Dean Michael Berris via llvm-dev
2016-Jun-22 16:14 UTC
[llvm-dev] x86: How to Force 2-byte `jmp` instruction in lowering
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! -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160622/8aa6fd99/attachment.html>
Nirav Davé via llvm-dev
2016-Jun-22 16:36 UTC
[llvm-dev] x86: How to Force 2-byte `jmp` instruction in lowering
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! >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160622/85b4ee04/attachment.html>
Dean Michael Berris via llvm-dev
2016-Jun-22 17:40 UTC
[llvm-dev] x86: How to Force 2-byte `jmp` instruction in lowering
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! >> > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160622/ebbc6ddd/attachment.html>
Reid Kleckner via llvm-dev
2016-Jun-29 16:27 UTC
[llvm-dev] x86: How to Force 2-byte `jmp` instruction in lowering
On Wed, Jun 22, 2016 at 9:36 AM, Nirav Davé <llvm-dev at lists.llvm.org> wrote:> 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. >Relaxation is the process of *shortening* jumps that can be shortened, and then re-running instruction layout to discover more relaxations until fixpoint. Removing the -relax-all flag in clang won't help here, it would hurt. I'm not exactly sure what the semantics of linker private symbols are, but using a normal assembler temporary label is probably the way to go anyway. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160629/87386802/attachment.html>
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