[llvm-dev] x86: How to Force 2-byte `jmp` instruction in lowering
Dean Michael Berris via llvm-dev
llvm-dev at lists.llvm.org
Wed Jun 22 13:36:38 PDT 2016
Peter suggested just writing out '.byte 0xeb, 0x09' and that allowed the
jump instruction to bypass the relaxation, so that fixes my immediate
problem. The question still stands though whether it should be possible to
do through the instruction builder interface.
Cheers
On Wed, Jun 22, 2016 at 10:40 AM Dean Michael Berris <dberris at google.com>
wrote:
> Thanks Nirav,
>
> I can confirm that this works when I do the compile with llc, but then
> when linking to an executable with clang (patched with
> http://reviews.llvm.org/D20352 and compiler-rt patched with
> http://reviews.llvm.org/D21612) on Linux, I'm getting something
> different. Here's a sample of the transcript, and what I'm seeing:
>
> --->8 clang invocation 8<---
> [16-06-23 3:33:42] dberris at dberris: ~/xray/llvm-build% ./bin/clang
> -fxray-instrument -x c++ -std=c++11 -o test.bin test.cc -g --verbose
> clang version 3.9.0 (http://llvm.org/git/clang.git
> 3ae26ac8b1c9c5db65f3dc0236139448b8b0520a) (http://llvm.org/git/llvm.git
> 8fd5dd6aa8a633eeb03b245cd0060479371fc521)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
> InstalledDir: /usr/local/google/home/dberris/xray/llvm-build/./bin
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7.3
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8.4
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.3
> Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
> Candidate multilib: .;@m64
> Candidate multilib: 32;@m32
> Candidate multilib: x32;@mx32
> Selected multilib: .;@m64
> "/usr/local/google/home/dberris/xray/llvm-build/bin/clang-3.9" -cc1
> -triple x86_64-unknown-linux-gnu -emit-obj -mrelax-all -disable-free
> -main-file-name test.cc -mrelocation-model static -mthread-model posix
> -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases
> -munwind-tables -fuse-init-array -target-cpu x86-64 -v -dwarf-column-info
> -debug-info-kind=limited -dwarf-version=4 -debugger-tuning=gdb
> -fxray-instrument -resource-dir
> /usr/local/google/home/dberris/xray/llvm-build/bin/../lib/clang/3.9.0
> -internal-isystem
> /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8
> -internal-isystem
> /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/x86_64-linux-gnu/c++/4.8
> -internal-isystem
> /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/x86_64-linux-gnu/c++/4.8
> -internal-isystem
> /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/backward
> -internal-isystem /usr/local/include -internal-isystem
> /usr/local/google/home/dberris/xray/llvm-build/bin/../lib/clang/3.9.0/include
> -internal-externc-isystem /usr/include/x86_64-linux-gnu
> -internal-externc-isystem /include -internal-externc-isystem /usr/include
> -std=c++11 -fdeprecated-macro -fdebug-compilation-dir
> /usr/local/google/home/dberris/xray/llvm-build -ferror-limit 19
> -fmessage-length 272 -fobjc-runtime=gcc -fcxx-exceptions -fexceptions
> -fdiagnostics-show-option -fcolor-diagnostics -o /tmp/test-03d46e.o -x c++
> test.cc
> clang -cc1 version 3.9.0 based upon LLVM 3.9.0svn default target
> x86_64-unknown-linux-gnu
> ignoring nonexistent directory "/include"
> ignoring duplicate directory
> "/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/x86_64-linux-gnu/c++/4.8"
> #include "..." search starts here:
> #include <...> search starts here:
> /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8
>
> /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/x86_64-linux-gnu/c++/4.8
> /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/backward
> /usr/local/include
>
> /usr/local/google/home/dberris/xray/llvm-build/bin/../lib/clang/3.9.0/include
> /usr/include/x86_64-linux-gnu
> /usr/include
> End of search list.
> "/usr/bin/ld" -z relro --hash-style=gnu --eh-frame-hdr -m elf_x86_64
> -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o test.bin
> /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crt1.o
> /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crti.o
> /usr/lib/gcc/x86_64-linux-gnu/4.8/crtbegin.o
> -L/usr/lib/gcc/x86_64-linux-gnu/4.8
> -L/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu
> -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu
> -L/usr/lib/gcc/x86_64-linux-gnu/4.8/../../..
> -L/usr/local/google/home/dberris/xray/llvm-build/bin/../lib -L/lib
> -L/usr/lib -whole-archive
> /usr/local/google/home/dberris/xray/llvm-build/bin/../lib/clang/3.9.0/lib/linux/libclang_rt.xray-x86_64.a
> -no-whole-archive /tmp/test-03d46e.o --no-as-needed -lpthread -lrt -lm
> -latomic -ldl -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc
> --as-needed -lgcc_s --no-as-needed
> /usr/lib/gcc/x86_64-linux-gnu/4.8/crtend.o
> /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crtn.o
> --->8 clang invocation 8<---
>
> The test.cc is simply:
>
> --->8 test.cc 8<---
> #include <cstdio>
> #include <cassert>
>
> [[clang::xray_always_instrument]] void foo() { std::printf("Hello,
> XRay!\n"); }
>
> void bar() { std::printf("Not instrumented\n"); }
>
> extern "C" {
> extern int __xray_patch();
> }
>
> int main(int argc, char* argv[]) {
> printf("main has started.\n");
> bar();
> foo();
> __xray_patch();
> foo();
> }
> --->8 test.cc 8<---
>
> A snippet of the disassembly (llvm-objdump -disassemble test.bin) looks
> like:
>
> --->8 disassembly 8<---
> _Z3foov:
> 400cb0: e9 09 00 00 00 jmp 9 <_Z3foov+0xE>
> 400cb5: 66 0f 1f 84 00 00 02 00 00 nopw 512(%rax,%rax)
> 400cbe: 55 pushq %rbp
> 400cbf: 48 89 e5 movq %rsp, %rbp
> 400cc2: 48 83 ec 10 subq $16, %rsp
> 400cc6: 48 bf c5 0e 40 00 00 00 00 00 movabsq $4198085, %rdi
> 400cd0: b0 00 movb $0, %al
> 400cd2: e8 a9 f9 ff ff callq -1623 <.plt+0x30>
> 400cd7: 89 45 fc movl %eax, -4(%rbp)
> 400cda: 48 83 c4 10 addq $16, %rsp
> 400cde: 5d popq %rbp
> 400cdf: c3 retq
> 400ce0: 2e 66 0f 1f 84 00 00 02 00 00 nopw %cs:512(%rax,%rax)
> 400cea: 66 0f 1f 44 00 00 nopw (%rax,%rax)
>
> --->8 disassembly 8<---
>
> Having looked at this a bit, I think you're right that the jumps are being
> relaxed, due to the -mrelax-all option being used by clang. The question
> becomes whether it's possible to inhibit relaxation for specific
> instructions at the LLVM level.
>
> Cheers
>
>
> On Wed, Jun 22, 2016 at 9:37 AM Nirav Davé <niravd at google.com> wrote:
>
>> Hmm. Odd. I just rebuilt from scratch and it seems to work with
>> the test/CodeGen/X86/xray-attribute-instrumentation.ll test case outputing
>> straight to obj:
>>
>> llc -filetype=obj -o ~/a.o -mtriple=x86_64-apple-macosx <
>> test/CodeGen/X86/xray-attribute-instrumentation.ll
>>
>> What test case are you using?
>>
>> In any case, the issue appears to be that llvm doesn't realize that the
>> target address is resolved and erroneously applies branch relaxation to the
>> jump. I don't know why a linker private symbol would make a difference.
>>
>> -Nirav
>>
>>
>>
>> On Wed, Jun 22, 2016 at 12:14 PM, Dean Michael Berris <dberris at google.com
>> > wrote:
>>
>>> On Wed, Jun 22, 2016 at 6:05 AM Nirav Davé <niravd at google.com> wrote:
>>>
>>>> This appears to work:
>>>>
>>>> auto Target = OutContext.createLinkerPrivateTempSymbol();
>>>>
>>>> with
>>>>
>>>> auto Target = OutContext.createTempSymbol();
>>>>
>>>> -Nirav
>>>>
>>>>
>>> Thanks Nirav -- I tried this but I'm still getting a "jmpq <address>"
>>> with this incantation when I load and disassemble from gdb. I'm seeing a
>>> 5-instruction jump, followed by the nops.
>>>
>>> If I disassemble with llvm-objdump though I see the following:
>>>
>>> _Z3foov:
>>> 400c10: e9 09 00 00 00 jmp 9 <_Z3foov+0xE>
>>> 400c15: 66 0f 1f 84 00 00 02 00 00 nopw 512(%rax,%rax)
>>>
>>> I'm not sure whether the extra 0's after '0xe9 0x09' are alignment
>>> padding (though I was expecing 0x90 to show up if this was an alignment
>>> issue).
>>>
>>> Is there anything else I can try here?
>>>
>>> Thanks in advance!
>>>
>>
>>
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