[Lldb-commits] [PATCH] Profile Assembly Until Ret Instruction
Tong Shen
endlessroad at google.com
Thu Jul 31 12:50:47 PDT 2014
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
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