy with the assembly analysis code.
>> > > >>>>>
>> > > >>>>>
>> > > >>>>>
>> > > >>>>>> On Jul 29, 2014, at 4:17 PM, Todd Fiala <tfiala at google.com>
>> wrote:
>> > > >>>>>>
>> > > >>>>>> Hey Jason,
>> > > >>>>>>
>> > > >>>>>> Do you have any feedback on this?
>> > > >>>>>>
>> > > >>>>>> Thanks!
>> > > >>>>>>
>> > > >>>>>> -Todd
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>> On Fri, Jul 25, 2014 at 1:42 PM, Tong Shen <
>> endlessroad at google.com> wrote:
>> > > >>>>>> Sorry, wrong version of patch...
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>> On Fri, Jul 25, 2014 at 1:41 PM, Tong Shen <
>> endlessroad at google.com> wrote:
>> > > >>>>>> Hi Molenda, lldb-commits,
>> > > >>>>>>
>> > > >>>>>> For now, x86 assembly profiler will stop after 10
>> "non-prologue" instructions. In practice it may not be sufficient. For
>> example, we have a hand-written assembly function, which have hundreds of
>> instruction before actual (stack-adjusting) prologue instructions.
>> > > >>>>>>
>> > > >>>>>> One way is to change the limit to 1000; but there will always
>> be functions that break the limit :-) I believe the right thing to do here
>> is parsing all instructions before "ret"/"call" as prologue instructions.
>> > > >>>>>>
>> > > >>>>>> Here's what I changed:
>> > > >>>>>> - For "push %rbx" and "mov %rbx, -8(%rbp)": only add first row
>> for that register. They may appear multiple times in function body. But as
>> long as one of them appears, first appearance should be in prologue(If it's
>> not in prologue, this function will not use %rbx, so these 2 instructions
>> should not appear at all).
>> > > >>>>>> - Also monitor "add %rsp 0x20".
>> > > >>>>>> - Remove non prologue instruction count.
>> > > >>>>>> - Add "call" instruction detection, and stop parsing after it.
>> > > >>>>>>
>> > > >>>>>> Thanks.
>> > > >>>>>>
>> > > >>>>>> --
>> > > >>>>>> Best Regards, Tong Shen
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>> --
>> > > >>>>>> Best Regards, Tong Shen
>> > > >>>>>>
>> > > >>>>>> _______________________________________________
>> > > >>>>>> lldb-commits mailing list
>> > > >>>>>> lldb-commits at cs.uiuc.edu
>> > > >>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-commits
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>>
>> > > >>>>>> --
>> > > >>>>>> Todd Fiala | Software Engineer | tfiala at google.com |
>> 650-943-3180
>> > > >>>>>>
>> > > >>>>>
>> > > >>>>>
>> > > >>>>>
>> > > >>>>>
>> > > >>>>> --
>> > > >>>>> Best Regards, Tong Shen
>> > > >>>>
>> > > >>>>
>> > > >>>>
>> > > >>>>
>> > > >>>> --
>> > > >>>> Best Regards, Tong Shen
>> > > >>>
>> > > >>>
>> > > >>>
>> > > >>>
>> > > >>> --
>> > > >>> Best Regards, Tong Shen
>> > > >>
>> > > >>
>> > > >>
>> > > >>
>> > > >> --
>> > > >> Best Regards, Tong Shen
>> > > >>
>> > > >>
>> > > >>
>> > > >> --
>> > > >> Best Regards, Tong Shen
>> > > >> <adjust_cfi_for_frame_zero.patch>
>> > > >
>> > >
>> > >
>> > >
>> > >
>> > > --
>> > > Best Regards, Tong Shen
>> > >
>> > >
>> > >
>> > > --
>> > > Best Regards, Tong Shen
>> >
>> >
>> >
>> >
>> > --
>> > Best Regards, Tong Shen
>> >
>> > _______________________________________________
>> > lldb-commits mailing list
>> > lldb-commits at cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-commits
>> >
>> >
>> >
>> >
>> > --
>> > Todd Fiala | Software Engineer | tfiala at google.com |
>> 650-943-3180
>> >
>> >
>> >
>> >
>> > --
>> > Todd Fiala | Software Engineer | tfiala at google.com |
>> 650-943-3180
>> >
>> >
>> >
>> >
>> > --
>> > Todd Fiala | Software Engineer | tfiala at google.com |
>> 650-943-3180
>> >
>> >
>> >
>> >
>> > --
>> > Best Regards, Tong Shen
>> > <augment_eh_frame.patch>_______________________________________________
>> > lldb-commits mailing list
>> > lldb-commits at cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-commits
>>
>>
>
>
> --
> Todd Fiala | Software Engineer | tfiala at google.com | 650-943-3180
>
--
Best Regards, Tong Shen
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