[Lldb-commits] [PATCH] Profile Assembly Until Ret Instruction
Tong Shen
endlessroad at google.com
Tue Aug 19 16:41:50 PDT 2014
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
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